title: Evolution of a unique mitotype-specific protein-coding extension of the cytochrome c oxidase II gene in freshwater mussels (Bivalvia: Unionoida).
abstract: A unique mode of mitochondrial DNA inheritance, designated doubly-uniparental inheritance (DUI), occurs in three bivalve subclasses (Pteriomorpha: Mytiloida, Palaeoheterodonta: Unionoida, Heterodonta: Veneroida), indicating that DUI may be a widespread phenomenon among bivalves. In mytiloids, breakdown of this pattern of inheritance (gender-switching) is observed in natural populations and in a phylogenetic context. In contrast, gender-switching has not occurred during the evolutionary history of unionoids. Here we present sequences for the male (M) and female (F) mitotypes from an additional 8 species of Unionoida. Consistent with previous observations, the M and F mitotypes of all species form reciprocally monophyletic clades supporting the hypothesis of taxon-specific rates of gender-switching. Coinciding with the absence of gender-switching is an approximately 185 codon extension of the cytochrome c oxidase II (MTCO2) locus in the male genome. The extension is present in all 12 unionoid species examined, including a representative of the family Margaritiferidae, indicating that this protein-coding polymorphism originated > or = 200 MYBP: . Although the extension is well conserved in length among 11 of the 12 species, one taxon has a significantly shortened extension. Lastly, examination of the rates and patterns of substitution indicate that the extension is evolving under relaxed purging selection, a pattern inconsistent with the conserved nature of MTCO2 or any cytochrome c oxidase locus.
authors.affiliation: Hubbard Center for Genome Studies, University of New Hampshire, Durham, New Hampshire, 03824, USA. jcurole@usc.edu
authors.forename: Jason P
authors.initials: JP
authors.lastname: Curole
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Kocher
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D049872:Bivalvia / Q000201:enzymology* / Q000235:genetics*; D003576:Electron Transport Complex IV / Q000737:chemistry* / Q000235:genetics*; D019143:Evolution, Molecular; D005618:Fresh Water; D008969:Molecular Sequence Data; D010802:Phylogeny; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: C117108:cytochrome C oxidase subunit II; D003576:Electron Transport Complex IV