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Terugkruising in Hibriede

Terugkruising in Hibriede


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Kan iemand my help om te verstaan ​​hoe terugkruising baster help om suiwerlyn te bereik?

Ek het gesoek na die verwysings wat ek besit, maar kon niks vind nie


Terugkruising is die proses waardeur 'n hibriede organisme teruggekruis word (teruggekruis) tot 'n suiwer organisme. Kom ons kyk na 'n voorbeeld


Organisme 1 is heterosigoties vir 'n enkele geen (bv. 'n baster), wat allele het wat ons $A$ en $a$ kan noem. Ons kan daardie organisme se genotipe skryf.

Organisme 1 Genotipe: $Aa$ (100% kans)

Op die oomblik is daar 'n 0% kans dat organisme 1 se genotipe $AA$ is.

Doelwit: verhoog waarskynlikheid dat organisme 1 se genotipe $AA$ is


Een manier om hierdie doel te bereik, is om oor te steek Organisme 1 met 'n ander organisme, kom ons noem dit Organisme 2, dit wil sê het 'n homosigotiese (bv. suiwer) genotipe van $AA$.

So as Organisme 1 is gekruis met Organisme 2 en produseer 'n nuwe Organisme 3 wat is sy genotipe? Wel, ons is nie seker nie.

Organisme 3 Genotipe: $AA$ (50% kans)

Organisme 3 Genotipe: $Aa$ (50% kans)

Nou doen ons weer dieselfde ding, om te kry Organisme 4

Organisme 4 Genotipe: $AA$ (75% kans)

Organisme 4 Genotipe: $Aa$ (25% kans).

Hierdie effek bly saamgestel, totdat daar 'n baie groot kans is dat die finale organisme 'n suiwer (bv. $AA$) genotipe het.


Wat is 'n toetskruising in biologie?

Mediese definisie van toetskruis : 'n genetiese kruising tussen 'n homosigotiese resessiewe individu en 'n ooreenstemmende vermeende heterosigoot om die genotipe van laasgenoemde te bepaal.

Net so, wat is Dihybrid-kruising in biologie? A dihibriede kruis beskryf 'n paringseksperiment tussen twee organismes wat identies baster vir twee eienskappe is. 'n Hibriede organisme is een wat heterosigoties is, wat beteken dat dit twee verskillende allele op 'n spesifieke genetiese posisie, of lokus, dra.

Behalwe hierbo, wat is toetskruis met voorbeeld?

In 'n toetskruis, word die individu met die onbekende genotipe gekruis met 'n homosigotiese resessiewe individu (Figuur hieronder). Oorweeg die volgende voorbeeld: Gestel jy het 'n pers en wit blom en pers kleur (P) is dominant vir wit (p). A toetskruis sal die organisme se genotipe bepaal.

Wat is die belangrikheid van toetskruising in genetika?

A toets kruis is mooi belangrik in genetika aangesien dit jou help om 'n onbekende genotipe te bepaal. In 'n toets kruis, word 'n homosigotiese resessiewe (albei allele is identies) individu gekruis met 'n individu met onbekende genotipe, wat 'n dominante fenotipe vertoon.


Wisselende pryse van spesies

Twee patrone word tans in die tempo van spesiasie waargeneem: geleidelike spesiasie en gepunktueerde ewewig.

Leerdoelwitte

Verduidelik hoe die interaksie van 'n organisme se bevolkingsgrootte in assosiasie met omgewingsveranderinge kan lei tot verskillende tempo's van spesiasie

Sleutel wegneemetes

Kern punte

  • In die geleidelike spesiasiemodel divergeer spesies stadig oor tyd in klein stappies, terwyl 'n nuwe spesie vinnig van die ouerspesie divergeer in die punktueerde ewewigsmodel.
  • Die twee sleutelfaktore wat die verandering in spesiasiekoers beïnvloed, is die omgewingstoestande en die bevolkingsgrootte.
  • Geleidelike spesiasie sal heel waarskynlik voorkom in groot populasies wat in 'n stabiele omgewing leef, terwyl die leesteken-ewewigmodel meer geneig is om in 'n klein populasie met vinnige omgewingsverandering te voorkom.

Sleutel terme

  • punktueerde ewewig: 'n evolusieteorie wat beweer dat evolusionêre verandering geneig is om gekenmerk te word deur lang periodes van stabiliteit, met ongereelde episodes van baie vinnige ontwikkeling
  • gradualisme: in evolusionêre biologie, geloof dat evolusie teen 'n bestendige pas voortgaan, sonder die skielike ontwikkeling van nuwe spesies of biologiese kenmerke van een generasie na die volgende

Wisselende pryse van spesies

Wetenskaplikes regoor die wêreld bestudeer spesiasie en dokumenteer waarnemings van beide lewende organismes en dié wat in die fossielrekord gevind word. Soos hul idees vorm aanneem en namate navorsing nuwe besonderhede openbaar oor hoe lewe ontwikkel, ontwikkel hulle modelle om die tempo van spesievorming te help verduidelik. In terme van hoe vinnig spesiasie plaasvind, word twee patrone tans waargeneem: die geleidelike spesiasiemodel en die punktueerde ewewigsmodel.

In die geleidelike spesiasiemodel divergeer spesies geleidelik oor tyd in klein stappies. In die gepunktueerde ewewigsmodel verander 'n nuwe spesie vinnig van die ouerspesie en bly dan grootliks onveranderd vir lang tydperke daarna. Hierdie vroeë veranderingsmodel word punktueerde ewewig genoem, want dit begin met 'n punktueerde of periodieke verandering en bly dan in balans daarna. Terwyl gepunktueerde ewewig 'n vinniger tempo suggereer, sluit dit nie noodwendig gradualisme uit nie.

Gegradueerde Spesiasie vs Gepunteerde Ekwilibrium: In (a) geleidelike spesiasie divergeer spesies teen 'n stadige, bestendige pas soos eienskappe inkrementeel verander. In (b) gepunktueerde ewewig divergeer spesies vinnig en bly dan onveranderd vir lang tydperke.

Die primêre beïnvloedende faktor op veranderinge in spesiasietempo is omgewingstoestande. Onder sekere toestande vind seleksie vinnig of radikaal plaas. Beskou ’n soort slakke wat al vir baie duisende jare met dieselfde basiese vorm geleef het. Lae van hul fossiele sou vir 'n lang tyd soortgelyk lyk. Wanneer 'n verandering in die omgewing plaasvind, soos 'n daling in die watervlak, word 'n klein aantal organismes binne 'n kort tydperk van die res geskei, wat in wese een groot en een klein bevolking vorm. Die klein bevolking staar nuwe omgewingstoestande in die gesig. Omdat sy genepoel vinnig so klein geword het, word enige variasie wat na vore kom en wat help om die nuwe toestande te oorleef, die oorheersende vorm.


Terugkruismetode: Betekenis en kenmerke | Oesverbetering | Plantkunde

In hierdie artikel sal ons bespreek oor:- 1. Betekenis en kenmerke van terugkruismetode 2. Genetiese basis van terugkruisingsmetode 3. Teelprosedure 4. Prestasies 5. Verdienstelikheid en nadele.

Betekenis en kenmerke van Backcross-metode:

Terugkruis verwys na kruising van F1 met een van sy ouers. Wanneer die F1 gekruis is met homosigotiese resessiewe ouer, staan ​​dit bekend as toetskruising.

’n Stelsel van teling waarin herhaalde terugkruisings gemaak word om ’n spesifieke karakter oor te dra na ’n goed aangepaste variëteit waarvoor die variëteit gebrekkig is, word terugkruisteling genoem. Die terugkruismetode van teling word algemeen gebruik in self- en kruisbestuifde spesies. In vegetatief vermeerderde gewasse soos suikerriet en aartappels word hierdie metode selde gebruik en dit ook met enkele wysigings.

Die belangrikste kenmerke van hierdie metode word kortliks hieronder aangebied:

Die terugkruismetode word oor die algemeen gebruik om spesifieke karakter van 'n goed aangepaste variëteit waarvoor dit 'n gebrek het, soos weerstand teen 'n spesifieke siekte, te verbeter. Hierdie metode word meer algemeen gebruik vir die oordrag van monogene of oligogeniese karakters as poligene karakters.

Met ander woorde, dit is meer suksesvol wanneer die karakter hoë oorerflikheid het. Oligogeniese karakters het 'n hoë oorerflikheid as poligene eienskappe. Terugkruismetode is van toepassing in al drie groepe gewasplante, nl. selfbestuif, kruisbestuif en ongeslagtelik voortgeplant.

Backcross metode behels twee tipes ouers, nl. ontvanger ouer en skenker ouer. Die ouer wat 'n gewenste karakter ontvang, staan ​​bekend as ontvanger ouer. Die ontvanger-ouer word herhaaldelik in die terugkruismetode gebruik, daarom word dit ook as herhalende ouer genoem.

Die ontvanger ouer is oor die algemeen 'n goed aangepaste hoë-opbrengs variëteit van 'n area wat 'n gebrek aan een of min karakters het. Die ouer wat die gewenste karakter skenk, staan ​​bekend as skenker-ouer. Aangesien skenkerouer slegs een keer in die kruising gebruik word, staan ​​dit ook bekend as nie-herhalende ouer. Die skenkerouer is oor die algemeen arm aan agronomiese karakters. Dus word terugkruismetode gebruik wanneer een van die ouers onaangepaste tipe is.

3. Genetiese Grondwet:

Terugkruismetode behou die genotipe van oorspronklike variëteit behalwe vir die karakter wat deur terugkruising verbeter word. Met ander woorde, die nuwe variëteit lyk soos die ouervariëteit in al die karakters behalwe vir die karakter onder oordrag.

4. Aantal terugkruisings:

Oor die algemeen is 5 tot 6 terugkruising voldoende om die genotipe van oorspronklike variëteit met nuwe karakter te behou.

Die basiese vereistes om 'n backcross-program te begin is:

(iii) Hoë oorerflikheid van die karakter onder oordrag.

Genetiese basis van terugkruisingsmetode:

Terugkruising verhoog die frekwensie van gewenste individue in 'n populasie. Byvoorbeeld, van 'n kruising wat enkellokus (AA x aa) behels, sal ons slegs 1/4 gewenste individue (AA) in F kry2 deur selfing (1AA: 2Aa: 1 aa). In die geval van terugkruising (AA x Aa), kry ons 1/2 wenslike individue in die BC F1 (1 AA: 1 Aa).

Dieselfde word vir elke geenpaar verwag. Die bevolking word geleidelik identies aan die herhalende ouer. Die populasie word nie in 2 n homosigotiese genotipes verdeel soos wat gebeur in geval van selfing nie.

By terugkruising word homosigositeit egter teen dieselfde tempo bereik as met selfing wat hieronder gegee word:

Proporsie homosigotiese individue = [(2 m – 1)/2 m ] n

m = aantal terugkruising of selfing en

Boonop is die kanse om die verband tussen wenslike en ongewenste gene te verbreek meer met terugkruising as met selfing. Gestel geen A is wenslik en dit is gekoppel aan ongewenste geen b. Die gewenste geen A moet van 'n skenker na 'n goed aangepaste variëteit oorgedra word.

Die kruising tussen aangepaste en skenkerouers sal AaBb-baster produseer. Die gene A en ‘a’ het die neiging om saam te erf om dit moeilik te maak om AB kombinasie te verkry. Aangesien geen B met elke terugkruising weer ingestel word, sal daar verskeie kanse wees dat die oorkruising plaasvind.

Dus is die waarskynlikheid van eliminasie van b geen soos hieronder gegee:

Waarskynlikheid van eliminasie van b geen = 1 – (1 – p) m+1

p = rekombinasie breuk en

Teelprosedure van Terugkruismetode:

Sommige karakters word deur dominante geen beheer en ander deur resessiewe geen. Die teelprosedure van terugkruismetode hang daarvan af of die karakter onder oordrag deur dominante of resessiewe geen beheer word.

Die teelprosedure vir beide die situasies word kortliks hieronder aangebied:

1. Oordrag van dominante geen:

Gestel verwelkweerstand in katoen word deur 'n dominante geen (RR) beheer. Die skenker ouer is 'n stam (B) van die kiemplasma. Die weerstand moet oorgedra word na 'n aangepaste variëteit (A) wat vatbaar is vir verwelk. Die aangepaste variëteit (A) sal as herhalende ouer gebruik word en stam (B) as skenker ouer.

Die F1 sal verwelkbestand maar heterosigoties (Rr) wees. Terugkruising van F1 (Rr) met vatbare variëteit (rr) sal weerstandbiedende en vatbare plante in gelyke aantal in BC produseer1 F1 (1Rr: 1rr). Die weerstandbiedende katoenplant (Rr) kan uitgeken word deur die materiaal in verwelk siek plot te laat groei. Die weerstandbiedende plante (Rr) word dan teruggekruis na die aangepaste variëteit.

Oor die algemeen is 6-8 terugkruisings voldoende om plante identies aan aangepaste variëteit te verkry, behalwe vir die bygevoegde gene vir verwelkweerstand. Die verwelkbestande plante is heterosigoties (Rr). Hulle word vir een generasie self geplant om homosigotiese (RR) weerstandbiedende plante te verkry. Al die weerstandbiedende egte teelplante word grootgemaak en nuwe variëteit word vrygestel. Die variëteit wat so ontwikkel is, is identies aan die aangepaste variëteit (A) wat vir verwelkweerstand verwag word (Tabel 20.1).

2. Oordrag van resessiewe geen:

Gestel verwelkweerstand in katoen word deur 'n resessiewe geen (rr) beheer. In so 'n geval sal die nageslag van elke terugkruising in twee genotipes (RR en Rr) skei wat nie geïdentifiseer kan word nie. Daarom is dit nodig om self die populasie na elke terugkruising te bekom om weerstandbiedende homosigotiese resessiewe plante (rr) te verkry.

Die weerstandbiedende plante word geïdentifiseer deur die F2 materiaal in verwelk siek plot. Die weerstandbiedende plante word teruggekruis met aangepaste variëteit. Hier word elke terugkruising gevolg deur een selfing, terwyl met dominante geen deurlopende terugkruisings gemaak word.

3. Oordrag van kwantitatiewe eienskappe:

Terugkruismetode word gewoonlik gebruik vir die oordrag van monogene of oliegogene karakters. Dit kan ook gebruik word vir die oordrag van poligeniese eienskappe. Oordrag van poligeniese karakters is egter ietwat moeilik as gevolg van lae oorerflikheid van sulke karakters en meer invloed van die omgewing in die uitdrukking van poligeniese karakters. Vir suksesvolle oordrag van poligeniese karakter moet die nie-herhalende ouer met uiterste fenotipe vir die poligeniese karakter onder oordrag gekies word.

As ons byvoorbeeld proteïenpersentasie van 20 tot 25% wil verbeter, moet ons nie-herhalende ouer met 30% proteïen kies. Dit sal die identifikasie van die karakter maklik maak. Bowendien, na elke terugkruising is een of twee generasies van selfing nodig om die gewenste segregante te kry. Verder moet groot bevolkings grootgemaak word om die gewenste kombinasie te bereik. Met ander woorde, die waarnemings moet op groot steekproewe gebaseer word.

Soms moet verskeie karakters deur terugkruising na 'n aangepaste kultivar oorgeplaas word.

Dit kan op twee maniere bereik word:

(i) Oordrag van gene in afsonderlike terugkruisingsprogram en kombineer dit dan in enkele genotipe, en

(ii) Gelyktydige oordrag van gene in enkele genotipe in een terugkruisingsprogram.

Vir gelyktydige oordrag van veelvuldige karakters, moet terugkruis sade in meer hoeveelheid geproduseer word om seker te wees om 'n genotipe met alle gewenste gene te kry.

Prestasies van Backcross-metode:

Terugkruismetode is wyd gebruik vir die ontwikkeling van siektebestande variëteite in beide self- en kruisbestuifde spesies. Dit is ook gebruik vir interspesifieke geenoordrag en ontwikkeling van multilynvariëteite in selfbestuifde spesies. Verskeie variëteite wat bestand is teen verskeie siektes is ontwikkel, deur hierdie metode in koring, katoen en verskeie ander gewasse.

In katoenvariëteite is V797, Digvijay, Vijalpa en Kalyan wat aan Gossypium herbaceum behoort ontwikkel deur terugkruismetode. 'n Kort vergelyking van stamboom-, grootmaat- en terugkruis-teelmetodes word in Tabel 20.2 aangebied.

Meriete en nadele van terugkruismetode:

1. Terugkruismetode behou alle wenslike handveste van 'n gewilde aangepaste variëteit en vervang ongewenste alleel by 'n spesifieke lokus.

2. Dit is 'n nuttige metode vir die oordrag van oliegogene karakter soos siekteweerstand. Dit is ook nuttig in die inkorporering van gene vir kwaliteit soos proteïeninhoud.

3. Hierdie metode word wyd gebruik in die ontwikkeling van variëteite met veelvuldige siekteweerstand. Multilyn-variëteite wat weerstandbiedende gene vir verskillende rasse van patogeen dra, word ook deur terugkruismetode ontwikkel. Dit word gebruik vir die ontwikkeling van isogeniese lyne en multilyn variëteite, 'n mengsel van verskeie isogeniese lyne.

4. Die manlike steriliteit en vrugbaarheid herstel gene word oorgedra na verskeie agronomiese basisse deur hierdie metode.

5. Dit is die enigste teelmetode wat vir interspesifieke geenoordrag gebruik word.

6. Die variëteit wat deur hierdie metode ontwikkel word, vereis nie multi-lokasie toetsing nie, want dit is identies aan ouer variëteit behalwe vir die karakter onder oordrag.

1. Hierdie metode word gebruik om die gebrek van 'n aangepaste variëteit reg te stel. Die nuwe variëteit verskil slegs van die ou een ten opsigte van gebrek wat reggestel is.

2. Dit behels baie kruiswerk. Die terugkruisings moet vir 6-8 generasies gemaak word. In stamboom- en grootmaatmetodes word hibridisering slegs een keer gedoen.

3. Soms is ongewenste karakter nou verbind met wenslike karakter, wat ook na die nuwe variëteit oorgedra word.


Bespreking

Die belangrikheid van paring tussen spesies in die natuur word meer duidelik namate molekulêre studies uitgebreide bewyse van hibridisasiegebeure toon. Dus, die studie van basters en reproduktiewe isolasie is nou sentraal tot ons begrip van die oorsprong en instandhouding van spesies [4, 6, 34]. In die huidige studie is die gebruik van eksperimentele basters tussen die helmknop smutswamme, M. lychnidis-dioicae en M. silenes-dioicae, help om die effek van paringstipe tydens reproduktiewe isolasie in onlangs gedivergeerde susterspesies te belig. Die resultate toon beduidende ondersteuning vir voorparingsversperrings wat afhang van die kombinasies van spesifieke paringstipe chromosome en minder ooglopende tekens vir na-paring isolasie gedryf deur die oorsprong van die paringstipes. In teenstelling met vorige hibriede studies in hierdie stelsel, openbaar die gebruik van gekontroleerde terugkruisingseksperimente verder effekte wat waarskynlik belangrike rol sal speel in beide die handhawing van spesieskeiding en die aard van terugkruisingsbasters afstammelinge wat in die teenwoordigheid van terugkruisingspotensiaal kan ontstaan.

Assortatiewe paring in Mikrobotryum

Voorparingsversperrings tussen susterspesies, wat direk tot hul evolusionêre lewensvatbaarheid en isolasie kan bydra, is 'n kwessie wat gekompliseer word deur die veelvuldige invloede op kontak en paring tussen individue. In die helmknop smutswamme wat die Caryophyllaceae infekteer, waar die patogene vir hul spesifieke gashere gespesialiseer is [18, 19, 22, 30], word die sterkte van voorparingshindernisse in simpatie swak verstaan. Simpatriese bevolkings van M. lychnidis-dioicae en M. silenes-dioicae is algemeen, maar die frekwensie van hibriede genotipes blyk laag te wees [23, 24]. Vorige studies het oor die algemeen nie oorsake vir pre-paring isolasie by kontak tussen spesies [30] opgespoor nie, afgesien van die potensiële bydrae van 'n ontwikkelingsbeïnvloedde hoë selfingtempo in kombinasie met broers en susters kompetisie [28]. Die huidige studie toon dat assortatiewe paring, in die vorm van die herkenning van spesiespesifieke variasie by die paringstipe lokus, kan dien as voorparingsversperring wat aktief is tussen susterspesies van Mikrobotryum. Alhoewel die sein betekenisvol is, is patrone vir assortatiewe paring onder die twee spesies swak. Dit kan wees as gevolg van die feit dat die naaste verwant Mikrobotryum spesies is gebruik - uit die noodsaaklikheid om lewensvatbare F1-hibriede meiotiese produkte te hê, en ons kan sterker MAT-voorspelde assortatiewe paring verwag in meer verafgeleë ander spesiepare wat in die natuur in aanraking kom. Boonop beklemtoon die groot variasie in vervoegingstempo's tussen replikate dat paring in Mikrobotryum kan ook beïnvloed word deur ander faktore wat nog nie geïsoleer is nie.

Byvoorbeeld, in die studie deur Le Gac et al. [30], assortatiewe paring is geëvalueer deur te toets vir die korrelasie tussen paringtempo's (intra- en interspesifiek) en die genetiese afstande tussen verskeie Mikrobotryum spesies. Wye variasie in paringtempo's is oor die spesiepare gesien, maar dit was nie met genetiese afstande gekorreleer nie. Die resultaat dui ook daarop dat spesieverskille behalwe die versoenbare paringstipes vervoegingstempo's kan beïnvloed (d.i. omgewingsreaksies, fenologie, ens.). In M. lychnidis-dioicae, het vorige studies getoon dat temperatuur, beskikbare voedingstowwe en die teenwoordigheid van die planteksudaat alfa-tokoferol die geneigdheid van haploïede selle om te paar [35-37] kan beïnvloed, aangesien ander organismes soortgelyk kan reageer op ekstrinsieke seine soos pH of lig [38, 39]. In die huidige studie is gamete van F1-basters geproduseer, waar die identiteit van die nie-rekombinerende paringstipe chromosoom beheer is en die outosomale komponent van die genoom word oor die algemeen verwag om 'n mengsel van die twee ouerspesies te wees, en dus potensieel homogeniseer die invloed van kontrasterende sellulêre reaksies op nie-feromoon-gebaseerde omgewingsaanwysings. Daarom, met hierdie benadering kan die invloed van paringstipes op gedrag waarskynlik beter opgelos word as in vorige studies om voorkeure in die paringsversoenbaarheidseine te openbaar.

Met bewyse dat M. lychnidis-dioicae en M. silenes-dioicae oorspronklik afgewyk het deur allopatriese isolasie [24], sien ons nou dat die patogene aanpassing toon aan spesifieke gashere wat verder kan bydra tot hul isolasie [40]. Neutrale divergensie of seleksie vir assortatiewe paring by sekondêre kontak (d.w.s. versterking, maar sien [41]), bly geloofwaardige verklarings vir die evolusie van die patrone wat hier waargeneem word.

Bronne van na-paring isolasie

Fiksheidsvermindering as gevolg van wanaanpassing by oueromgewings en genomiese vlak onverenigbaarheid wat tipies van basters is, is eksperimenteel gedemonstreer deur gebruik te maak van kruisings tussen Mikrobotryum spesies. Interspesifiek Mikrobotryum basters is minder suksesvol om gasheerplante te besmet as die nageslag van intraspesifieke kruisings [30]. Ook, basters toon dikwels onvolledige sporulering op gasheerplante [31, 32]. Boonop het die studie van Le Gac et al. [30] het die bestaan ​​van gasheerafhanklike faktore aan die lig gebring wat basters fiksheid beïnvloed, waar identiese F1-baster genotipes tussen M. lychnidis-dioicae en M. silenes-dioicae verskil in infeksievermoë op hul twee gashere, S. latifolia en S. dioica. Ons resultate kan die gevolgtrekking van gasheer-afhanklike effekte op baster fiksheid ondersteun, waar terugkruising homospesifiek was vir die M. lychnidis-dioicae paringstipe is aansienlik bevoordeel S. latifolia maar nie aan nie S. colorata, in ooreenstemming met die verwagting wat gasheer aanpassing by S. latifolia is 'n ekstrinsieke post-paringsfaktor [42]. Dit val saam met die meta-analise studie van Giraud en Gourbier [43] wat ook beklemtoon dat die voorkoms van post-paring hindernisse in Mikrobotryum is meer waarskynlik veroorsaak deur ekstrinsieke faktore as genetiese onverenigbaarheid.

In ons ontwerp kon die tweede natuurlike ouergasheeromgewing nie gebruik word nie, (d.w.s. Silene dioica), maar dit sal baie insiggewend wees om kragte van ekstrinsieke isolasie te toets in modelle wat beide ouergashere as omgewings kan insluit. Die gebruik van 'n nuwe gasheer-omgewing het ons studie egter toegelaat om paringsversoenbaarheid te bepaal gebaseer op die spesie-spesifieke paringstipe chromosome. In die onbevooroordeelde nuwe gasheer omgewing het patogene teruggekruis met genome met 'n hoër persentasie van 'n enkeling Mikrobotryum spesies (m.a.w. homospesifieke terugkruising) behoort beter te presteer as nageslag met 'n meer mosaïekgenoom (heterospesifieke terugkruisings), maar hierdie studie het nie bewyse vir so 'n effek gelewer nie. Die gebrek aan bewyse vir negatiewe epistatiese interaksies in hierdie terugkruisings kan redelik wees met inagneming van die baie klein genetiese afstand tussen M. silenes-dioicae en M. lychnidis-dioicae[21, 44] al toon hierdie twee swamspesies verminderde hibriede fiksheid in die vorm van steriliteit [32].

Daarbenewens dui resultate verkry op die nuwe gasheer omgewing daarop dat groter genetiese bydrae van die M. lychnidis-dioicae spesies het 'n infeksievoordeel gebied. Die rigting van terugkruising na M. lychnidis-dioicae was hoër in beide van die paringstipe behandelings, waar die mees suksesvolle infeksie paringstipe heterospesifiek was, en aansienlik hoër in een geval as die 0.5 neutrale verwagting. N groter infeksie potensiaal van M. lychnidis-dioicae as ander Mikrobotryum spesies is voorheen waargeneem [21, 44]. Dus, met betrekking tot na-paring isolasie in Mikrobotryum, is dit belangrik dat spesie-spesifieke eienskappe naas die klassifikasie van intrinsieke en ekstrinsieke faktore in ag geneem word, en so 'n effek kan ook bygedra het tot hoër infeksiekoerse d.m.v. M. lychnidis-dioicae-terugkruis patogene op die S. latifolia gasheer.

Potensiaal vir verbastering en terugkruising in Mikrobotryum

Terwyl 'n groot aantal studies molekulêre gereedskap gebruik vir die ontleding van huidige en vorige hibridisasie, neem die huidige studie 'n ander benadering om die potensiële impak van interspesifieke paring deur beheerde terugkruisingseksperimente te belig. F1-basters en terugkruisings tussen die twee wat nou verwant is Mikrobotryum spesies M. lychnidis-dioicae en M. silenes-dioicae is hoogs lewensvatbaar op 'n natuurlike gasheer en 'n nuwe gasheer, wat die idee ondersteun dat hibridisering en introgressie die potensiaal het om natuurlike te beïnvloed Mikrobotryum bevolkings. Daar is verskeie voorbeelde in plante en diere waar hibridisasie blykbaar nuwe evolusionêre afstammelinge fasiliteer [4, 6, 45], en in swamme is hibriede spesiasiegebeure ook beskryf [1, 46, 47]. Die huidige studie verskaf insigte in die potensiaal vir basterspesiasie in Mikrobotryum en vir introgressie deur terugkruising van allele van een spesie na 'n ander, wat albei voorgestel is deur molekulêre analise van natuurlike Mikrobotryum bevolkings [24, 44].

Reproduktiewe isolasie van die ouerspesie is noodsaaklik vir die opkoms van 'n nuwe basterspesie. Dit kan bereik word deur veranderinge in ekologie of genetika (d.w.s. ploïdie) wat die produksie van nageslag tussen hibriede genotipes bevoordeel [4, 48, 49]. Die voorkeur deur F1 Mikrobotryum basters vir vervoeging met versoenbare paringstipe allele van dieselfde ouerspesie kan eerder terugkruising bo basterselfing bevoordeel, omdat F1-basterselfing noodwendig heterospesifiek is by die paringstipe terwyl terugkruising as homospesifiek bevoordeel kan word. Daar moet egter op gelet word dat dit slegs die potensiële invloed van die paringstipe op die proses van terugkruising sal wees, wat dalk nie sterk genoeg is om teengeprogrammeerde effekte op ontwikkeling wat selfvorming in hierdie organisme bevoordeel nie.


TERUGKRUIS ROETE

Dominante AAB basters met CaMa sitotipe

Om die afwesigheid van 'n B-genoom plastied en mitochondrion bydrae tot die sitotipe van AB en AAB tipes te verduidelik, het Carreel (1994) voorgestel dat 'n vrugbare primêre AB baster van 'n kruis AAvroulik × BBmanlik met CaMb sitotipe kan bestuif gewees het deur 'n AA skenker van sitotipe CaMa. Dit sou verseker het dat sy AA of AB nageslag almal CaMa was. Dit is bekend dat die bestuiwing van 'n AB-tipe deur AA lewensvatbare diploïede nageslag produseer, maar hul frekwensie is vermoedelik afhanklik van die genotipe van die primêre AB-diploïed se B-genoom-voorloper (Shepherd, 1999). Die sekondêre AB baster van sitotipe CaMa kon dan ABA (AAB) nageslag van CaMa sitotipe produseer wanneer dit deur 'n AA tipe bestuif word.

Die rariteit van eetbare AB-tipes laat die vraag ontstaan ​​of die (AB) × AA-roete (hakies dui die bron van vroulike meiotiese restitusie aan) in werklikheid 'n bydrae kon gemaak het tot die voorkoms van die AAB-tipes wat oorheers onder Afrika en Stille Oseaan plantains. Die oënskynlike afwesigheid van eetbare AB-tipes buite Indië maak die roete nogal ongeloofwaardig. Die alternatief, vanaf 'n minder vrugbare eetbare AA en via die (AA) × BB kruising, blyk meer realisties te wees, aangesien die AAB baster die CaMb sitotipe sal hê, en sy bestuiwing deur 'n manlik-vrugbare AA ouer sal 'n AAB met die CaMa sitotipe. So 'n scenario is meer as haalbaar in die situasie (soos in die laaglande en eilande van Suidoos-Asië) waarin 'n klein aantal wilde BB-tipes omring word deur baie AA-tipes. Die 'wilde' BB is waarskynlik in die afgeleë verlede deur menslike ingryping in hierdie streek ingebring en sedertdien genaturaliseer (De Langhe en de Maret, 1999).

AAB basters met CbMa sitotipe is ongewoon

Tot op datum blyk dit dat slegs een toetreding ('Pisang Radjah') die CbMa-sitotipe besit. 'n Moontlike oorsprong vir hierdie tipe het moontlik deur 'n primêre BA-diploïed gegaan wat gevorm is deur die kruis (wilde)BB × (eetbare)AA (CbMa), met die eetbaarheid en vroulike restitusie van die triploïede BAA (CbMa) wat van die AA-stuifmeelouer geërf is betrokke by die (BA) × AA-kruising.

Veelvuldige oorsprong van ABB basters

Boonruangrod et al. (2008) het twee sitotipes onder ABB-toetredings waargeneem: CaMb in 'Pelipita', 'Saba', 'Monthan', 'Ney Mannan' en 'Bluggoe' en CbMb in 'Pisang Awak', 'Peyan' en 'Klue Teparod' (Tabel 1). Die Indiese toetredings 'Monthan', 'Ney Mannan' en 'Bluggoe' sou uit die kruis (AB) × BB gegenereer gewees het. Vir die Filippynse kultivars 'Pelipita' en 'Saba' is die (AB) × BB-roete egter onwaarskynlik, aangesien geen eetbare AB-tipes in hierdie streek aangeteken is nie. Omdat eetbare AA-tipes endemies is, is die waarskynlike oorsprong [(AA) × BB] → (AAB) × BB → ABB.

Dit laat die probleem van die ABB (CbMb) tipes. Die teenwoordigheid van Cb bepaal dat 'n BB-tipe die moederlike ouer was. As die vaderlike ouer van die primêre baster 'n AA-tipe was, sou hierdie BBA-baster 'n CbMa-sitotipe hê, wat tot op hede nog nie onder ABB-tipes waargeneem is nie. 'n Teoretiese roete kan voorgestel word, wat deur 'n BA-diploïed gaan wat van 'n kruising (BB × AA) afgelei is, en sy terugkruising na BB om BAB (CbMb) nageslag te produseer. Alhoewel hierdie roete denkbaar is vir die Indiese ABB-toetreding 'Peyan' en miskien ook vir 'Klue Teparod', bied dit nie 'n aanvaarbare verduideliking van die oorsprong vir 'Pisang Awak' nie, aangesien geen eetbare AB-tipes in Thailand bekend is nie, terwyl die uitsonderlike somaklonale diversiteit van 'Pisang Awak' dui op sy moontlike oorsprong as 'n triploïede baster in hierdie streek.

In 'n alternatiewe skema sou ons aanvaar dat eetbare BB-tipes met vroulike restitusie wel bestaan. Dan kon die stamboom van die BBA (CbMb) tipes via 'n [(BB) × AA] baster (BBA van sitotipe CbMa) gewees het, gevolg deur sy bestuiwing met BB. Die onderliggende aanname bly egter kontroversieel balbisiana-agtige plante wat min of meer pitlose vrugte dra, is beskryf. Dus, Swangpol et al. (2007) het cpDNA-volgorde-gebaseerde analise verskaf om te wys dat sommige ABB-toetredings moet hê M. balbisiana as 'n moederlike voorouer. Verder 'n DNS-analise, gebaseer op ses onderskeidende kerngeenfragmente, van vier balbisiana-agtige eetbare piesangmonsters van Noord-Thailand (waarvan een diploïed en die ander triploïed was) het getoon dat geen diagnostiese A-genoomvolgorde teenwoordig was nie (G. Volkaert, Kasetsart Universiteit, Thailand, pers. komm.).


Hibriede organismes is dié wat gebore word as gevolg van die kombinasie van die eienskappe van twee organismes van verskillende variëteite, rasse of spesies deur seksuele voortplanting. Nie net plante nie, maar diere vorm ook basters in die natuur. Byvoorbeeld, wanneer 'n leeumannetjie met 'n tierwyfie paar, is die gevolglike nageslag 'n baster en 'n liger.

Liger, 'n leeu/tierbaster wat in aanhouding geteel is (Fotokrediet: Ali West / Wikimedia Commons)

Neem eweneens die voorbeeld van kappies en aas kraaie. Dit is verskillende groepe kraaie wat gewoonlik binne hul eie groep paar, maar soms paar hulle met mekaar en verbaster. Die nageslag van so 'n unie besit gewoonlik fisiese eienskappe van beide kappies en aas kraaie.

Dit is belangrik om daarop te let dat nie alle basterorganismes, of bloot basters (of kruisrasse), tussengangers tussen hul ouers is nie, sommige basters toon net basterkrag, wat beteken dat hulle langer of korter kan groei, of ander eienskappe teen 'n ander mate van intensiteit kan demonstreer as hul ouers,


Metodes wat in plantteling gebruik word (met diagram)

Die onderstaande artikel verskaf 'n oorsig oor die drie metodes wat in plantteling gebruik word. Die metodes wat in plantteling gebruik word, is: (I) Seleksie (II) Hibridisering en (III) Mutasieteling.

Metode # I. Keuse:

Die oudste metode van plantverbetering is deur die keuse van die beste plante en deur slegs die saad daaruit te kweek. Dit is slegs nuttig solank die populasie van plante 'n mengsel van suiwer lyne is. Seleksie binne 'n suiwer lyn is nutteloos volgens Mendelian Genetics.

Die suiwer lynkonsep: suiwer lynkeuse:

Louis de Vilmorin (Frankryk, 1856) het die metode van nageslagtoets ontwikkel. Individuele plante word geïsoleer en hul nageslag getoets om uit te vind of al die nageslag eenvormige karakters toon (bv. suikerinhoud van suikerbeet), dit wil sê of karakters skei. By strict progeny tests W. Johannsen (Denmark, 1857-1927) established the Pure Line Theory. He took a commercial variety of beans, selected for the weight of beans and found that a single variety could be broken up into 19 pure lines.

Each one of the 19 pure lines had a constant average weight of beans (different for the different lines) and this average within a line could not be altered by further selection. This is because the pure line is homozygous for the character or characters studied, no further segregation takes place and, therefore, any further selection within it is futile.

The Lysenko school denied the existence of pure lines since, according to them, heredity is not anything fixed but varies with a variable and unpredictable environment. Even in the Mendelian sense it is not possible to get a perfectly homozygous plant for all characters although one may get a plant homozygous for all predominant characters.

The case becomes even more complicated if polygenes or modifying genes be present. The term pure line should, therefore, be used in a relative sense.

Pure line selection is important whenever a new variety of uncertain origin is obtained, or when investigations are begun on a new crop. It loses its importance, as soon as all the pure lines are isolated. But, it has already been pointed out that the word pure line is relative. Whenever investigations are taken in hand for a new character, fresh pure line selection is necessary. Thus, one may even now carry on pure line selection for protein content or vitamin content of rice.

In the methods of selection two courses are usually followed:

Not individual plants but whole groups of plants are selected out. This is the simpler method.

2. Individual Plant selection or Pedigree selection:

Individual plants are selected out, isolated, and its seed grown separately (Progeny test). The same process may be repeated for a few generations. The process is naturally more rigorous but yields better result. A very well-known method is the ear-to-row or panicle-row method. Ears or panicles of cereal crops are selected out and each ear or panicle is grown in a separate row for future selection.

Clonal Selection:

Clones are plants propagated vegetatively from single original stocks and it has already been pointed out that the genotypic constitution of plants propagated in this way is not likely to change. They are as stable as pure lines and no segregation or varia­tion is expected among them. So, selection within a clone is not likely to be fruitful. But, in nature, bud mutations have been found to occur occasionally and the selection of such bud mutations has played an important role in the breeding of clonal crops like potato, sugarcane, pineapple, apple, citrus, etc.

In clonal crops, even better results are expected if clonal propagation be combined with actual hybridisation—sometimes in places far away from the actual fields. That is why there are special sugarcane and potato breeding stations on the hills where very important work is done on the hybridisation of such clonal crops.

Variety Test:

It can now be realised that all plant breeding stations must maintain different sections for different purposes. First, there must be the variety plots where hundreds and thousands of varieties (introductions and selections of varieties usually grown) are grown year after year as a living herbarium from which seed of any variety may be obtained for further selection or hybridisation. Secondly, there are the pedigree culture or progeny test plots where the pure lines are found out. Thirdly, there must be the variety test plots.

In whatever way may a variety be obtained (by introduction or selection or hybridisa­tion), its performances must be tested before it can be recommended to the farmers. For this, the varieties under test are grown side by side with standard varieties and their qualities compared. There may be different plots for testing different qualities, viz., yield trial plots, disease nursery plots, etc.

Selection played a very important part in the early days of plant breeding. It was largely employed in the selection of Indian wheat varieties by Mr. and Mrs. Howard. Valuable strains have similarly been obtained in India by pure line selection in rice, millets, cotton, etc.

Method # II. Hybridisation:

Hybridisation is a very important method in plant breeding. It has played a big role in the development of improved sugarcane strains at Coimbatore where sugarcane has been experimentally crossed with sorghum, maize and even with arundinacia bamboo. Hybridisation has also been usefully employed in getting good strains of wheat, rice, cotton, etc. There was phenomenal improvement of wheat varieties of England by hybridisation.

There may be a local variety which is good in all respects but inferior in one particular character, e.g., the grain may have an unwelcome red colour. Let it have the constitution AABBCCRR (A, B & C are good qualities and R for red grain). The plant breeder will then find out another pure line which may not be a good variety but it will have white grains. Its constitution may be aabbccrr (a, b & c are bad recessives and r white recessive).

If a cross be made, the F1 hybrid plant will show the dominant characters and will have the constitution AaBbCcRr. In die F2 and sub­sequent generations the characters will segregate and recombine in all different ways. Some will be inferior types with white grains, some superior with red grains but there will be a very few superior types with white grains.

The plant breeder will now go on selecting for a few generations only the desirable combinations, i.e., good qualities with white grains (phenotype A-B-C-rr). The progeny will be all white as it is homozygous for rr but there will be segregation for the A, B and C characters as most of the selected plants will be heterozygous for these. If he goes on selecting for a few more generations he will ultimately find out an AABBCCrr plant which will be a pure line, being homo­zygous, and this will be the desired combination.

The plant breeder may carry on this selection of the F1 progeny by two well established methods:

The plant breeder grows every one of the F2 selected plants separately. In die F3 he again selects the suitable plants (A-B-C-rr) and grows every one of the selected plants separately keeping clear pedigree records of each plant. The advantage of this method lies in the quickness with which he will get the true breeding AABBCCrr plant. Among the F2 plants that he selects (A-B-C-rr), there may be some AABBCCrr plants which he will be able to identify in 3 or 4 generations if he is lucky enough from the simple fact that these pedigreed lines will not show any segregation.

While the pedigree method is advantageous in being very quick it is disadvanta­geous in demanding much more attention and labour. In plant breeding stations, usual­ly a large number of crosses are handled simultaneously and it is impossible to carry on with all those crosses by the pedigree method. The second method (Bulk method) is, therefore, used to save labour.

This is a mass selection method. Die F2 selected plants are not grown separately but are bulked together to form a single F3 bevolking. In die F3 again, the suitable (A-B-C-rr) plants are collected and bulked together. This goes on for a few years after which the A-B-C-rr plants are tested separately to find out the true breeding AABBCCrr plant. This method involves more time but minimises labour as the plants do not need individual attention. Very often the plant breeder has no other option but to follow this method.

The table below shows two programmes, one according to pedigree method and the other according to bulk method, which are meant for rice- but may also be followed for other small grains like wheat or barley.

Hybridisation Technique:

When a plant breeder wants to hybridise between two varieties he must first gather all information about the flowers, viz., the time of flowering, the exact time when the anther and the stigma become mature for pollination, which flowers give healthy seeds, how long do the pollens remain viable, etc. He must take all precautions so that hybridisation takes place only in the way he desires, precluding all chances of self- pollination and must ensure that no foreign pollen can contaminate the result.

He should follow the following stages:

1. Selection and preparation of parents: Isolation:

The plant breeder first selects the plant that he will use as the mother parent and keeps the male parent ready so that the anther will be ripe just at the desired time. If there are too many flowers on the branch of the mother parent he clips off a number of them.

This is specially true in the cereals (wheat or rice) where there are a big number of flowers on the spike or panicle. In rice, about ten or twelve flowers of the same age are kept. It is necessary to isolate the female parent and, sometimes, even the male parent, by growing on isolated plots or by bagging or caging. Necessity of isolation increases with the percentage of natural cross-pollination.

The anthers must be plucked off the female flowers just before the anthers are ripe (anthesis) without causing injury to the flowers and, specially, the car­pels. Care should also be taken not to break the anthers. This is easily done with a pair of fine pointed forceps in the case of larger flowers like those of tomato. Rectified spirit should be used freely in sterilising the instruments during crossing.

In the case of small flowers the process is rather painstaking. In ordinary cereals where the bracts are not brittle (e.g., wheat or oats) the process is simpler but it is rather different in rice. Fig. 882 shows the plant breeder’s kit specially necessary for emasculation.

After the flowers are emasculated they are to be kept isolated which may be done either by keeping the whole plant in a muslin cage or by enclosing the flowers in muslin or oil paper or plastic bags so that foreign pollens may not come in contact with the stigma. Fig. 883 shows different types of bagging. Usually these bags are kept till seed-setting is complete.

When the stigma of the emasculated flower is mature the bag is temporarily removed and the stigma pollinated by dusting with complete broken anthers or pollens from the male parent. Special study should be made as to the viability of the pollens. Flowers are bagged again after pollination.

Care must always be taken to keep the crossed flowers properly labelled or tagged. The label should be as brief as possible but complete. It should bear the names of the parents (female parent first) and, at least, a number referring to the field record book as shown in Fig. 882. All or her necessary particulars should be entered in a handy field record book with a number and the number referred to on the tag.

The hybridisation technique must be adapted to the particular crop on which work is being done.

Techniques usually followed for three important crops—rice, wheat and cotton are give below:

In rice, the lemma and palea are rather hard and the flowers remain open for only about half an hour, sometimes after the sun warms up. Emasculation must be done before the time of an thesis which can be easily ascertained by placing the closed flowers against the sun (Fig. 884) and looking through the semitransparent lemma-palea.

Just before anthesis the anthers rise from the base to the top of the closed spikelet.

Emas­culation may be done in four ways:

(1) By slightly forcing open the lemma and palea just before the opening of the flower. This is the method for wheat and other cereals and this is the method usually adapted for rice in India.

(2) By clipping off the tip of the open flower with a pair of scissors and taking out the anthers through the opening by a pair of fine pointed forceps or a needle. Care must taken not to break the anthers and to take out all the six anthers of rice.

(3) The panicle is covered in the early morn­ing, before blooming, by a dark or brown paper bag (Ramiah). The heat inside forces the flowers open and emasculation is carried on as usual.

(4) The last method of hot water emasculation, developed by N. E. Jodon, is very interesting and is now widely used in the U.S.A. It has also been found very fruitful in India (Gangulee 1959).

Warm water (about 43°C.) is taken in a thermoflask, a mature tiller of rice is tilted and a whole panicle kept immersed within the warm water for about 10 minutes (Fig. 884). Within a few minutes of taking the panicle out of the water, just the mature flowers open out. Not only that, the temperature renders all the anthers sterile while the carpels are not injured, thus causing automatic emasculation. Pollination is to be completed within the next half an hour after which the flowers automatically close.

For pollination whole broken anthers, from flowers in which anthesis is about to take place, are used. When emasculation is carried on by forcing the flowers open, the latter are kept closed by small rubber rings so that bagging is unnecessary. Bagging is necessary for the -second and the third methods of emasculation in rice and necessary only for the half an hour while the flowers are open after hot water treatment. Only a few flowers in a panicle should be pollinated and all others scissored off.

After the female spike is selected, 5 to 10 healthy flowers are chosen and the rest scissored off. Emasculation is easily done with a pair of narrow forceps by gently opening the lemma and palea as the latter are soft and not brittle as in rice. The emasculated spikes are kept bagged with paper or plastic bags supported on stakes. Pollination is done after two days with whole broken anthers.

Emasculation is done on the afternoon of the day previous to the normal opening of the flower. An incision is carefully made round the base of the corrolla and the latter is removed taking care not to injure the pistil. The anthers are removed by plucking or scraping them off very carefully. Anthers must not be broken in the process. Emas­culated flowers are now bagged.

A still simpler process is to take a small bit of drinking straw, to seal one of its ends and to slip the open end on the emasculated pistil so that it fits tightly on the ovary. With straw it is not necessary to scrape off the lower anthers which get rubbed off when the straw is fitted. The straw is now fastened to the stem with soft and fine copper wire. Pollination is done next morning by plucking whole flowers from previously bagged (to ensure purity of pollen) male plants and dusting the pollens on the stigma.

Special Methods Involving Hybridisation:

1. Breeding for Disease Resistance:

Disease often causes serious havoc among plants. Effective control of such diseases by germicides, etc., is often too expensive and inconvenient. On the other hand, strains of crop plants (which may even be wild, distantly related varieties or species) are some­times found to be naturally immune (resistant) while other strains are normally sus­ceptible for specific diseases.

This resistance and susceptibility to diseases (e.g., rust in wheat or Helminthosporium = Ophiobolus in rice) are usually genetic factors. Sometimes this resistance is found in wild varieties which are otherwise useless. When this is so, it is possible to get the resistance factor from the resistant variety combined with the good qualities of some suitable cultivated variety.

The cultivation of the new resistant variety will then be an efficient as well as cheap measure in controlling the disease. In breeding disease resistant strains, every hybrid generation is to be subjected to artificially created disease producing conditions. Special inoculation tents are sometimes found useful for this purpose. All plants are artificially inoculated with fungal spores, etc., and the proper environment (specially, humidity) created.

The resistant segregates are then easily spotted in every generation. Selection is carried on for a number of years till the homo­zygous strain is obtained. Breeding for disease resistance is not always simple as it is difficult to get strains resistant under all conditions and the inheritance of disease resistance is often multi-factorial. Moreover, most important diseases have a large num­ber of strains of the pathogens and it is difficult to get any variety of the crop resistant to all the strains. Some success has been attained in breeding disease resistant strains of various Indian crop plants.

One of the serious limitations of the present method of raising disease resistant varieties is that it does not take into account the potentiality of the disease producing organisms to undergo mutation. That is why it has often been found that a disease resistant variety once obtained does no remain so for a long time. Later, it becomes susceptible due to mutation of the organism causing the disease, which can then infect the so-called resistant plant.

2. Backcross and Testcross Methods:

It has already been shown how the crossover percentage of two linked genes may be determined by the testcross, i.e., by backcrossing the hybrid F1 plant back to the recessive parent. This testcross method is very usefully utilised by the plant breeder and the geneticist in determining the genotypic constitution of any plant.

As backcrossing takes place with the recessive parent, the latter does not show itself in the progeny and the backcross segregation ratio represents the gametic ratio of the plant in question and, hence, its genotypic constitution.

Thus, Fig. 885 shows the result of a testcross with a tall x dwarf heterozygous pea plant. The backcross ratio can only be explained by assuming that there are T and d gametes on the plant in equal proportions, i.e., it is a Td plant. If the backcross plant is a pure line (with only one type of gametes), the backcross generation must show plants of one type only.

Another use of the backcross method in plant breeding is introduction of a character from either (i.e., recessive or dominant) of the parents more quickly. Thus in breeding for disease resistance, the hybrid disease resistant plants of successive generations are repeatedly backcrossed with the disease resistant parent. In this way homozygosity for disease resistance may be attained a few years earlier. Backcrossing is rather easy when a monogenic character is to be transferred while it may also be adapted for multi-factorial characters.

3. The use of Hybrid Seed:

F1 plants are always more vigorous because of heterosis. Yield of a crop can be greatly increased if F1 seeds can be directly used as seed by the farmers. But, production of such seed is usually so costly that it can only be used for experimental purposes and is hopelessly uneconomic if used as the farmers seed.

However, an exception has been found in the use of hybrid com (maize) as seed. Methods have been developed in the U.S.A. for obtaining hybrid corn seed in large scales at low cost. Production of maize has been greatly increased in recent years by this process.

Production of hybrid corn has now taken hold in India. The author’s own experiments show that it is possible to cultivate hybrid rice on a small plot and obtain a much higher yield.

Maize is a naturally cross-fertilised crop. So, the first step towards producing hybrid corn is isolating the pure lines by inbreeding. The inbred pure lines are rather weak.

Two such pure lines are planted in the field alternately, one as the male stock or polli­nator and the other as the female stock or the seed producer. Such fields must be isolated from all other varieties of maize to get good uncontaminated hybrid corn The-usual method is to plant one pollinator row for every two seed rows. Male flowers of maize are borne on apical panicles or ‘tassels’ while female cobs are borne on the axils.

So, emasculation is rather easy by simply lopping off these panicles and it is now possible to carry on large-scale emasculation or ‘detasseling’ mechanically. All kernels developing on the detasseled variety are hybrids, pollination being possible only from the other variety. But, as the inbred pure line plants are weak, the production of single- cross grains on them is rather low. So, double-crossing is resorted to by making a hybrid of the second order out of two F1 single-cross hybrids in order to get a large quantity of double-cross grains.

The cobs on the double hybrids (Fig. 886) are even larger than on the single hybrids. For double crossing, two rows of pollinator single-cross plants are planted for six to eight rows of single-cross seed rows. As the single-cross plants are much more Vigorous than the pure line plants, the production of double-cross grains is much more bountiful. Four pure lines are involved in producing a double-cross hybrid and the method of such production is diagrammatically represented in Fig. 887. The fanner gets a heavy yield when he uses double-cross grains as his seed.

A new development in the production of hybrid corn is the utilisation of the ‘male- sterile’ character. Plants having this gene or character have sterile pollens so that they are naturally emasculated. If a strain having this character is used as the seed parent then it is no longer necessary to emasculate or detassel it. If it be possible to get such suitable ‘male-sterile’ genes in other crops it may be possible to get cheap hybrid F1 seeds as the labour involved in emasculating will be reduced to the minimum.

Method # III. Mutation Breeding:

A new line of plant breeding has opened up in recent years—that of mutation breeding. Important crop varieties are known to be mutants. Mint- zing thinks that more than half the species of flowering plants are polyploids. A plant breeder has to keep his eyes open to select out any naturally occurring mutant (point mutation or intergenic mutation) that may look promising. Selection within clones and pure lines is possible only when such mutants occur.

Artificial induction of mutations has been extensively employed in recent years. The most important agents for such mutation induction are (i) X-rays and other types of ionising radiation explained in (giving rise to the science of Radiation Biology) and (ii) Chemical mutagens like mustard gas or colchicine.

All growing organs, seeds, pollens, eggs, etc., may be subjected by such irradiation or chemical treatment. There is no fundamental difference between natural and artificial mutations. It is true that most mutations are of no practical importance or even harmful and also that there is no way of predicting what type of mutation one is going to get.

But, this should not give rise to any scepticism and a plant breeder should be satisfied if he gets a new beneficial character in a million. Some very useful radiation-induced mutations have already come to the use of agriculturists and horticulturists. Sweden has greatly advanced in mutation breeding since 1929 with workers like Gustafsson and Miintzing at their Svaloff station. They have obtained improved ‘ereictoid’ stiff-strawed barley varieties by X-ray treat­ment and similar treatment has yielded better varieties of white mustard, Phaseoltis, ground-nut, oats, peas, etc.

Similarly, there, are reports of improved barley from Germany rust resistant wheat from Austria improved barley, peanut and short-strawed rice (by Beachell) from the U.S.A. and improved wheat varieties in India. Radiation induction has been even more fruitful in horticulture. New flower varieties have been raised and then propagated vegetatively. In India attempts have also been made to get better varieties of rice, sesame and jute in this way. A short-strawed, high-tillered rice mutant has been reported by Ramanujam and Parthasarathy. Other results are still under investigation.

Induction of mutation with chemical mutagens has become even more popular as the method is accessible to all types of workers. Colchicine has been extensively used in the production of tetraploids and amphidiploids involving hundreds of species. Although most of these polyploids are of no practical value, they have been found to be very useful material which can possibly be improved by further breeding.

Thus, some useful Triticale (wheat X rye, i.e., Triticum x Sec ale amphidiploid) varieties have been obtained by hybridising different types of Triticales. Muntzing has obtained tetraploid winter rye. Kihara and his associates in Japan have reported triploid sugar beet with higher sugar content, triploid water-melon, tetraploid radish, etc. Tetraploid grapes, also, have been produced.

Cytogenetics has placed more synthetic breeding material in the hand of the plant breeders in the form of plants with altered genomes or substituted chromosomes and the study of trisomies, monosomies, nullisomics, etc., has enabled them to work on the definite positions of the desirable genes in the chromosomes.

Finally, the latest attempt of inducing mutations in a novel way is well worth mentioning. It has been mentioned that the most important chemical constituents of genes are nucleic acids and there is evidence that free nucleic acid may control heredity.

There has been a claim from France that deoxyribose-nucleic acid (known so cytogeneticists as DNA) extracted from the eggs of the Khaki Campbell variety of ducks when injected into the Peking variety, has induced some Khaki Campbell characters to the Peking ducks and this change has been found to be hereditary.

As DNA has been proved to be the gene substance, it is quite likely that in near future it may be possible to use just the DNA extract from one of the desirable parents in the hybridisation of higher plants as has already been done in the case of Pneumococcus strains of bacteria. If this method of changing heredity be established and improved, possibly it will be the most important tool in the hands of the plant and animal breeders, (c.f. Genetic Engi­neering).

Afsluiting:

The Importance of Plant Breeding in Modern Agriculture:

With the present population explosion the cry in the densely populated under­developed countries, specially of Asia, is for food and more food. Many, such countries like India does not produce enough cereals to feed their own populations. The need is to develop more amount of food on the same area of land.

For this, besides improved methods of agriculture, better and high yielding varieties and strains must be developed. Fortunately, great success has been achieved in this direction in recent years. A green revolution has taken place in India and some other count­ries so that these countries would soon produce more food that what they presently need.

In this connection one must men­tion Norman E. Borlaug (Fig. 888), the plant pathologist plant breeder devoting his life at the International Maize and Wheat Improvement Centre at Sonora in Mexico. His splendid work in developing new high yielding, rust resistant, non-lodging dwarf wheat varieties and strains, which are now being cultivated in many countries, is the basis of the present ‘green revo­lution’.

His Sonora 64, with Sonalika, Kalyansona and other varieties developed in India, is working a miracle. Most deservirtgly, Borlaug was awarded a Nobel Prize for Peace in 1970. Nothing contri­butes more to peace than self-sufficiency in food.

Similarly, in the field of rice also high-yielding varieties have been developed by plant breeders. IR-8 developed in the International Rice Research Institute located in the Philippines and the strains Padma and Jaya developed in India are contributing towards ‘green revolution’ here.


Backcrossing in Hybrid - Biology

Abstrak

Speciation is the underlying process that leads to formation of new species, and therefore is the basis of biodiversity. Genes involved in each stage of speciation, such as those involved in interspecies sterility, remain elusive. Male hybrid sterility and postzygotic isolation between Drosophila pseudoobscura en D. persimilis was examined in this study through backcrossing of female hybrids into each parental line (introgression), selecting for a sterile sperm phenotype, needle-eye sperm. Sperm phenotypes did not separate through backcrossing instead, males presented with multiple sperm phenotypes. A relationship between the phenotypes observed and the potential genes involved was examined through whole genome sequencing and SNP analysis of the DNA of 20 introgressed male hybrid samples. One finding was SNPs for hybrid sperm sterility were species specific. Also, sperm sterility and heteromorphism appear to be controlled by many loci. Further analysis of SNPs isolated in this study has the strong potential to identify candidates for loci involved in formation of needle-eye sperm, and postzygotic male hybrid sterility in other species.

Summary for Lay Audience

Speciation is the process of two populations of organisms of the same species evolving over time until they are unable to reproduce with each other. Some species have not completely separated, and are still able to create viable, but oftentimes sterile, hybrid offspring. A common example of hybrid sterility comes from horses and donkeys, who separated approximately 7.7-15 million years ago (Huang et al. 2015). When a male donkey and a female horse reproduce, they sire a mule. All male mules are sterile and most female mules are sterile. In rare cases female mules are fertile when mated to a horse or donkey (Savory 1970).

Similar to horses and donkeys, the crossing of two species of fruit flies, Drosophila pseudoobscura en D. persimilis, produce all sterile male hybrids. However, in the case of these fruit flies, all female hybrids are fertile. These two species of fruit flies also diverged more recently, 0.55 million years ago. These sterile hybrid male fruit flies can still produce sperm, but these sperm are not able to fertilize female eggs to make more hybrids. Fruit flies are used because they are less expensive to maintain, have shorter life cycles, and can be in a tightly controlled environment. My research focused on genetic differences cause the male fruit flies to be sterile. Hybrids receive genetic material (DNA) from both parent species. The DNA of both fly species studied here is split into two pairs of five separate chromosomes, X/Y, 2, 3, 4, and dot. The pairs of each chromosome can interact with each other through proteins. Instead of ten separate assembly lines for proteins, pairs of chromosomes are connected to each other by networks integral to protein production and cell function. In hybrids, the chromosomes are unlikely to all function properly because each species has differentiated chromosomes that might not be able to form proper pairs. The failure of some of these networks could be the basis of sterility. My study supported the species-specific differences in the pieces of the network contributing to hybrid sterility. This work can be continued to identify specific points in the DNA that lead to hybrid sterility and applied to other species.


Achieving Consistency When Breeding Marijuana Plants

If you have ever purchased a hybrid strain from a dispensary, you should appreciate the effort that has gone into ensuring your weed has consistently desirable traits. In all likelihood, the weed you buy has gone through generations of breeding to ensure it doesn’t carry unwanted characteristics.

The seeds created from cross-pollination will have different attributes to their parent strains. Every seed is unique with various characteristics and different combinations of traits derived from its parent. Seeds with different expressions of traits are known as fenotipes. If you purchase cannabis seeds , the best kind is ‘homozygous’ that means they have the same set of genes.

When you have homozygous seeds, you know that your plants will consistently produce seeds with the same genetic makeup every time. This consistency is highly desirable because it means breeders and consumers know what to expect 100% of the time. Heterozygous seeds produce a wide variety of phenotypes which makes them a lot less predictable. When you cross a strain, you need to select the phenotype you prefer.


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In: Plant Breeding , Vol. 97, No. 4, 12.1986, p. 315-323.

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T1 - Simple Genetic Control of Hybrid Plant Development in Interspecific Crosses between Phaseolus vulgaris L. and P. acutifolius A. Gray

N2 - Crosses were performed between nine Phaseolus vulgaris lines (as females) and seven P. acutifolius lines (as‐ male to examine parental compatibility for the production of vigorous hybrid And backcross plants, in vitro embryo rescue techniques were required to secure hybrid and backcross proseny following interspecific crossing. Seedling development appeared to be dependent on which allele the P. vulgaris parent carried at an interspecific incompatibility locus. Seven of the nine P. vulgaris lines tested carried an allele at this locus which interacted with a nuclear factor in the P. acutifolius genome resulting in stunted, sub‐lethal hybrids. The lines, ICA pijao' and ‘Sacramento Light Red Kidney’ did not carry this allele and produced vigorous hybrid progeny in combination with all P. acutifolius parents. Intensive backcrossing produced progeny which also segregated for sub‐lethal and viable plant development. The observed segregation patterns suggest that a bridge crossing scheme would facilitate the introgression of P. acutifolius germplasm into incompatible P. vulgaris lines. Similarities, with an intraspecific incompatibility system are discussed.

AB - Crosses were performed between nine Phaseolus vulgaris lines (as females) and seven P. acutifolius lines (as‐ male to examine parental compatibility for the production of vigorous hybrid And backcross plants, in vitro embryo rescue techniques were required to secure hybrid and backcross proseny following interspecific crossing. Seedling development appeared to be dependent on which allele the P. vulgaris parent carried at an interspecific incompatibility locus. Seven of the nine P. vulgaris lines tested carried an allele at this locus which interacted with a nuclear factor in the P. acutifolius genome resulting in stunted, sub‐lethal hybrids. The lines, ICA pijao' and ‘Sacramento Light Red Kidney’ did not carry this allele and produced vigorous hybrid progeny in combination with all P. acutifolius parents. Intensive backcrossing produced progeny which also segregated for sub‐lethal and viable plant development. The observed segregation patterns suggest that a bridge crossing scheme would facilitate the introgression of P. acutifolius germplasm into incompatible P. vulgaris lines. Similarities, with an intraspecific incompatibility system are discussed.


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