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

Abstract

Abstract With the identification of an archaeal Last Universal Common Ancestor phylogenetically related to the archaeon Methanopyrus, the origin of Bacteria becomes a choice between independent emergence versus descent from Archaea. The similarity bitscores between paralogous valyl-tRNA synthetase (VARS) and isoleucyl-tRNA synthetase (IARS) indicated that an Ancestral Bacteria Cluster centred at clostridial Mahella australiensis (Mau) and Thermincola potens (Tpo) were the oldest bacteria. Overall, the high-bitscore bacteria dominated by Clostridia included a number of hydrogen producers. A search for archaea capable of hydrogen production that might be ancestral to the Bacteria domain yielded candidate Archaeal Progenitors led by Thermococci which, like Clostridia, form hydrogen through dark fermentation. A two-domain VARS tree based on Mahella, Thermincola, a broad spectrum of archaea together with a range of well known as well as newly detected species of Thermococci and Euryarchaeota allocated the two Clostridia to a minor-Thermococcal division on the tree containing Thermococi and Euryarchaeota species isolated from high-biodiversity environments. The kinship between Thermoccoci and Clostridia suggested by this allocation was substantiated by highly conserved oligopeptide segments on their VARS sequences, leading to the proposal that a Thermococci-to-Clostridia evolutionary pathway mediated the emergence of the Bacteria domain under conditions of elevated biodiversity.