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Background  Freshwater unionoidean bivalves, and species representing two marine bivalve orders (Mytiloida and Veneroida), exhibit a mode of mtDNA inheritance involving distinct maternal (F) and paternal (M) transmission routes concomitant with highly divergent gender-associated mtDNA genomes. Additionally, male unionoidean bivalves have a ~550 bp 3' coding extension to the  cox2  gene (M cox2 e), that is apparently absent from all other metazoan taxa.    Results  Our molecular sequence analyses of M COX2 e indicate that both the primary and secondary structures of the M COX2 e region are evolving much faster than other regions of the F and M  COX2-COX1  gene junction. The near N-terminus ~2/3 of the M COX2 e region contains an interspecifically variable number of predicted transmembrane helices (TMH) and interhelical loops (IHL) whereas the C-terminus ~1/3 is relatively conserved and hydrophilic while containing conserved functional motifs. M COX2 e displays an overall pattern of purifying selection that leads to the preservation of TMH/IHL and C-terminus tail sub-regions. However, 14 amino acid positions in the M COX2 e TMH/IHL sub-region might be targeted by diversifying selection, each representing a site where there exists interspecific variation for the constituent amino acids residing in a TMH or IHL.    Conclusion  Our results indicate that M cox2 e is unique to unionoidean bivalves, likely the result of a single insertion event that took place over 65 MYA and that M COX2 e is functional. The predicted TMH number, length and position variability likely stems from substitution-based processes rather than the typically implicated insertion/deletion events. M COX2 e has relatively high rates of primary and secondary structure evolution, with some amino acid residues potentially subjected to site-specific positive selection, yet an overall pattern of purifying selection leading to the preservation of the TMH/IHL and hydrophilic C-terminus tail subregions. The more conserved C-terminus tail (relative to the TMH/IHL sub-region of M COX2 e) is likely biologically active because it contains functional motifs. The rapid evolution of primary and secondary structure in M COX2 e, combined with the action of both positive and purifying selection, provide supporting evidence for the hypothesis that M COX2 e has a novel reproductive function within unionoidean bivalves. All tolled, our data indicate that unionoidean bivalve M COX2  is the first reported chimeric animal mtDNA-encoded protein.

bmc evolutionary biology

Extreme primary and secondary protein structure variability in the chimeric male-transmitted cytochrome c oxidase subunit II protein in freshwater mussels: Evidence for an elevated amino acid substitution rate in the face of domain-specific purifying selection