Marine mussels of the genus Mytilus posses a system of transmission of mitochondrial genomes different from other animals. According to this system, termed "doubly uniparental inheritance" (DUI), both sexes inherit an F genome from the mother, but males also inherit an M genome from the father. In crosses, DUI is associated with a great variation in the sex-ratio. Unpublished results obtained in M. galloprovincialis indicate that sex-ratio is controlled by the mother, and that males that produce sons without the M genome (SMI type males) occur frequently in wild populations. In this project, crosses and PCR-based molecular techniques will be used to test the roles of mitochondrial and nuclear genomes in determining the DUI or SMI condition and the sex-ratio bias. The results will also test the hypothesis that phylogenetic relationship of F and M genomes in Bivalves are governed by occasional breakdowns of DUI.