The Mytilus edulis complex comprises three closely related marine mussel species (M.edulas, M. galloprovincialis  and M trossulus). In areas where species distributions overlap a varying degree of hybridisation occurs, yet genetic differences between allopatric populations are maintained. These mussels have an unusual mode of mitochondrial DNA (mtDNA) inheritance termed Doubly Uniparental Inheritance (DUI). Females are homoplasmic for the F mitotype which is inherited maternally, whereas males are heteroplasmic for this and the paternally inherited M mitotype. Observations from natural populations and previous laboratory experiments suggest that DUI may be disrupted by hybridisation, giving rise to heteroplasmic females and homoplasmic males. To investigate this, controlled laboratory crosses were carried out to produce pure species and hybrid larvae of known parentage. Species identification was confirmed using the Me 15/16 nuclear DNA marker which amplifies part of the adhesive protein gene sequence. This marker reliably identified European M edulis and M. galloprovincialis and their hybrids, but results for Canadian M edulis and for M trossulus from Canada
and the Baltic Sea contradicted those from allozyme loci. MtDNA markers were used to follow the fate of the F and M mitotypes through larval development. The results confirmed that the M mitotype enters all eggs at fertilisation in M. edulis, M galloprovincialis and their hybrids and is later eliminated in a proportion of pure species larvae. Disruption of the mechanism which determines whether the M mitotype is retained or eliminated occurred in an estimated 40% of M. edulis x M. galloprovincialis hybrid larvae, a level double that previously observed in adult mussels from natural M. edulis x M.galloprovincialis populations. Furthermore, reciprocal hybrid crosses exhibited contrasting types of DUI disruption. The results indicate that disruption of DUI in hybrid mussels may be a factor in the maintenance of genetic integrity for each species.