Abstract
The mechanisms proposed for eukaryogenesis are divisible into mitochondria-early and mitochondria-late ones, where the mitochondriate-eukaryotes were evolutionary precursors or products of the amitochondriate-eukaryotes respectively. Analysis of prokaryote-to-eukaryote gene transfers in eukaryogenesis showed two tranches of high-intensity transfers from prokaryotes to eukaryotes mediated by the 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 of bacteria to the amitochondriate eukaryotes. 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 or endosymbioses, pointing instead to the participation of a huge number of ectosymbiotic transfers. The euryarchaeon Aciduliprofundum boonei and the gammaproteobacterium Escherichia coli were among the foremost contributors of archaeal and bacterial genes to the eukaryotic DNA-apparati respectively, and the ratios of the genes in different eukaryotes indicated that Microsporidia have retained more of the genomic imprint of Aciduliprofundum than all other eukaryotes. These findings supported an ectosymbiosis-based mechanism of eukaryogenesis with Aciduliprofundum as the Archaeal Parent of Eukarya, and Microsporidia as the eukaryotes phylogenetically closest to the Last Eukaryotic Common Ancestor.