An ectosymbiosis-based mechanism of eukaryogenesis

Abstract

The findings of a deep branching Microsporidia clade on the SSU rRNA tree, and diversity of sequence motifs in eukaryotic Hsp70s rendered invalid the endosymbiosis-first theory that mitosome- and hydrogenosome-containing amitochondriate eukaryotes (AMIs) arose from mitochondriate eukaryotes (MTEs) via reductive evolution. Instead, evidence of widespread ectosymbioses indicated that eukaryogenesis was started by an archaeal parent via its acquisition of archaeal proteins through ‘accelerated gene adoption’, and bacterial proteins from ectosymbionts including a clostridial ectosymbiont that supplied its [Fe] hydrogenase and pyruvate:ferredoxin oxidoreductase genes to the AMIs. Subsequent endosymbiosis with Tistrella gave rise to mitochondria with the participation of other alphaproteobacteria. The high frequencies of top similarity bitscores displayed by Giardia, Edhazardia and Trichomonas toward Aciduliprofundum boonei (Abo) pertaining to the enzymes of DNA biology, far surpassing the frequencies toward any Asgard or TACK archaeon, established Abo as the source of these enzymes in eukaryotes, and the archaeal parent of Eukarya.

Thermococci-to-Clostridia Pathway for the Evolution of the Bacteria Domain

“…Complementary searches of primitive Clostridia/Firmicutes with elevated VARS-IARS similarity bitscores may likewise lead to species with increasingly close phylogenetic relationships with LBCA. Moreover, in view of the privotal roles played by both ectosymbiotic and endosymbiotic gene transfers from prokaryotes to eukaryotes in eukaryogenesis 50 , it would be useful to examine whether such transfers might have contributed exogenous prokaryotic genes to facilitate the maturation of either the Archaeal Progenitor or the Last Bacterial Common Ancestor. Altogether, a combination of multiple approaches will be required to meet the unique challenge of understanding how the complex transformation of an archaeal genome into a bacterial one could be achieved.…”