An ectosymbiosis-based mechanism of eukaryogenesis

The mechanisms proposed for eukaryogenesis are divisible into mitochondria-early and mitochondria-late ones, where the mitochondriate-eukaryotes (MTEs) were evolutionary precursors or products of the amitochondriate-eukaryotes (AMIs) respectively. Analysis of prokaryote-to-eukaryote gene transfers in eukaryogenesis showed two tranches of high-intensity transfers from prokaryotes to eukaryotes mediated by endosymbioses that gave rise to mitochondria and chloroplasts, and hundreds of medium-intensity transfers which included the transfer of hydrogenase and pyruvate: ferredoxin oxidoreductase genes from the Thermoanaerobacter-Hungateiclostridium-Sporanaerobacter group to the AMIs. Since 94.5% of these medium-intensity transfers generated more than 100 inter-proteome similarity hits between each donor-recipient pair, they were not readily explicable by horizontal gene transfers (HGTs) or endosymbioses, pointing instead to the participation of a large number of ectosymbiotic transfers. The euryarchaeon Aciduliprofundum boonei (Abo) and the gammaproteobacterium Escherichia coli (Eco) were the foremost contributors of archaeal and bacterial genes for the eukaryotic DNA-apparatus respectively, and the Abo/Eco ratios for the prokaryote-derived genes in eukaryotes decreased from Microsporidia to other AMIs, and even more so in the MTEs. These findings supported an ectosymbiosis-based mechanism of eukaryogenesis with Abo as the Archaeal Parent of Eukarya, and Microsporidia as the eukaryotes phylogenetically closest to the Last Eukaryotic Common Ancestor (LECA).