Figure 3. Eukaryotic traits in light of housing an endosymbiont.

Each segment highlights an eukaryotic (Eu) trait and the comparable, if present, situation in bacteria (Ba) and archaea (Ar). (i) Prokaryotes secrete outer membrane vesicles (OMVs) and an endosymbiont (mitochondrion) secreting OMVs could have (ii) given rise to a dynamic endomembrane system within the archaeal host and explaining the transition from archaeal to bacterial lipids. N-glycosylation has been identified in all domains of life, but the eukaryotic N-glycosylation pathway is homologous to that of archaea. (iii) A specialized extension of the endoplasmic reticulum (ER), the nucleus prevents co-transcriptional translation of proteins – as is the rule in prokaryotes – to allow for the splicing of introns. (iv) Prokaryotes constantly shed and acquire DNA from the environment, and often promiscuously by transformation, transduction, and conjugation. In the absence of such dedicated mechanisms, eukaryotes avoid Muller’s ratchet through sex and meiosis whose origin might be linked to coordinating the merging of two genomes and synchronizing nuclear and mitochondrial division. (v) Peroxisomes also form through mitochondria-derived vesicles and house enzymes of alphaproteobacterial origin. (vi) Eukaryotes perform autophagy using membranes and proteins of the ESCRT machinery to surround and digest internal membrane compartments including the mitochondrion. (vii) While bacteria use homologs of tubulin to perform fission, eukaryotic fission utilizes actin and components of the ESCRT machinery similar to archaea, whereas the tubulin in eukaryotes is for instance used to separate chromatin and intracellular compartments.