Mar Biol (2009) 156:913–925 DOI 10.1007/s00227-009-1137-x 123 ORIGINAL PAPER Molecular population genetics of male and female mitochondrial genomes in European mussels Mytilus Beata Jmietanka · Artur Burzyjski · Roman Wenne Received: 6 August 2008 / Accepted: 12 January 2009 / Published online: 11 February 2009  Springer-Verlag 2009 Abstract The doubly uniparental system of mitochon- drial inheritance (DUI) is best known in marine mussels Mytilus. Under DUI there are two types of mitochondrial DNA (mtDNA). The female type (F) is transmitted to oVspring of both genders and the male type (M) exclusively to sons; consequently two distinct mtDNA lineages exist. The M lineage evolves under more relaxed selection than the F lineage resulting in higher polymorphism within the M lineage. Though this polymorphism is expected to make inferences on Wne population structure easier using M instead of F data, no comprehensive comparative data exist to support this claim. We sequenced a 1,205 bp fragment of M and F mtDNA comprising parts of the COIII and ND2 genes, and analysed 204 individuals representing three Mytilus species: M. edulis, M. galloprovincialis and M. trossulus from 13 European sampling sites. A clear dis- tinction between Mediterranean and Atlantic populations was found with both M and F data, but much better geo- graphic diVerentiation was found within the Atlantic using F rather than M data. In particular, Atlantic M. galloprovin- cialis can be diVerentiated from Atlantic M. edulis, and fur- ther subdivision of Atlantic M. edulis is possible using the F data but not the M data. Multiple tests of selection were carried out to attempt to explain this paradox. We con- cluded that the overall pattern of polymorphism is consis- tent with strong purifying selection; not only is this selection relaxed in the M lineage in comparison with the F lineage, but it is also more frequently interrupted by peri- odic selective sweeps within the M lineage. Introduction The Mytilus edulis species complex (Rawson and Hilbish 1995) is composed of three phylogenetically young species, widely distributed in boreal and temperate waters of the northern and southern hemispheres (McDonald et al. 1991; Gosling 1992). Surveys based on allozymes and DNA indi- cate the presence of all members of this complex in Euro- pean waters (Varvio et al. 1988; Gosling 1994; Jmietanka et al. 2004). M. edulis occurs from the White Sea and Barents Sea in the North, to the Atlantic coast of southern France in the South. The range of the more thermophilic M. galloprovincialis extends from the Black Sea to the British Isles, as far north as the Orkney Islands. M. trossulus inhabits the brackish Baltic Sea. All three taxa hybridise in nature within their contact zones (Gosling 1992; Gardner et al. 1993; Hilbish et al. 2002; Bierne et al. 2003), unlike their closest related species M. californianus (Ort and Pogson 2007). The hybrid zone between Atlantic M. edulis and Baltic M. trossulus in the Danish Straits is apparently very permeable, as strong but unidirectional genetic intro- gression takes place into the M. trossulus populations (Borsa et al. 1999; Riginos et al. 2002; Kijewski et al. 2006). The introgression of mtDNA between species of the M. edulis complex complicates analyses based on mito- chondrial (mt) markers. In North America this phenomenon is generally limited (Rawson et al. 1996; Saavedra et al. 1996; Rawson et al. 1999), but in Europe extensive mtDNA introgression has been observed (Quesada et al. 1995; Jmietanka et al. 2004). Sequence analysis has additionally conWrmed that mtDNA of M. edulis replaced the native Communicated by T. Reusch. B. Jmietanka (&) · A. Burzyjski · R. Wenne Department of Genetics and Marine Biotechnology, Institute of Oceanology, Polish Academy of Sciences, Powstajców Warszawy 55, 81-712 Sopot, Poland e-mail: bsmietanka@iopan.gda.pl