1. ¹School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, Wales LL59 5EY, UK
2. ²School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea, Wales SA2 8PP, UK

Correspondence: AR Wood, School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand. E-mail: ann.wood@vuw.ac.nz

³Current address: School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand

Received 29 March 2003.

Abstract

Blue mussels of the genus Mytilus have an unusual mode of mitochondrial DNA 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. In areas where species distributions overlap a varying degree of hybridization occurs; yet genetic differences between allopatric populations are maintained. Observations from natural populations and previous laboratory experiments suggest that DUI may be disrupted by hybridization, giving rise to heteroplasmic females and homoplasmic males. We carried out controlled laboratory crosses between Mytilus edulis and M. galloprovincialis to produce pure species and hybrid larvae of known parentage. DNA markers were used to follow the fate of the F and M mitotypes through larval development. Disruption of the mechanism which determines whether the M mitotype is retained or eliminated occurred in an estimated 38% of M. edulis M. galloprovincialis hybrid larvae, a level double that previously observed in adult mussels from a natural M. edulis M. galloprovincialis hybrid population. Furthermore, reciprocal hybrid crosses exhibited contrasting types of DUI disruption. The results indicate that disruption of DUI in hybrid mussels may be associated with increased mortality and hence could be a factor in the maintenance of genetic integrity for each species.