engleski

Reconstruction of the Last Eukaryotic Common Ancestor by cladistic and phylogenetic approach

Organisation and diversity of the eukaryotic cells are well documented phenomena today, so it is already known scientific theory that LECA (the Last Eukaryotic Common Ancestor) harboured mitochondrion, nucleus, endoplasmic reticulum made of bacterial lipids, sex, meiosis, and eukaryotic life cycle. None of the aforementioned traits has been found in prokaryotes. The origin of those eukaryotes traits has, on the other side, been an unresolved issue for many years. It is questionable whether the mitochondrion (bacterial endosymbiont) entered the archaeal host cell prior to the formation of the nucleus or later ; it has never been systematically studied if LECA had a single or many nuclei ; it has not been quantified how many gene duplications there were in LECA, and what is their origin. Furthermore, cladistic terminology is often misapplied in molecular evolution and studies are rarely based on taxon- rich sampling, meaning that eukaryotic diversity is not well covered. This dissertation, hence, aims to discuss cladistics and its interpretation in theory, with emphasis on the definition of eukaryotes ; and practically, by analyses, to shed some light on the genome organization, morphology, and physiology of LECA. Eukaryotes/Eukarya are defined as a monophyletic, holophyletic group with polyphyletic, reticulated origin. Because of the reticulated origin of eukaryotes and because mitochondria and the nucleus are not regarded as prokaryotes anymore, both Bacteria and Archaea are paraphyletic, i.e., monophyletic groups. Many duplications were present in LECA and the Bacteria-derived ones were found to be prevalent what suggests that mitochondria-early hypothesis might be correct. Except for the genes that originated from plastid acquisition (Cyanobacteria), no specific genes were found within eukaryotic supergroups, suggesting that differential loss and genome duplications are the major forces of the eukaryotic evolution. Ubiquity of the multinucleate state across the eukaryotic domain is presented. Traits annotated on the eukaryotic tree were multinucleate state presence and absence, open vs. closed nuclear division, as well as 'control traits' for which it is known to be ancestral to LECA (presence of sex, mitochondria) or for which it is known not to be ancestral to LECA (plastid, polyploidy). Ancestral state reconstruction did not reject the hypothesis that LECA was multinucleated, similar to modern aseptate fungi or myxomycetes, and exhibited closed nuclear division. It is confirmed (i.e., not rejected) that LECA was sexual, had mitochondria, did not have plastid and was not polyploid. The results of ancestral state investigations presented in this work indicate that, contrary to popular beliefs, LECA was likely not a uninucleate cell, from which it follows that uninuclear eukaryotes possibly represent highly specialized forms, of which some, such as Excavata might even have originated long time ago from LECA's gametes.

evolution, eukaryogenesis, LECA, gametes, duplications, syncytium, coenocyte

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