Fig. 3. Evolutionary history of MCR/ECR/ACR-based anaerobic alkane metabolisms.

(A) A scenario for the evolutionary history of MCR/ECR/ACR-encoding genes. After the emergence of an ancestral MCR that reduced methylated compounds, in the common ancestor of Euryarchaeota, TACK, and Asgard, MCR was inherited by each of the two lineages. ECR/ACR evolved before the radiation of extant Euryarchaeota on the euryarchaeotal or TACK stem or potentially earlier. MTR originated in Euryarchaeota and was subsequently horizontally transferred to Ca. Nezhaarchaeia in TACK. Before the radiation of Euryarchaeota, a putative duplication of MCR-encoding genes created the ECR-encoding genes, which later evolved to metabolize longer-chain alkanes and spread by both vertical descent and horizontal transfer in the domain Archaea. “l.” refers to long chain, and “s.” refers to short chain. (B) A cartoon phylogenetic tree for archaeal evolution and MCR/ECR/ACR divergence. MCR-encoding gene might have originated by the time of the last archaea common ancestor (LACA), assuming that DPANN had lost their MCR-encoding gene during evolution. ECR/ACR might have been present in LACA or the stem lineage of Euryarchaeota, TACK, and Asgard but underwent multiple HGT processes. (C) Predicted ancestral MCR/ECR/ACR-based anaerobic alkane metabolism evolution in Archaea. First, a methyl-reducing methanogen or an ethyl-metabolizing archaea appeared. Second, the origin of MTR complex created the carbon dioxide-reducing methanogen; then, the introduction of beta-oxidation pathway enabled the anaerobic multicarbon alkane-metabolizing archaea.