Eukaryotes

Abstract

The eukaryote pangenome has a chimeric structure encompassing genes unique to eukaryotes (~41%), and genes of bacterial (~50% of total) and archaeal ancestry (~9%). Crucially, bacterial sequences far exceed the set acquired from the proteobacterial ancestor of mitochondria. Extant eukaryotic lineages share a common ancestor (LECA) that had all the fundamental traits of eukaryotes including the mitochondrion. The current debate about eukaryote origins revolves around two competing scenarios. The fusion model posits that the eukaryotes derive from the “fusion” of an archaeon and a bacterium, and that the acquisition of the mitochondrion was pivotal to the evolution of other eukaryotic traits. The neomuran model maintains that the archaea and eukaryotes are sister groups devived from a bacterial ancestor, and that fundamental eukaryotic traits including phagocytosis were already in place before the evolution of the mitochondrion by endosymbiosis. Eukaryote placement within the archaea in phylogenomic analysis supports the fusion scenario. The predominance of bacterial sequences in the eukaryote pangenome, the bacterial stereochemistry of eukaryote membrane lipids, and similar trajectories in mitochondrial and chloroplast evolution favour the neomuran scenario. Phylogenomic analysis resolves two major eukaryotic domains, the Amorphea and Diaphoretika, with traditional Excavata being probably paraphyletic. The root of the eukaryote tree remains elusive. Paleontological evidence and molecular clock analysis date the eukaryote lineage to at least 1.5 GYA, the concestor of extant eukaryotes to about 1.2 GYA, and major extant lineages to 900 MYA or less. The chapter includes a review of mitochondrial properties and of locomotor organelles in bacteria, archaea and eukaryotes.

If you find the story confusing, welcome to the club.

Maynard Smith and Szathmàry (1995)