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Hoe het die kern van eukariotiese selle ontwikkel?

Hoe het die kern van eukariotiese selle ontwikkel?


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Wat is/is die gewildste teorie/teorieë oor hoe die kern ontwikkel het? Ek weet mitochondria kom van alfa-proteobakterieë, chloroplaste van sianobakterieë en dat eukariote direk uit archaea ontwikkel het, maar wat van die kern?


Ek hoop hierdie referate deur Wilson en Dawson (2011) en Devos et al. (2014) sal jou help.

Opsommend verskaf hierdie twee oorsigte bewyse vir die volgende stellings:

  1. Die kernporieëkompleks is goed bewaar met sommige streke van divergensie.
  2. Die kernlaag verskyn redelik veranderlik tussen groot supergroepe.
  3. Sentrosome is antieke strukture, maar vertoon 'n komplekse evolusionêre geskiedenis.
  4. Daar is bewyse vir prokariotiese voorouers van sommige kernkomponente.
  5. Ontleding van uiteenlopende organismes is noodsaaklik om kernbiologie en die oorsprong daarvan ten volle te verstaan.

Funksionele evolusie van kernstruktuur

Die evolusie van die kern, die bepalende kenmerk van eukariotiese selle, was lank in spekulasie en misterie gehul. Daar is nou sterk bewyse dat kernporiekomplekse (NPC's) en kernmembrane saam met die endomembraanstelsel ontwikkel het, en dat die laaste eukariotiese gemeenskaplike voorouer (LECA) ten volle funksionele NPC's gehad het. Onlangse studies het baie komponente van die kernomhulsel in lewende Opisthokonts geïdentifiseer, die eukariotiese supergroep wat swamme en metazoïese diere insluit. Hierdie komponente sluit in diverse chromatien-bindende membraanproteïene, en membraanproteïene met kleef lumenale domeine wat moontlik bygedra het tot die evolusie van kernmembraanargitektuur. Verdere ontdekkings oor die nukleoskelet dui daarop dat die evolusie van kernstruktuur styf gekoppel was aan genoomverdeling tydens mitose.

Inleiding

Die kern, 'n dubbelmembraangebonde kompartement wat die kerngenoom bevat, is die essensiële morfologiese en funksionele kenmerk van eukariote (Wilson en Berk, 2010). Behalwe chromatien, is die mees prominente struktuur van die kern die kernomhulsel (NE): twee grensmembrane met groot kernporiekomplekse (NPC's) wat molekules toelaat om die kern binne te gaan en te verlaat (Strambio-De-Castillia et al., 2010) . Nog 'n ooglopende kenmerk is die nukleolus, wat die plek is van rDNA-geenuitdrukking en ribosoomsamestelling (Németh en Längst, 2011). Minder voor die hand liggend, dus eers onlangs erken, is die dinamiese en komplekse interne argitektuur van die kern, konseptueel genoem die nukleoskelet, wat intermediêre filamente, aktien en titien insluit, en ook funksioneer tydens mitose (Simon en Wilson, 2011). Hoe het hierdie strukturele kompleksiteit ontstaan?

Die klein subeenheid RNA (SSU)-gebaseerde filogenetiese "boom van die lewe" dui op drie domeine - Bacteria, Archaea en Eucarya (Woese et al., 1990) - wat almal uiters diep oorsprong het (Pace, 2009), en deel oorvleuelende stelle gene. Het hierdie drie afstammelinge onafhanklik van die presellulêre fase van biologiese evolusie ontstaan ​​(Pace, 2009), of het die eukariotiese voorloper ontstaan ​​deur samesmelting van bakteriële en argaeale selle? Laasgenoemde moontlikheid is aantreklik gegewe die dwingende genomiese bewyse vir twee primêre simbiotiese gebeurtenisse: die endosimbiose van 'n alfaproteobakterium wat uiteindelik aanleiding gegee het tot mitochondria, en die endosimbiose van 'n sianobakterie wat aanleiding gegee het tot chloroplaste (Margulis, 1970 Pace, 2009). Daar is ook sterk filogenetiese en genomiese bewyse vir sekondêre en tersiêre endosimbiose van plastiede in sommige eukariotiese afstammelinge (Palmer en Delwiche, 1996). In teenstelling met mitochondria, chloroplaste en plastiede, is bewyse vir die betrokkenheid van endosimbiose by die evolusie van die kern egter yl of ontbreek.

Die vroeë evolusie van die eukariotiese afkoms bly troebel, grootliks omdat die genetiese diversiteit van bestaande – veral eensellige – eukariote onduidelik bly (Dawson en Pace, 2002). Die grootste genetiese diversiteit word inderdaad onder mikrobiese (enkelsellige) eukariote gesien (Sogin en Silberman, 1998). Soos Pace (2009) egter daarop gewys het, verskaf genoomvolgorde-vergelykings van lewende eukariote "geen bewyse hoegenaamd nie" of die vroegste eukariote werklik kernmembrane of NPC's as morfologiese kenmerke gehad het. Hierdie eenvoudige idee, dat die eerste eukariotiese gemeenskaplike voorouer (FECA) nie kernmorfologie gehad het nie, maak 'n mens vry om te oorweeg hoe spesifieke tipes proteïene in die FECA kon bygedra het tot die daaropvolgende inkrementele evolusie van kernstruktuur teenwoordig in die laaste gemeenskaplike eukariotiese voorouer (LECA Fig. . 1). Soos in hierdie oorsig bespreek, dui nuwe bewyse gebaseer op die voorvaderlike aard van endomembraanproteïene daarop dat die eukariotiese endomembraanstelsel saam met die kernmembrane en NPC's ontwikkel het, wat blykbaar ten volle funksioneer in die LECA (Neumann et al., 2010).

Die LECA het daarna aanleiding gegee tot ses groot eukariotiese supergroepe, wat elk mikrobiese eukariote insluit: Opisthokonts (bv. swamme, diere, protiste), Amoebozoa (bv. Dictyostelium), Uitgrawings (bv. Trypanosome, Giardia), Chromoalveolate (bv. Plasmodium), Archaeplastiede (bv. plante) en Rhizaria (Hampl et al., 2009). Alhoewel die mees basale takke ietwat kontroversieel is (Rogozin et al., 2009 Parfrey et al., 2010), laat hierdie klassifikasiestelsel 'n mens die genome van diverse eukariote binne elke supergroep vergelyk en skep inventarisse van gene wat kodeer vir bekende kernstruktuurproteïene. Vergelykings tussen supergroepe kan dan in teorie kerngene identifiseer wat afgelei word as teenwoordig in die LECA (Keeling, 2007). Hierdie benadering word egter tans beperk deur die gebrek aan annotasie van die meeste nukleoskeletale proteïene (Simon en Wilson, 2011), en deur 'n gebrek aan kennis oor die kernmembraanproteïenkomponente in die meeste eukariote.

Die geweldige diversiteit van mikrobiese eukariote word nog nie weerspieël in voltooide genoomprojekte nie (Dawson en Fritz-Laylin 2009), wat oorweldigend (meer as 80%) gefokus het op die Opisthokont (veral diere en swamme, wat nie baie gene [“sekondêr verminderde”) genome het nie. ]) en Argeplastiede (plant) afstammelinge. Net so kom die meeste funksionele kennis oor kernstruktuur van modelstelsels (diere, swamme, plante) wat slegs twee van die ses eukariotiese supergroepe verteenwoordig. Meer opeenvolgende genome, en kernomhulselproteome, van ander eukariotiese supergroepe sal noodsaaklik wees om te verstaan ​​hoe kerne ontwikkel het. Alle eukariotiese afstammelinge word gekenmerk deur die verlies, wins, uitbreiding en diversifikasie van geenfamilies (Fritz-Laylin et al., 2010). Die geskiedenis van kernstruktuur na die LECA het dus ongetwyfeld baie verskillende paaie in die ses belangrikste eukariotiese afstammelinge gevolg. Om hierdie verskille en gedeelde kenmerke te verstaan, sal ongekende insig gee in die mees fundamentele aspekte van kernstruktuur en genoomorganisasie, en kan ook terapeutiese molekulêre teikens in parasitiese eukariote voorstel.

Mede-evolusie van NPC's en endomembrane: Die proto-coatomeer hipotese

Twee dekades van intensiewe ondersoek het 'n magdom inligting oor die NPC opgelewer, insluitend sy ~30 samestellende proteïene (nukleoporiene) en hul stoïgiometrie, biochemie, samestelling en driedimensionele posisies binne die NPC (Doucet en Hetzer, 2010 Fichtman et al., 2010 Wente en Rout, 2010). Hierdie kennis sluit die funksies van spesifieke gevoude domeine binne elke nukleoporien in (Devos et al., 2006). Merkwaardig, die komponente en struktuur van een NPC-subkompleks (gewerwelde Nup107-160-kompleks) lyk soos die membraanbuigende proteïenjasse wat vesikels in die sekretoriese en endomembraanweë genereer (Fig. 2 Devos et al., 2004). Hierdie verstommende bevinding het gelei tot die proto-koatomeer hipotese, wat daarop dui dat beide strukture ontwikkel het uit 'n voorvaderlike membraan-kromme proteïen(e) (Fig. 2 Devos et al., 2004 Hsia et al., 2007 Debler et al., 2008 Leksa en Schwartz, 2010).

Om die proto-coatomeer hipotese te toets, is NPC proteïene gesuiwer uit die divergerende basale Excavate eukariote Trypanosoma brucei, 'n belangrike menslike patogeen. Gedetailleerde funksionele kennis oor spesifieke gevoude polipeptieddomeine was deurslaggewend vir die identifisering van Trypanosoom-nukleoporiene omdat die ooreenstemmende Trypanosoom-gene onherkenbaar was deur DNA-volgorde en aminosuurvergelykings alleen (DeGrasse et al., 2009). Die Trypanosome NPC-proteoom stel die bewaring van NPC-proteïene en NPC-argitektuur in die LECA voor, en ondersteun die proto-coatomeer-hipotese (DeGrasse et al., 2009). Hierdie hipotese is aansienlik uitgebrei deur 'n ontleding van 60 eukariotiese genome wat vyf supergroepe verteenwoordig, wat ten minste 23 en soveel as 26 (uit 30) nukleoporiene in die LECA geplaas het (Neumann et al., 2010). Hierdie gevolgtrekking is nie beïnvloed deur die posisie van die eukariotiese wortel nie. Onder vyf bekende transmembraannukleoporiene is twee (gp210, Ndc1) geïdentifiseer as sleutelkomponente wat NPC's aan die membraan anker in al vyf supergroepe. Ook bewaar in al vyf supergroepe was NPC "mandjie" nukleoporiene Tpr en Nup50 die derde mandjie proteïen, Nup153, wat in gewerwelde diere lamine direk bind (Smythe et al., 2000), is in vier uit vyf supergroepe bewaar (Neumann et al. , 2010). Hierdie bewaarde proteïene, wat waarskynlik teenwoordig was in die LECA, het implikasies buite NPC struktuur en funksie soos bespreek in die volgende afdeling, Tpr en Nup153 het ook funksies wat verband hou met chromatien en geen uitdrukking.

Watter ander kernstrukturele proteïene was teenwoordig in die LECA?

Die teenwoordigheid van oënskynlik funksionele NPC's in die LECA laat 'n interessante vraag ontstaan: het hierdie voorvaderlike kern ander kernstrukturele proteïene gehad, en indien wel, watter? Om hierdie vraag te beantwoord het 'n mens leidrade nodig oor watter proteïene om in diverse genome te soek. Gelukkig het baie proteïene wat moontlik bygedra het tot die evolusie van kernstruktuur uit funksionele studies in Opisthokonts en plante ontstaan. Hierdie proteïene van belang sluit in groeiende getalle kernmembraanproteïene, wat vervolgens bespreek word, en funksioneel diverse nukleoskeletale proteïene, insluitend aktien, molekulêre motors, spektrienherhalingsproteïene, opgerolde spoelproteïene en kernporiekompleksgekoppelde filamente (Simon en Wilson, 2011) , wat later in hierdie resensie bespreek word.

Kernmembraanproteïene: Onbekende gebied

Soogdiere kodeer groot repertoriums (waarskynlik honderde) ongekarakteriseerde kernomhulsel transmembraan (NET) proteïene (Wilson en Berk, 2010). Hierdie onverwagte kompleksiteit is die eerste keer aan die lig gebring in 'n landmerk-proteomiese studie wat meer as 60 verskillende NET-proteïene in gesuiwerde rotlewersel-kernkoeverte geïdentifiseer het (Schirmer et al., 2003), en is bevestig en uitgebrei deur studies in ander soogdierselle (Wilkie et al. , 2011). Die meeste kernmembraanproteïene in Opisthokonts is óf ongekarakteriseerd óf word nog nie verstaan ​​op die vlak van strukturele of funksionele detail wat nodig mag wees om ortoloë gene in diverse eukariote te identifiseer nie. Om hierdie probleem ten minste gedeeltelik te omseil, kan 'n mens die NET-proteome van diverse eukariote bepaal, en daardeur spesifieke relevante gene identifiseer. Verdere ontleding van bewaarde NET-proteïene, selfs in ander Opisthokonts, lewer verrassings oor kernstruktuur op. Byvoorbeeld, die swam Schizosaccharomyces pombe kodeer 'n kern binneste membraan proteïen genaamd Ima1, geïdentifiseer as 'n potensiële analoog van soogdier NET5 (King et al., 2008). Funksionele studies toon dat Ima1 heterochromatien aan die NE heg en (deur onbekende verbindings) aan mikrotubuli (King et al., 2008). Die Ima1-proteïen bind direk aan sentromere en telomere, en die eienskappe daarvan dui daarop dat heterochromatien 'n meganiese "moer" verskaf wat die NE versterk teen kragte wat deur mikrotubules gegenereer word (King et al., 2008). Dit stem ooreen met biomeganiese bewyse dat heterochromatien self as 'n kragdraende struktuur funksioneer (Dahl et al., 2005). Kernproteïene wat óf meganies versterk chromatien, óf chromatien teen krag beskerm het, kon die vermoë van selle beïnvloed het om nie net eksterne meganiese uitdagings te oorleef nie, maar ook om krag op die buitewêreld uit te oefen (Dahl et al., 2008). Die toekomstige karakterisering van bewaarde NET-proteïene het dus die potensiaal om nuwe aspekte van kernstruktuur in lewende eukariote te openbaar, asook nuwe beginsels oor hoe hierdie struktuur ontwikkel het.

Klewerige membraanproteïene help om die koevert te stabiliseer.

Ons stel voor dat membraanproteïene met "klewende" ekstrasellulêre domeine bygedra het tot die evolusie van kernstruktuur deur die parallelle organisasie van geboë of ingevoude plasmamembrane te stabiliseer (Fig. 2). Hierdie idee is gebaseer op die ontdekking van Opisthokont-kernmembraanproteïene wat adhesie óf homotipies (tussen twee of meer kopieë van een proteïen) óf heterotipies (tussen verskillende proteïene) bemiddel. Let daarop dat die lumenale domeine van membraanproteïene na 'n kompartement kyk wat topologies gelykstaande is aan die buitekant van die sel. Die lumenale domeine van gis-transmembraanproteïen Pom152 (potensiële gewerwelde ortoloog: gp210) word voorspel om self-interaksie te hê via 'n cadherin-vou (Devos et al., 2006). Hierdie vou is kenmerkend van die ekstrasellulêre domeine van sekere seloppervlakproteïene (bv. cadheriene) en bemiddel homotipiese adhesie aan naburige selle (Franke, 2009). Alhoewel kadherienvou-bevattende kernmembraanproteïene tot dusver slegs in Opisthokonts gesien is, word ander tipes kleefdomeine wyd bewaar. Byvoorbeeld, Pom121, 'n bewaarde NPC-membraanproteïen, word na chromatien gewerf deur die DNA-bindende nukleoporien ELYS en die Nup107-160-kompleks (Lau et al., 2009). Hierdie interaksies veroorsaak op een of ander manier kleefmiddel Pom121-gemedieerde samesmelting van parallelle membrane om nuwe porieë te skep (Fichtman et al., 2010). Benewens hierdie bewyse vir homotipiese adhesie, is daar groeiende bewyse dat sekere kernmembraanproteïene adhesie heterotipies bemiddel.

Ten minste drie tipes NE-membraanproteïene word benodig vir die parallelle organisasie van die binne- en buitenste kernmembrane. Een familie bestaan ​​uit die binnemembraanproteïen lamina-geassosieerde polipeptied 1 (LAP1) en 'n verwante buitenste membraanproteïen, LULL1, wat interaksie het via hul groot lumenale domeine in samewerking met oplosbare lumenale proteïene genaamd torsinA en torsinB (Nery et al., 2008 Vander. Heyden et al., 2009 Kim et al., 2010). Omdat LAP1 en LULL1 verwant is, kan hul adhesie as homotipies beskou word. Daarenteen tree twee ander kleefmiddelfamilies, bestaande uit KASH-domeinproteïene en SUN-domeinproteïene, heterotipies in wisselwerking via hul lumenale domeine (Crisp et al., 2006 Starr en Fridolfsson, 2010). KASH- en SUN-domeinproteïene het ook bykomende domeine wat óf selfinteraksie óf direkte binding aan spesifieke sitoskeletale proteïene, nukleoskeletale proteïene of NE membraanproteïene bemiddel (Wilson en Berk, 2010). Alhoewel die eksperimentele prentjie nog lank nie volledig is nie, dui huidige bewyse daarop dat KASH-domein- en SUN-domeinproteïene 'n verskeidenheid meganies robuuste adhesiekomplekse by die NO vorm, genaamd LINC-komplekse (skakel die nukleoskelet en sitoskelet Crisp et al., 2006), waarvan sommige is noodsaaklik vir chromosoomparing tydens meiose en seksuele rekombinasie (Fridkin et al., 2009 Hiraoka en Dernburg, 2009). In ons beperkte soektog is die SUN-domein in elke getoetste supergroep opgespoor (Fig. 3, Tabel I). Dit dui sterk daarop dat die LECA 'n SUN-domeinproteïen gehad het. Daarenteen het ons die KASH-domein slegs in Opisthokonts gevind (Fig. 3, Tabel I). Die potensiële teenwoordigheid van 'n KASH-domeinproteïen in die LECA kan nie tans uitgesluit word nie. As SUN-domeinproteïene egter meer oud is, kan hulle ook antieke (KASH-onafhanklike) rolle dien. Hierdie rolle kan NPC's behels (Liu et al., 2007) omdat SUN1 'n vroeë rol in NPC-samestelling speel (Talamas en Hetzer, 2011). SUN-domeinproteïene werk ook met meiotiese telomere, histoon H2A.Z, en koppel die kern aan die mikrotubule-organiseringsentrum (Hiraoka en Dernburg, 2009 Gardner et al., 2011). Hierdie bevindings ondersteun die idee dat membraanproteïene, insluitend kleefmembraanproteïene, die evolusie van kernstruktuur beïnvloed het.

Membraanproteïene wat chromatien of nukleoskeletale vennoot(e) bind.

Die evolusie van kernstruktuur is moontlik sterk beïnvloed deur membraanproteïene wat aan DNA of chromatienproteïene kan bind (Wilson en Foisner, 2010). Een so 'n familie van proteïene in metazoane het 'n kenmerkende "LEM-domein"-vou, wat eers in LAP2, emerin en MAN1 geïdentifiseer is (Lin et al., 2000 Laguri et al., 2001 Wagner en Krohne, 2007). Trouens, die eerste van twee LEM-domeine in LAP2 verleen direkte binding aan dsDNA (Cai et al., 2001). Ander getoetste LEM-domeine verleen binding aan versperring-tot-outo-integrasiefaktor (BAF), 'n gekonserveerde metazoan-proteïen wat ook direk aan dsDNA, histoon H3 en lamine bind (Margalit et al., 2007 Montes de Oca et al., 2009), en beïnvloed histoon posttranslasionele modifikasies (Montes de Oca et al., 2011). LEM-domeinproteïene het ander domeine wat een of meer komponente van die nukleoskelet bind, of verskeie sein- of geenregulerende proteïene (Wagner en Krohne, 2007 Wilson en Berk, 2010). Die LEM-domeinproteïen-emerien bind ook KASH-domein- en SUN-domeinproteïene direk (Simon en Wilson, 2011), en koppel op een of ander manier meganiese krag aan stroomafwaartse veranderinge in geenuitdrukking, 'n verskynsel bekend as meganotransduksie (Lammerding et al., 2005). Gewerwelde kerne druk ook 'n gespesialiseerde niemembraan LEM-domeinproteïen genaamd LAP2α uit wat met homself (as trimere), lamine A, chromatien en telomere in wisselwerking tree, en wat nodig is om A-tipe lamine in die kernbinne te organiseer (Snyers et al., 2007 Gotic en Foisner, 2010 Dechat et al., 2011). Gis, wat nie lamine en BAF het nie, kodeer nietemin 'n LEM-domein ("HEH"-domein) binnekernmembraanproteïen genaamd Src1, wat assosieer met en telomere, subtelomere en rDNA-gene onderdruk (Grund et al., 2008). Interessant genoeg funksioneer Src1 ook tydens mitose selle wat nie Src1 het nie, het korter anafase en langer telofase (Rodríguez-Navarro et al., 2002). Beide die LEM-domeinvou (Cai et al., 2001) en 'n gekonserveerde C-terminale "MSC" (MAN1–Src1p–C-terminaal) domein wat deur Src1 en menslike MAN1 gedeel word (Mans et al., 2004) word in bakterieë bewaar. en kan funksioneer om nukleïensure te bind. Onder eukariote het ons beperkte soektog LEM-domeinverwante ORF's slegs in Opisthokonts opgespoor (Fig. 3, Tabel I). Vorige meer uitgebreide belynings het egter proteïene met beide kenmerke (LEM/HEH-domein MSC-domein) in alle getoetste eukariotiese supergroepe gevind, wat daarop dui dat die LECA 'n LEM-domeinproteïen het (Mans et al., 2004).

Die nurim-proteïen het vier membraan-oorspanende domeine (en min). Hierdie proteïen lokaliseer by die kern binnemembraan deur onbekende meganismes omdat dit geen waarneembare binding aan NPC's, lamine of ander intranukleêre komponente toon nie (Rolls et al., 1999). Daar word voorgestel dat Nurim funksioneer in 'n pad wat nuut gesintetiseerde kernmembraanproteïene verby die NPC na die binnemembraan sorteer (King et al., 2006 Braunagel et al., 2007). Ons soektog het nurim-verwante ORF's in twee eukariotiese supergroepe aan die lig gebring (Fig. 3). 'n vorige studie (Mans et al., 2004) het nurim in 'n proteïensuperfamilie gegroepeer wat die binneste kernmembraanproteïen van soogdiere LBR ('n sterolreduktase Holmer et al., 1998) en verwante ensieme in bakterieë insluit, wat potensieel antieke oorsprong voorstel.

Die LUMA-proteïen (gekodeer deur TMEM43) kruis die kernbinnemembraan vier keer, het 'n groot ongekarakteriseerde lumenale domein, assosieer met SUN2, lamine en emerien, en vorm homo-oligomere (Bengtsson en Otto, 2008 Liang et al., 2011). LUMA-oligomerisasie word ontwrig deur 'n mutasie wat Emery-Dreifuss spierdistrofie veroorsaak (Liang et al., 2011). LUMA word bewaar in drie eukariotiese supergroepe (Fig. 3) en veral ook in bakterieë (Bengtsson en Otto, 2008), wat daarop dui dat LUMA in die LECA teenwoordig kon gewees het. Die idee dat LUMA, LEM-domeinproteïene, nurim en waarskynlik ander kernmembraanproteïene potensieel antieke rolle in kernstruktuur en -funksie het, sal dit selfs meer interessant maak om hul rolle in lewende eukariote te ontsyfer.

Aktien en miosiene: Antieke komponente van kernstruktuur?

Eukariotiese aktien en aktienafhanklike motors (miosiene) is bekende sitoskeletale komponente wat relevant is vir selmotiliteit, en hul evolusionêre betekenis word byna altyd eksklusief in hierdie terme bespreek (Fritz-Laylin et al., 2010). Minder waardeer word hul fundamentele en potensieel antieke rolle in kernstruktuur en genoomfunksie. Polimeriseerbare aktien en miosiene is betrokke by transkripsie deur al drie DNA-afhanklike RNA-polimerases, bemiddel RNA-uitvoer vanaf die kern, en word benodig vir die langafstandbeweging van spesifieke lokusse binne die kern (Gieni en Hendzel, 2009 Hofmann, 2009 Mekhail en Moazed, 2010 Skarp en Vartiainen, 2010). Ten minste ses verskillende miosienmotors (Pestic-Dragovich et al., 2000 Salamon et al., 2003 Hofmann et al., 2006, 2009 Vreugde et al., 2006 Cameron et al., 2007 Pranchevicius et al., 2008 en McCrey Lindsay. , 2009) en vier verskillende kinesienmotors (Macho et al., 2002 Levesque et al., 2003 Mazumdar et al., 2004 Wu et al., 2008 Cross and Powers, 2011 Zhang et al., 2011) is teenwoordig in dierekerne , met rolle wat transkripsie, intranukleêre beweging van chromatien insluit, of uitvoer langs porie-gekoppelde filamentnetwerke wat die nukleolus met NPC's verbind (Simon en Wilson, 2011). Miosien I-motors word bewaar in diverse eukariote (Foth et al., 2006 Hofmann et al., 2009) insluitend die basale uitgrawing Naegleria gruberi (Goodson en Dawson, 2006), wat ses miosien I-homoloë het (Fritz-Laylin et al., 2010). Watter (indien enige) Naegleria miosiene funksioneer eintlik in die kern is onbekend.

Ander nukleoskeletale proteïene sluit in kern mitotiese apparaat (NuMA interfase rolle onduidelik, maar self saamstel in 3D ruimtevul strukture Harborth et al., 1999 Radulescu en Cleveland, 2010), kern spektriene (bv. Kothary, 2005 Zhang et al., 2010), en kernproteïen 4.1 (bind NuMA-assosiasies met porie-gekoppelde filamente, help om die nukleoskelet en verskeie NE membraanproteïene te organiseer Meyer et al., 2011 Simon en Wilson, 2011). Daar word inderdaad voorgestel dat die voorvaderlike spektrien-herhaalproteïen in die kern gefunksioneer het (Young en Kothary, 2005). Kerntitien kan direk aan kern-intermediêre filamentproteïene bind (Zastrow et al., 2006), en is noodsaaklik vir chromosoomkondensasie tydens mitose (Machado et al., 1998 Machado en Andrew, 2000 Zhong et al., 2010). Daar is eers onlangs erken dat proteïene soos aktien, miosiene, NuMA, spektriene en titien fundamentele rolle in kernstruktuur en genoomfunksie in lewende eukariote het (Simon en Wilson, 2011), en om historiese redes bly hul rolle in die nukleoskelet grootliks on - geannoteer. Kinesins, waarvan die LECA voorgestel word om ~11 te besit (Wickstead et al., 2010), is ook teenwoordig in die kern (Simon en Wilson, 2011). Kernkinesiene assosieer met chromatien, en een (Kif4A) is betrokke by die reaksie op DNA-skade (Wu et al., 2008) anders is min bekend oor hul kernfunksies.

Opgerolde spoel nukleoskeletale proteïene

DNA-bindende opgerolde spoel nukleoskeletale proteïene is teenwoordig in twee meersellige lyne, diere (Opisthokonts) en plante (Archaeplastides), maar het onafhanklik ontwikkel. In die geval van diere bestaan ​​hierdie proteïene (lamine) uit die voorouerlike tussenfilament (Prokocimer et al., 2009) waaruit sitoplasmiese intermediêre filamente later ontwikkel het. Kernintermediêre filamente (lamine filamente) is belangrike strukturele komponente van die dierlike nukleoskelet, met genetiese skakels met 'n verskeidenheid menslike siektes (Dittmer en Misteli, 2011). Lamins ondersteun of beïnvloed byna elke aspek van genoombiologie, insluitend replikasie, transkripsie, sein, ontwikkeling en chromosoomorganisasie (Prokocimer et al., 2009 Dechat et al., 2010, 2011 Wilson en Berk, 2010). Alle metazoane het een of twee gene wat kodeer vir "B-tipe" lamine, terwyl komplekse diere (insekte, gewerwelde diere) 'n bykomende geen het wat kodeer vir 'n onafhanklike netwerk van "A-tipe" lamine filamente wat nodig is vir die fisiologie van baie uitnemend meganosensitiewe seltipes, soos bv. as spiere en been (Dahl et al., 2008). Ander bekende opgerolde spoel-strukturele proteïene in Opisthokonts sluit in die bewaarde NPC-mandjieproteïen Tpr (Krull et al., 2004) en Smc (strukturele instandhouding van chromosome Wong, 2010). In meersellige diere, waar aktiewe gene diep binne die kern geleë kan wees, is Tpr en verwante proteïene voorgestelde komponente van nukleoskeletale porie-gekoppelde filamente wat NPC's verbind met aktiewe gene en die nukleolus, en aktien- en miosienafhanklike uitvoer vanaf die kern vergemaklik. (Simon en Wilson, 2011). In gis, wat kleiner kerne het, word aktiewe gene aan die NPC vasgemaak deur direkte binding van bewaarde promotorelemente (bv. "DNA-poskodes" Ahmed et al., 2010) aan spesifieke nukleoporiene (Casolari et al., 2004 Kalverda en Fornerod. , 2010). Of ander eukariotiese supergroepe porie-gekoppelde filamentnetwerke het, is onbekend.

Hoër plante het nie intermediêre filamente nie (Rose et al., 2005), maar het wel funksioneel analoog filamente insluitend dié wat gevorm word deur die dsDNA-bindende opgerolde spoel-proteïen MPF1 (Samaniego et al., 2006 Fiserova et al., 2009 Meier en Brkljacic, 2009). Daar word dus voorgestel dat gene wat kodeer vir verskillende groot gedraaide spoel-nukleoskeletale proteïene onafhanklik, na die LECA, in die Opisthokont- en Archaeplastid-lyn ontstaan ​​het. Hierdie gene het moontlik genoomorganisasie en kernstruktuur ingrypend beïnvloed omdat hulle korreleer met die opkoms van meersellige organismes.

Voorgestelde impak van genoombinding aan die selmembraan

Evolusie het duidelik die stabilisering van positief geboë en negatief geboë membrane deur onderskeidelik proto-coatomeer proteïene (Devos et al., 2004) en ESCRT proteïene (Samson en Bell, 2009) bevoordeel. Hierdie molekules is egter nie verantwoordelik vir die fundamenteel genoom-geassosieerde aard van NPC's, of hul intieme rolle in mitose en chromosoomsegregasie nie. Om hierdie rede stel ons voor dat die oorgang van FECA na LECA deels deur verskeie tipes membraanproteïene aangedryf is. Van die vroegste, stel ons voor, was dié wat DNA of chromatien gebind het en daardeur die genoom aan die selmembraan vasgemaak het. Stabiele binding van relatief digte chromatien sou 'n meganiese las op die selmembraan geplaas het (Fig. 2). Hierdie las het moontlik min effek gehad op membraankromming in selle met sterk membraanhegtings aan 'n eksterne selwand. Daarenteen, in selle met swakker eksterne versterking (bv. FECA en miskien die voorloper van planctomycetes-bakterieë, wat hul genoom omsluit binne 'n kernagtige struktuur Fuerst en Sagulenko, 2011), kon die chromatien-gelaaide membraan omvattend ingevou het, wat bedreig is. selstruktuur en moontlik inmeng met chromosoomsegregasie of seldeling. Ons stel dus voor dat die inkrementele molekulêre evolusie van kernomhulsel en nukleoskeletale struktuur styf gekoppel was aan die evolusie van chromosoomsegregasie en mitose. Hierdie idee word ondersteun deur bewyse van lewende eukariote dat baie NPC en nukleoskeletale proteïene noodsaaklik is vir chromosoomsegregasie en mitose, soos bespreek in die volgende afdeling. In parallel stel ons voor dat daar sterk positiewe seleksie was, beide vir kleefproteïene wat die struktuur van ingevoude membrane gestabiliseer en meganies versterk het (Fig. 2) en vir fusogeniese proteïene wat verhoed het dat ingevoude membrane met mitose inmeng.

Was evolusie van die NPC/nukleoskelet gekoppel aan chromosoomsegregasie en mitose?

Daar is toenemende bewyse dat nukleoporiene aktiewe gene kontak, heterochromatien organiseer en mRNA-sintese koppel aan kernuitvoer (Strambio-De-Castillia et al., 2010 Liang en Hetzer, 2011). Opisthokont-nukleoporiene kom ook na vore as sentrale spelers in mitose, betrokke by chromosoomkondensasie en susterchromatiedkohesie (Nakano et al., 2011), kinetochore-samestelling (Salina et al., 2003 Rasala et al., 2006 Roux en Burke, 2006), regulering van mikrotubule-afhanklike motors (mandjieproteïen Tpr Nakano et al., 2010), regulering van mikrotubuli-polimerisasie by kinetochore (Mishra et al., 2010), mitotiese kontrolepuntregulering (De Souza en Osmani, 2009 Lussi et al., 2010 Wozniak. et al., 2010), en spilsamestelling (Nakano et al., 2011). Hierdie bevindinge dui sterk daarop dat NPC's nie net as "portale" ontwikkel het nie, maar as membraan-gebinde strukturele spilpunte vir die genoom en mitose.

Net so is die handboekprent van mitose, waarin chromosome hoofsaaklik deur spilmikrotubuli geskei word, onvolledig. ’n Aparte spasievulstruktuur, die spil “matriks” met voorgestelde elastiese hidro-gel eienskappe, is nou bekend om die spil te ondersteun (Zheng, 2010 Johansen et al., 2011). Hierdie matriks sluit beide mitoties herorganiseerde nukleoporiene in (bv. Tpr Ding et al., 2009 Lince-Faria et al., 2009) en nukleoskeletale proteïene (bv. NuMA, B-tipe lamine Simon en Wilson, 2011). Interessant genoeg word chromosoomsegregasie in oösiete aangedryf deur die sametrekking van 'n kernaktiennetwerk (Lénárt et al., 2005). Net so kan chromosoomsegregasie in gis plaasvind in die afwesigheid van spilmikrotubuli deur 'n kernsplytingsproses wat aktien vereis (Castagnetti et al., 2010). Hierdie bevindinge openbaar aktien en moontlik ander komponente van die Opisthokont-nukleoskelet in 'n nuwe lig: as genoom-partisionerende proteïene. Was chromosoomsegregasie (mitose) 'n dryfkrag in die evolusie van die nukleoskelet?

Die idee dat die nukleoskelet uit antieke genoom-segregerende proteïene ontwikkel het, word ondersteun deur die bewaring van baie verwante proteïene as komponente van genoomverdeling (“par”) sisteme in bakterieë. Bakteriese verdeling word die beste verstaan ​​vir plasmiede, en behels drie eenvoudige komponente: 'n herhaalde DNA-volgorde (sentromere DNA), 'n sentromeer-bindende proteïen en 'n gepaardgaande kraggenererende ("motoriese") proteïen wat die twee sentromere skei deur polimere te vorm ( Schumacher, 2008). Van hierdie komponente is slegs die motors - vier soorte - aansienlik bewaar. Die meeste bakterieë gebruik 'n motor met 'n Walker-tipe ATPase-motief (tipe I par stelsel) of 'n actin/hsp70 superfamilie proteïen (tipe II Schumacher, 2008). Other bacteria use a tubulin/FtsZ GTPase superfamily protein (type III) or an unusual (type IV) protein that is predicted to form coiled-coil polymers and also has a predicted DNA-binding domain, potentially uniting both the centromere-binding and motor functions in a single polypeptide (Simpson et al., 2003 Schumacher, 2008). Eukaryotes express proteins related to potentially all four bacterial par motors. Actin is both a major component of the interphase nucleoskeleton (as discussed earlier) and essential for chromosome segregation (Castagnetti et al., 2010). The Smc family of Walker-type ATPases are conserved in all living cells (Hirano, 2005). Tubulin forms intranuclear microtubules in eukaryotes with “closed” mitosis, or “spindle matrix-associated” microtubules in eukaryotes with open mitosis, and is mitotically regulated by nucleoporins. Less clear is whether any nucleoskeletal protein(s) are related to coiled-coil (type IV par) proteins, but candidates include lamins, Tpr, and Smc. Interestingly, certain nuclear membrane proteins also appear to function during mitosis: Samp1, a nuclear inner membrane protein, colocalizes with the mitotic spindle (Buch et al., 2009).

Concluding remarks

The “conserved protein fold” strategy, coupled to purification of NPCs from diverse eukaryotes, yielded brilliant insight into the early coevolution of NPCs with endomembranes (DeGrasse et al., 2009). Further explorations of nuclear transmembrane and nucleoskeletal proteins purified from diverse eukaryotes may yield fascinating insights into the LECA nucleus, and the human cell nucleus. Current limitations include lack of knowledge about most Opisthokont nuclear membrane proteins, and a paucity of sequenced genomes from diverse eukaryotes. Genome analysis of the free-living predatory amoebo-flagellate Naegleria gruberi, which diverged from other eukaryotic lineages over a billion years ago, reveals a rich repertoire of proteins involved in cell structure, signaling, metabolism, and sexual recombination (Fritz-Laylin et al., 2010). This organism has a typical-appearing nucleus and can be cultured in the laboratory (Fulton et al., 1984 Fritz-Laylin et al., 2011). Naegleria, other diverse Excavates including Giardia en Trypanosoma, and laboratory-friendly members of other eukaryotic supergroups including Amoeba (Dictyostelium) and Archaeplastids/Plants (Chlamydomonas) are all available to explore the evolution of nuclear structure. Additional clues to the early evolution of nuclear structure, whether independent or based on shared genes, may come from an unlikely source: planctomycetes bacteria, which enclose their genome within a double membrane, express a clathrin-related protein, and have an endocytosis-like pathway (Fuerst and Webb, 1991 Fuerst and Sagulenko, 2011). It will be exciting to understand how different kinds of proteins and possibly other types of molecules including noncoding RNAs (Pauli et al., 2011) and ADP-ribose chains (Chang et al., 2005) might have shaped the evolution of nuclear structure before the LECA, and to this day.


Eukaryotic origins

The origin of the eukaryotes is a fundamental scientific question that for over 30 years has generated a spirited debate between the competing Archaea (or three domains) tree and the eocyte tree. As eukaryotes ourselves, humans have a personal interest in our origins. Eukaryotes contain their defining organelle, the nucleus, after which they are named. They have a complex evolutionary history, over time acquiring multiple organelles, including mitochondria, chloroplasts, smooth and rough endoplasmic reticula, and other organelles all of which may hint at their origins. It is the evolutionary history of the nucleus and their other organelles that have intrigued molecular evolutionists, myself included, for the past 30 years and which continues to hold our interest as increasingly compelling evidence favours the eocyte tree. As with any orthodoxy, it takes time to embrace new concepts and techniques.

Sleutelwoorde: dawn cell eocytes eukaryotes evolution nucleus origin.

Syfers

This first ‘eocyte tree’ was…

This first ‘eocyte tree’ was reconstructed based on the presence and absence of…

The sister group relationship of…

The sister group relationship of the eocytes to the eukaryotes is illustrated by…


Endosymbiosis & The Origin of Eukaryotic Cells

This lecture will help Advanced Biology students understand how mitochondria and chloroplasts evolved. It includes the three domains of life, evidence for serial endosymbiotic theory, and advantages of multicellularity.

Nothing in molecular biology makes sense in light of the evolutionary history of organims in specific paleoenvironments

1960’s scientist Lynn Margulis studied cell structure
Thought mitochondria looked like baccteria- mito evolved from bacteria that lived in permanent symbiosis within the cells of animals and plants

Symbiotic events have a profound impact on the organization and complexity of many life forms

Living relics
Of the 5000 species of bacteria and archaea that have been described almost all were disc when isoalted from natural habitatsa snd grown under controlled conditions in the lab.

Taq polymerase- enzyme stable up to 95ºC- used to run PCR in research and commercial settings-
research- likely that first life forms lived at high temperatures and high in anoxic environments (no oxygen), PCR is necessary research tool that allows investigation in forensic crime scenes, genetic diseases, inheritance patterns, sex determination of embryos, drug discovery and detection of pathogens.

Woese's work on Archaea is also significant in its implications for the search for life on other planets. Before the discovery by Woese and Fox, scientists thought that Archaea were extreme organisms that evolved from the organisms more familiar to us. Now, most believe they are ancient, and may have robust evolutionary connections to the first organisms on Earth.[28] Organisms similar to those archaea that exist in extreme environments may have developed on other planets, some of which harbor conditions conducive to extremophile life.[29]

In addition to a nucleus, eukaryotic cells contain a variety of membrane-enclosed organelles within their cytoplasm. These organelles provide compartments in which different metabolic activities are localized. Eukaryotic cells are generally much larger than prokaryotic cells, frequently having a cell volume at least a thousandfold greater.

The compartmentalization provided by cytoplasmic organelles is what allows eukaryotic cells to function efficiently. Two of these organelles, mitochondria and chloroplasts, play critical roles in energy metabolism. Mitochondria, which are found in almost all eukaryotic cells, are the sites of oxidative metabolism and are thus responsible for generating most of the ATP derived from the breakdown of organic molecules. Chloroplasts are the sites of photosynthesis and are found only in the cells of plants and green

Despite their many similarities, mitochondria (and chloroplasts) aren't free-living bacteria anymore. The first eukaryotic cell evolved more than a billion years ago. Since then, these organelles have become completely dependent on their host cells. For example, many of the key proteins needed by the mitochondrion are imported from the rest of the cell. Sometime during their long-standing relationship, the genes that code for these proteins were transferred from the mitochondrion to its host's genome. Scientists consider this mixing of genomes to be the irreversible step at which the two independent organisms become a single individual.

Single eukaryotic cells became living in close association- colonies

Volvox
Somatic cells- swim and keep it near the light to PS, cannot divide – colonies of up to 50,000 individuals, cannot live alone
characteristics of a single individual. Multicellularity has arisen many times among the eukaryotes. Practically every organism big enough to see with the unaided eye is multicellular, including all animals and plants. The great advantage of multicellularity is that it fosters specialization some cells devote all of their energies to one task, other cells to another. Few innovations have had as great an impact on the history of life asspecialization made possible by multicellularity

Multicellular organisms need specialised organ systems, whereas all the life processes in a unicellular organism take place in that one cell. Multicellular organisms need organ systems to carry out functions such as:
Communication between cells, eg the nervous system and circulatory system
Supplying the cells with nutrients, eg the digestive system
Controlling exchanges with the environment, eg the respiratory system and excretory system


Poxviruses and the Origin of the Eukaryotic Nucleus

A number of molecular forms of DNA polymerases have been reported to be involved in eukaryotic nuclear DNA replication, with contributions from α-, δ-, and ε-polymerases. It has been reported that δ-polymerase possessed a central role in DNA replication in archaea, whose ancestry are thought to be closely related to the ancestor of eukaryotes. Indeed, in vitro experiment shown here suggests that δ-polymerase has the potential ability to start DNA synthesis immediately after RNA primer synthesis. Therefore, the question arises, where did the α-polymerase come from? Phylogenetic analysis based on the nucleotide sequence of several conserved regions reveals that two poxviruses, vaccinia and variola viruses, have polymerases similar to eukaryotic α-polymerase rather than δ-polymerase, while adenovirus, herpes family viruses, and archaeotes have eukaryotic δ-like polymerases, suggesting that the eukaryotic α-polymerase gene is derived from a poxvirus-like organism, which had some eukaryote-like characteristics. Furthermore, the poxvirus's proliferation independent from the host-cell nucleus suggests the possibility that this virus could infect non-nucleated cells, such as ancestral eukaryotes. I wish to propose here a new hypothesis for the origin of the eukaryotic nucleus, posing symbiotic contact of an orthopoxvirus ancestor with an archaebacterium, whose genome already had a δ-like polymerase gene.

Dit is 'n voorskou van intekeninginhoud, toegang via jou instelling.


In ancient giant viruses lies the truth behind evolution of nucleus in eukaryotic cells

DNA exchange between ancient giant viruses and ancient biological cells might have been the key to the evolution of nuclei in eukaryotic cells Credit: Tokyo University of Science

Perhaps as far back as the history of research and philosophy goes, people have attempted to unearth how life on earth came to be. In the recent decades, with exponential advancement in the fields of genomics, molecular biology, and virology, several scientists on this quest have taken to looking into the evolutionary twists and turns that have resulted in eukaryotic cells, the type of cell that makes up most life forms today.

The most widely accepted theories that have emerged state that the eukaryotic cell is the evolutionary product of the intracellular evolution of proto-eukaryotic cells, which were the first complex cells, and symbiotic relationships between proto-eukaryotic cells and other unicellular and simpler organisms such as bacteria and archaea. But according to Professor Masaharu Takemura of the Tokyo University of Science, Japan, "These hypotheses account for and explain the driving force and evolutionary pressures. But they fail to portray the precise process underlying eukaryotic nucleus evolution."

Prof Takemura cites this as his motivation behind his recent article published in Grense in mikrobiologie, where he looks into the recent theories that, in addition to his own body of research, have built up his current hypothesis on the subject.

In a way, Prof Takemura's hypothesis has its roots in 2001 when, along with PJ Bell, he made the revolutionary proposal that large DNA viruses, like the poxvirus, had something to do with the rise of the eukaryotic cell nucleus. Prof Takemura further explains the reasons for his inquiry into the nucleus of the eukaryotic cell as such: "Although the structure, function, and various biological functions of the cell nucleus have been intensively investigated, the evolutionary origin of the cell nucleus, a milestone of eukaryotic evolution, remains unclear."

The origin of the eukaryotic nucleus must indeed be a milestone in the development of the cell itself, considering that it is the defining factor that sets eukaryotic cells apart from the other broad category of cells—the prokaryotic cell. The eukaryotic cell is neatly compartmentalized into membrane-bound organelles that perform various functions. Among them, the nucleus houses the genetic material. The other organelles float in what is called the cytoplasm. Prokaryotic cells do not contain such compartmentalization. Bacteria and archaea are prokaryotic cells.

The 2001 hypothesis by Prof Takemura and PJ Bell is based on striking similarities between the eukaryotic cell nucleus and poxviruses: in particular, the property of keeping the genome separate in a compartment. Further similarities were uncovered after the discovery and characterization of a type of large DNA virus called "giant virus," which can be up to 2.5 μm in diameter and contain DNA "encoding" information for the production of more than 400 proteins. Independent phylogenetic analyses suggested that genes had been transferred between these viruses and eukaryotic cells as they interacted at various points down the evolutionary road, in a process called "lateral gene transfer."

Viruses are "packets" of DNA or RNA and cannot survive on their own. They must enter a "host" cell and use that cell's machinery to replicate its genetic material, and therefore multiply. As evolution progressed, it appears, viral genetic material became integrated with host genetic material and the properties of both altered.

In 2019, Prof Takemura and his colleagues made another breakthrough discovery: the medusavirus. The medusavirus got its name because, like the mythical monster, it causes encystment in its host that is, it gives its host cell a 'hard' covering.

Via experiments involving the infection of an amoeba, Prof Takemura and his colleagues found that the medusavirus harbors a full set of histones, which resemble histones in eukaryotes. Histones are proteins that keep DNA strands curled up and packed into the cell nucleus. It also holds a DNA polymerase gene and major capsid protein gene very similar to those of the amoeba. Further, unlike other viruses, it does not construct its own enclosed 'viral factory' in the cytoplasm of the cell within which to replicate its DNA and contains none of the genes required to carry out the replication process. Instead, it occupies the entirety of the host nucleus and uses the host nuclear machinery to replicate.

These features, Prof Takemura argues, indicate that the ancestral medusavirus and its corresponding host proto-eukaryotic cells were involved in lateral gene transfer the virus acquired DNA synthesis (DNA polymerase) and condensation (histones) genes from its host and the host acquired structural protein (major capsid protein) genes from the virus. Based on additional research evidence, Prof Takemura extends this new hypothesis to several other giant viruses as well.

Thus, Prof Takemura connects the dots between his findings in 2019 and his original hypothesis in 2001, linking them through his and others' work in the two decades that come in between. All of it taken together, it becomes clear how the medusavirus is prime evidence of the viral origin of the eukaryotic nucleus.

Takemura says, "This new updated hypothesis can profoundly impact the study of eukaryotic cell origins and provide a basis for further discussion on the involvement of viruses in the evolution of the eukaryotic nucleus." Indeed, his work may have unlocked several new possibilities for future research in the field.


Lynn Margulis

Ons redakteurs sal nagaan wat jy ingedien het en bepaal of die artikel hersien moet word.

Lynn Margulis, (born March 5, 1938, Chicago, Illinois, U.S.—died November 22, 2011, Amherst, Massachusetts), American biologist whose serial endosymbiotic theory of eukaryotic cell development revolutionized the modern concept of how life arose on Earth.

Margulis was raised in Chicago. Intellectually precocious, she graduated with a bachelor’s degree from the University of Chicago in 1957. Soon after, she married American astronomer Carl Sagan, with whom she had two children one, Dorion, would become her frequent collaborator. The couple divorced in 1964. Margulis earned a master’s degree in zoology and genetics from the University of Wisconsin at Madison in 1960 and a Ph.D. in genetics from the University of California, Berkeley, in 1965. She joined the biology department of Boston University in 1966 and taught there until 1988, when she was named distinguished university professor in the department of botany at the University of Massachusetts at Amherst. She retained that title when her affiliation at the university changed to the department of biology in 1993 and then to the department of geosciences in 1997.

Throughout most of her career, Margulis was considered a radical by peers who pursued traditional Darwinian “survival of the fittest” approaches to biology. Her ideas, which focused on symbiosis—a living arrangement of two different organisms in an association that can be either beneficial or unfavourable—were frequently greeted with skepticism and even hostility. Among her most important work was the development of the serial endosymbiotic theory (SET) of the origin of cells, which posits that eukaryotic cells (cells with nuclei) evolved from the symbiotic merger of nonnucleated bacteria that had previously existed independently. In this theory, mitochondria and chloroplasts, two major organelles of eukaryotic cells, are descendants of once free-living bacterial species. She explained the concept in her first book, Oorsprong van eukariotiese selle (1970). At the time, her theory was regarded as far-fetched, but it has since been widely accepted. She elaborated in her 1981 classic, Symbiosis in Cell Evolution, proposing that another symbiotic merger of cells with bacteria—this time spirochetes, a type of bacterium that undulates rapidly—developed into the internal transportation system of the nucleated cell. Margulis further postulated that eukaryotic cilia were also originally spirochetes and that cytoplasm evolved from a symbiotic relationship between eubacteria and archaebacteria (sien archaea).

Her 1982 book Five Kingdoms, written with American biologist Karlene V. Schwartz, articulates a five- kingdom system of classifying life on Earth—animals, plants, bacteria (prokaryotes), fungi, and protoctists. The protist kingdom, which comprises most unicellular organisms (and multicellular algae) in other systems, is rejected as too general. Many of the organisms usually categorized as protists are placed in one of the other four kingdoms protoctists make up the remaining organisms, which are all aquatic, and include algae and slime molds. Margulis edited portions of the compendium Handbook of Protoctista (1990).

Another area of interest for Margulis was her long collaboration with British scientist James Lovelock on the controversial Gaia hypothesis. This proposes that the Earth can be viewed as a single self-regulating organism—that is, a complex entity whose living and inorganic elements are interdependent and whose life-forms actively modify the environment to maintain hospitable conditions.

In addition to Margulis’s scholarly publications, she wrote numerous books interpreting scientific concepts and quandaries for a popular audience. Among them were Mystery Dance: On the Evolution of Human Sexuality (1991), What Is Life? (1995), What Is Sex? (1997), and Dazzle Gradually: Reflections on Nature in Nature (2007), all cowritten with her son. She also wrote a book of stories, Luminous Fish (2007). Her later efforts were published under the Sciencewriters Books imprint of Chelsea Green Publishing, which she cofounded with Dorion in 2006.


General Overviews

For reviews of eukaryogenesis, refer to Martin, et al. 2015 and Baum 2015. Martin 2005 provides an older, but still useful review, whereas Harold 2014 is an accessible, book-length exploration of cell evolution. Gould, et al. 2008 focuses on the acquisition of plastids and subsequent additional endosymbiotic events. Koonin, et al. 2010 and Lombard, et al. 2012 discuss protein regulatory networks and membrane chemistry across the three domains of life and their implications for eukaryogenesis. Eme, et al. 2014 links molecular data, which drive much of the field, to fossil evidence. Williams, et al. 2013 summarizes phylogenetic arguments for the phylogenetic model that dominates current thinking, namely that archaea/eukarya and bacteria represent the two primary domains of life. Eme, et al. 2017 builds upon the relationship between archaea and models of eukaryogenesis.

Baum, D. A. 2015. A comparison of autogenous theories for the origin of eukaryotic cells. Amerikaanse Tydskrif vir Plantkunde 102:1954–1965.

Compares different autogenous theories for the origin of the nucleus and eukaryogenesis in general, framed in the context of cellular topology, consilience with modern cell biology and the timing of mitochondrial acquisition.

Eme, L., S. C. Sharpe, M. W. Brown, et al. 2014. On the age of eukaryotes: Evaluating evidence from fossils and molecular clocks. Cold Spring Harbor Perspectives in Biology 6:a016139.

Reviews the phylogenetic and fossil evidence on the age of eukaryotes.

Eme, L., A. Spang, J. Lombard, C. W. Stairs, and T. J. G. Ettema. 2017. Archaea and the origin of eukaryotes. Nature Reviews Microbiology 15:711–723.

Discusses models for eukaryogenesis in the light of newly discovered and characterized archaeal lineages.

Gould, S. B., R. F. Waller, and G. I. McFadden. 2008. Plastid evolution. Annual Review of Plant Biology 59:491–517.

Provides a comprehensive overview of plastid evolution, encompassing primary and secondary endosymbioses, protein targeting to plastids and plastid metabolism.

Harold, F. M. 2014. In search of cell history: The evolution of life’s building blocks. Chicago: Chicago Univ. Druk.

Provides a synthetic overview of the origin and evolution of cells, with a major focus on the origin of eukaryotes.

Koonin, E. V., J. Dacks, W. Doolittle, et al. 2010. The origin and early evolution of eukaryotes in the light of phylogenomics. Genome Biology 11:209.

Analyzes the origins of key eukaryotic protein regulatory modules using comparative genomics.

Lombard, J., P. López-García, and D. Moreira. 2012. The early evolution of lipid membranes and the three domains of life. Nature Reviews Microbiology 10:507–515.

Reviews the molecular composition of archaeal and bacterial phospholipid membranes and consequences for models of eukaryogenesis.

Martin, W. F. 2005. Archaebacteria (Archaea) and the origin of the eukaryotic nucleus. Huidige mening in mikrobiologie 8:630–637.

Summarizes the diversity of models for the origin of the nuclear compartment, arguing against nuclear endosymbiotic models.

Martin, W. F., S. Garg, and V. Zimorski. 2015. Endosymbiotic theories for eukaryote origin. Philosophical Transactions of the Royal Society B: Biological Science 370.1678: 20140318.

Surveys models of eukaryogenesis with a historical slant, focusing on origins of the nuclear and mitochondrial compartment as well as metabolic considerations.

Williams, T. A., P. G. Foster, C. J. Cox, et al. 2013. An archaeal origin of eukaryotes supports only two primary domains of life. Natuur 504:231–236.

Summarizes support for having only two primary domains of life, with eukaryotes being embedded within a paraphyletic Archaea.

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The eukaryotic ancestor shapes up

Laura Eme is in the Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, 75123 Uppsala, Sweden.

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Thijs J. G. Ettema is in the Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, 75123 Uppsala, Sweden.

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Eukaryotic cells, which carry their DNA in a nucleus, are thought to have evolved from a merger between two other organisms — an archaeal host cell 1 – 3 and a bacterium from which eukaryotic organelles called mitochondria emerged 4 . Some insights into the biological properties of the host have come from the closest known archaeal relatives of eukaryotes, the Asgard superphylum 5 , 6 . The genomes of organisms belonging to this archaeal group encode a suite of proteins typically involved in functions or processes thought to be eukaryote-specific. The functions of these ‘eukaryotic genes’ in Asgard archaea have been elusive, but in a paper in Natuur, Akıl and Robinson 7 provide evidence that some of them encode proteins that are structurally and functionally similar to their eukaryotic counterparts.

Read the paper: Genomes of Asgard archaea encode profilins that regulate actin

Apart from their nucleus and energy-producing mitochondria, eukaryotic cells are characterized by a complex internal system of membrane-bound compartments (the endomembrane system), and by a dynamic network of proteins such as actin, called the cytoskeleton. The latter gives the cells their shape and structure, but is also involved in a variety of cellular processes specific to eukaryotes 8 . These features are thought to have been present in the last common ancestor of all eukaryotes, which lived about 1.8 billion years ago 9 , but no life forms have been found that represent an intermediate between eukaryotes and their bacterial and archaeal ancestors. The seemingly sudden emergence of cellular complexity in the eukaryotic lineage is a conundrum for evolutionary biologists.

Several of the proteins produced by Asgard archaea are evolutionarily related to proteins that in eukaryotes modulate complex cellular processes 5 , 6 . The identification of these proteins raised the question of whether Asgard archaea have some primitive versions of certain eukaryotic properties. If they do, it would suggest that the last archaeal ancestor of eukaryotes already displayed a certain — albeit probably limited — degree of cellular complexity reminiscent of eukaryotes.

Experiments to support such ideas are complicated by the fact that evidence for the existence of the four known Asgard lineages (Lokiarchaeota, Odinarchaeota, Thorarchaeota and Heimdallarchaeota) 5 , 6 is based solely on metagenomics analyses. The cells have yet to be observed under a microscope, and have not been cultured in vitro. Nevertheless, Akıl and Robinson were determined to gain insight into the properties of Asgard proteins related to the eukaryotic proteins actin and profilin. In eukaryotes, profilin regulates the polymerization of actin into filaments of the cytoskeleton. These filaments have pivotal roles in processes that include vesicle and organelle movement, cell-shape formation and phagocytosis 8 , in which cells ingest foreign particles or other cells.

To produce Asgard profilins, Akıl and Robinson expressed these proteins in the bacterium Escherichia coli using a circular DNA molecule called a plasmid that harboured the profilin-encoding genes. They then purified the proteins and studied their structures using X-ray crystallography. Asgard profilins share limited amino-acid sequence identity with their eukaryotic counterparts. Nonetheless, the authors found that the structure of lokiarchaeal profilin is topologically similar to that of human profilin, although some structural divergences could be observed. This confirms that Asgard and eukaryotic profilins are indeed evolutionarily related, albeit distantly.

Next, the researchers set out to investigate whether Asgard profilins could interact with Asgard actins. Unfortunately, despite considerable efforts, they were unable to produce functional Asgard actin. As an alternative, they therefore carried out in vitro and co-crystallization experiments to test whether Asgard profilins could interact with eukaryotic actins. Remarkably, despite being separated by 2 billion to 3 billion years of evolution 9 , several of the Asgard profilins bound to mammalian actin and regulated its polymerization kinetics. Asgard and mammalian profilins seem to have similar effects on mammalian actin, although the Asgard proteins act less efficiently. These results suggest that Asgard archaea harbour a profilin-regulated actin cytoskeleton — a cellular feature generally regarded as a defining characteristic of eukaryotic cells (Fig. 1).

Figuur 1 | Cellular complexity along the tree of life. The Eukarya (organisms whose cells harbour DNA in a nucleus) are thought to have arisen from a merger between their last archaeal ancestor and a bacterium. In addition to a nucleus, eukaryotes have several characteristics that are thought to separate them from archaea, including: a complex internal system of membranes called endomembranes a structural feature called the actin cytoskeleton, the dynamics of which are regulated by the protein profilin and energy-producing organelles called mitochondria, which arose from the bacterial partner. But Akıl and Robinson 7 provide evidence that members of the Asgard superphylum — an extant group of archaea thought to be related to eukaryotes — harbour a primitive profilin-regulated actin cytoskeleton. If the last archaeal ancestor of eukaryotes had this feature, it might have enabled the cell to wrap around its presumed bacterial partner. In addition, it is possible that Asgard archaea and the last archaeal ancestor of eukaryotes carry primitive endomembrane systems. (Cells and cellular features are not drawn to scale.)

The inference of a primitive dynamic actin cytoskeleton in Asgard archaea sheds light on the biological properties of the ancestor of eukaryotes. In eukaryotic cells, the energy required to dynamically regulate actin is mainly provided by mitochondria 10 . Although the energetic and metabolic properties of Asgard archaea are currently unknown, they certainly lack the firepower that mitochondria provide. A profilin-regulated actin cytoskeleton in the archaeal ancestor of eukaryotes is therefore unlikely to sustain energy-consuming processes such as phagocytosis.

But was the energy provided by mitochondria necessarily the ultimate driving force for the emergence of complex cellular features in eukaryotes? Archaea such as Ignicoccus hospitalis, along with several types of bacterium, have independently evolved endomembrane systems 11 . Because these lineages lack mitochondria, energetic constraints can be ruled out as a limiting factor in the emergence of such a system. It is therefore feasible that Asgard archaeal cells produce sufficient energy to harbour both a primitive endomembrane system and undergo actin-driven membrane and cell-shape deformation. Perhaps the latter ability could have facilitated the symbiotic interaction between the Asgard-related host cell and the bacterial ancestor of mitochondria, for example by optimizing the membrane surface area for metabolic exchange between the two cells. Once mitochondria became an intrinsic part of eukaryotic cells, their capacity for energy production could have conferred selective advantages on their host. However, the exact contribution of these organelles to the emergence of the complex features of eukaryotic cells remains unresolved.

Future efforts to elucidate the biological and physiological properties of Asgard archaea will be essential to increase our understanding of the emergence of eukaryotes. Although biochemical and structural studies of individual Asgard proteins, such as those by Akıl and Robinson, are likely to provide piecemeal insights, it is the ability to grow Asgard archaeal lineages in vitro that will ultimately unravel their obscure biology.

Natuur 562, 352-353 (2018)


How did prokaryotic cells evolve into eukaryotic cells?

Is there any fossil or biochemical evidence showing how or when prokaryotic cells evolved into eukaryotic cells? I know mitochondria and chloroplasts for example are the result of symbiotic relationships but do we know when or how this happened?

Also, are archaea linked to this transition or do they have eukaryotic features due to convergent evolution?

One idea is that eukaryotes evolved from the union of archaea and bacteria into one single organism.

This would manifest itself as the overall cell housing the descendants of bacteria in the form of mitochondria and chloroplasts. There is some genetic evidence supporting the idea and often phylogenetic trees position eukaryotes and archaea closer together than bacteria to reflect this idea.

However, if true, the tree of life would need to be modified to allow the convergence of branches into one singular organism.

Edit: There is quite of bit of evidence suggesting that mitochondria and chloroplasts are the descendants of bacteria. However, the idea that the the larger cell which engulfed those early bacteria was an archaea and not another bacteria is not as well established.

The way I learned it at university is this:

At first there was an ancient procaryotic cell with ring-like DNA and without any endomembranes. Then the cytomembrane (with some attached ribosomes, but thats not important yet) startet to fold in and began to surround the DNA molecule. At one point, the membrane divided from the outer membrane and suddenly you had a DNA with a surrounding membrane system, the nucleus. This was the first, ancient eucaryotic cell.

The problem that remained was, that this cell was anaerobic and therefore couldn't really do many highly energy consuming reactions. The theory of endosymbiosis says that such an anaerobic ancient pre-eucaryotic cell engulfed an aerobic procaryotic cell by phagocytosis and somehow got into a symbiotic relationship with it - meaning it would supply the procaryote with raw material (i.e. glucose, or better the catabole pyruvate) and the aerobic procaryote supplies the cell with the energy (in form of ATP) it generated from this pyruvate. Over time, the procaryote transferred most of its genes into the eucaryotic DNA, and thats about the point where you can speak of an early aerobic eucaryotic cell. The procaryote had become a mitochondrium/chloroplast.

Whats interesting is, that not every eucaryotic cell today is aerobic, there are still some anaerobic eucaryotes left (some protozoes, for example chlamydia). Scientists are not sure whether they lost their procaryotic endosymbionts or whether they never had them in the first place. And it is proven that chloroplasts are derived from cyanobacteria. When you compare chloroplastide DNA with the DNA from modern cyanobacteria, they are really really similar. Source: Alberts et al.: Molecular Biology of the Cell


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