Presence of a unique male‐specific extension of C‐terminus to the cytochrome c oxidase subunit II protein coded by the male‐transmitted mitochondrial genome of Venustaconcha ellipsiformis (Bivalvia: Unionoidea)

Abstract

Analyses of unionoidean bivalve male-transmitted (M) mtDNA genomes revealed an approximately 555 bp 3 0 coding extension to cox2. An antibody was generated against this predicted C-terminus extension to determine if the unique cox2 protein is expressed. Western blot and immunohistochemistry analyses demonstrated that the protein was predominantly expressed in testes. Weak expression was detected in other male tissues but the protein was not detected in female tissues. This is the first report documenting the expression of a cox2 protein with a long C-terminus in animals. Its universal presence in unionoidean bivalve testes suggests a functional significance for the protein.

Comparative Mitochondrial Genomics of Freshwater Mussels (Bivalvia: Unionoida) With Doubly Uniparental Inheritance of mtDNA: Gender-Specific Open Reading Frames and Putative Origins of Replication

“…Presently, it is possible to suggest only that selection might maintain the general characteristics (i.e., one TMH followed by positively charged amino acids) of two functional F and M ORFs in the face of an extremely high overall amino acid substitution rate. This was also the suggestion for the M-specific cox2 extension, which, as mentioned above, appears to have a reproductive function (Chakrabarti et al 2006(Chakrabarti et al , 2007Chapman et al 2008). Given that relatively rapid rates of evolution are frequently observed for proteins involved in reproduction (e.g., Metz et al 1998;Swanson and Vacquier 2002;Clark et al 2006), it is possible that the F and M ORFs might also function in reproduction.…”

“…A role in the differential segregation patterns of sperm mitochondria in mussel embryos has already been proposed for the M-specific cox2 extension that is present only in unionoid bivalve M genomes (Breton et al 2007;Chakrabarti et al 2007;Chapman et al 2008). The unionoid MCOX2 protein, which is extremely variable in both its amino acid sequence and number of near C-terminal transmembrane helices among different species Kocher 2002, 2005;Chapman et al 2008), has been localized to both inner and outer mitochondrial membranes in sperm (Chakrabarti et al 2007) and appears to function in reproduction (Chakrabarti et al 2006(Chakrabarti et al , 2007Chapman et al 2008). It has been proposed that the localization of MCOX2 to the outer mitochondrial membranes likely ''tags'' the outer surface of unionoid M genome-bearing mitochondria and facilitates the distinct movements of the M genome-containing mitochondria, derived from the fertilizing sperm, in male and female embryos [as observed in Mytilus (Cao et al 2004b;Cogswell et al 2006)].…”

“…Transmembrane helices and other potential protein features of gender-specific ORFs were identified using ConPred II (Arai et al 2004) and PredictProtein (Rost et al 2004). Western blot analyses of eggs and testes extracts of the species V. ellipsiformis were performed as described by Chakrabarti et al (2006).…”

Characteristics of mitochondrial DNA of unionid bivalves (Mollusca: Bivalvia: Unionidae). I. Detection and characteristics of doubly uniparental inheritance (DUI) of unionid mitochondrial DNA

“…Sex-dependent heteroplasmy with two different forms of mtDNA transmitted to the next generations both maternally and paternally was first described in marine bivalves of the genus Mytilus in 1990 and called DUI, doubly uniparental inheritance (FISHER & SKIBINSKI 1990). Besides three species of Mytilus, DUI was detected in a few species representing six bivalve families, and is the most frequent in Unionidae, which have recently been intensely studied in this respect (HOEH et al 1996a, b, LIU et al 1996, PASSAMONTI & SCALI 2001, CUROLE & KOCHER 2002, KREBS 2004, MOCK et al 2004, WALKER et al 2006a, CHAKRABARTI et al 2006, THEOLOGIDIS et al 2008, SOROKA 2008a, b, BRETON et al 2009, DOUCET-BEAUPRÉ et al 2010). …”

“…Among unionids, which comprise more than 600 species, DUI has been described in over 50 species based on analyses of single mitochondrial genes, cox1, cox2, cytb and 16S rRNA (HOEH et al 1996b, 2002b, LIU et al 1996, CUROLE & KOCHER 2002, KREBS 2004, MOCK et al 2004, CHAKRABARTI et al 2006, WALKER et al 2006a, b, SOROKA 2008a. Among these species only Anodonta anatina and Sinanodonta woodiana (sometimes included in the genus Anodonta) occur in Poland and data on these species come from the author's earlier studies (Tables 1, 2) (SOROKA 2008a, b).…”