Diversity and evolution of DNA polymerase θ in eukaryotes and the origin of mitochondrion-localized DNA polymerase...

Pioneering phylogenetic studies recovered the sister relationship between two distinct types of DNA polymerase (DNAP)-one is nucleus-localized Polθ with a pan-eukaryotic distribution, and the other is mitochondrion-localized PolIA found exclusively in Euglenozoa. Nevertheless, previous phylogenetic analyses considered Polθ sequences from a highly restricted set of eukaryotes, leaving the precise evolutionary trajectory of PolIA unsettled. In this study, we conducted phylogenetic analyses using a large collection of Polθ sequences from diverse eukaryotes to examine the evolutionary relationship between Polθ and PolIA more rigorously than previously reported. We recovered the maximum likelihood tree in which the clade of PolIA was nested within the clade of Polθ. In the radiation of Polθ sequences, the PolIA clade branched specifically with one of the two types of Polθ identified in euglenozoans. These results suggest that a gene duplication of one of the two Polθ types in Euglenozoa produced PolIA specific to the members of this phylum. We also propose, based on the putative distributions of the two Polθ types and PolIA, the vertical inheritance of the three DNAPs from the ancestral euglenozoan to its descendants with multiple losses of one of the two Polθ types.