Doubly Uniparental Inheritance in Bivalvia: a noteworthy variation to matrilinear inheritance of the Mitochondrial Genome.

Doubly Uniparental Inheritance (DUI) represents an outstanding exception to matrilinear inheritance of Mitochondria, typical of most Metazoa. Some bivalve mollusks posses two different mitochondrial DNA genomes (the so-called M and F mtDNAs) and realize a double mechanism of transmission in which M and F mtDNAs are inherited by father to sons and by mother to daughters, respectively. DUI provides an intriguing system for addressing aspects of molecular evolution and intermolecular recombination of Mitochondrial DNA, in both DUI and non-DUI species. Therefore, studies on DUI bivalve species may have a pivotal role in understanding the evolution of the metazoan mitochondrial genome. DUI was found in a few mussel species (Mytilidae), as well as in several unionids (Unionidae). In our lab, we started a large analysis aimed to detect gender-associated mtDNAs in additional bivalves. A venerid (Tapes philippinarum) and a mytilid species (Musculista senhousia) showed a mitochondrial heteroplasmy pattern, which is in line with a DUI model of mtDNA inheritance. Analyses on T. philippinarum DUI system were useful to test for selection on mtDNA genes under DUI, as well as for mitochondrial DNA recombination. Furthermore, the finding of mtDNA heteroplasmy in a venerid showed that DUI also occurs in phylogenetically distant families and suggested that it might be widely distributed among bivalves. The DUI system of M. senhousia revealed some unexpected traits, different from any previously known ones. Because of its peculiarity, this system challenged most of the rationales proposed to account for mitochondrial genome evolution under DUI.