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Many bivalve species, including mussels of the genus Mytilus, are unusual in having two mtDNA genomes, one inherited maternally (the F genome) and the other inherited paternally (the M genome). The sequence differences between the genomes are usually great, indicating ancient divergence predating speciation events. However, in Mytilus trossulus from the Baltic, both genomes are similar to the F genome from the closely related M. edulis. This study analyzed the mtDNA control region structure in male and female Baltic M. trossulus mussels. We show that a great diversity of structural rearrangements is present in both sexes. Sperm samples are dominated by recombinant haplotypes with M. edulis M-like control region segments, some having large duplications. By contrast, the rearranged haplotypes that dominate in eggs lack segments from this M genome. The rearrangements can be explained by a combination of tandem duplication, deletion, and intermolecular recombination. An evolutionary pathway leading to the recombinant haplotypes is suggested. The data are also considered in relation to the hypothesis that the M. edulis M-like control region sequence is necessary to confer the paternal role on genomes that are otherwise F-like. S TRICTLY uniparental inheritance of organelle genomes is a rule in nearly all anisogamic organisms. One of the most prominent exceptions is the mitochondrial inheritance system of mussels of the family Mytilidae in which separate maternal (the F genome) and paternal (the M genome) routes of mtDNA inheritance occur (for review, see Skibinski et al.1994a,b; Zouros et al. 1994a,b;Zouros 2000). This system, called doubly uniparental inheritance (DUI) (Zouros et al. 1994a), has also been observed in freshwater mussels of the family Unionidae (Hoeh et al. 1996;Liu et al. 1996) and clams of the family Veneridae (Passamonti and Scali 2001). Phylogenetic analysis indicates that divergence of the F and M genomes can be great, predating speciation events, and that role reversal or masculinization events, whereby the F genome takes on the role of the previous M genome, has occurred repeatedly in the evolution of marine mussels (Hoeh et al. 1996(Hoeh et al. , 1997, but is absent or less frequent in freshwater mussels (Hoeh et al. 2002).A hybrid zone separates Baltic Mytilus trossulus from North Sea M. edulis populations (see Riginos and Cunningham 2005 for review). Although there is little introgression of mtDNA between American M. trossulus and M. edulis (Saavedra et al. 1996;Comesana et al. 1999) in Baltic M. trossulus, the mtDNA in heteroplasmic male individuals is similar to that in the F genome from M. edulis (Quesada et al. 1995(Quesada et al. , 2003Wenne and Skibinski 1995; Zbawicka et al. 2003a,b). It appears that there has been complete asymmetric introgression of M. edulis F mtDNA into Baltic M. trossulus, accompanied by role reversal and masculinization (Rawson and Hilbish 1998;Quesada et al. 1999). These processes might be coupled and associated with cytonuclear incompatibilities that ...

Environmental conditions such as a pronounced salinity gradient and postglacial history make the Baltic Sea a suitable area for studying how selection and gene flow affect genetic differentiation in marine species. A cDNA library was used to identify new single nucleotide polymorphisms (SNPs) in Baltic populations of Mytilus spp. mussels. Sixty polymorphic SNPs were used to genotype 642 individual mussels from the inner Baltic, Danish Straits, northwest Denmark and a population of the northeast Pacific. We characterized 49 novel SNP markers that differentiate the populations of the North and Baltic Sea areas. Concordant narrow clines were observed at the entrance of the Baltic Sea for most of these markers. Considerable variance of hybrid index scores was observed in populations with intermediate allele frequencies within the hybrid zone, e.g. in Gedser and Hjelm. The presented results are in accordance with the existence of strong reproductive isolation, probably caused by a combination of exogenous (e.g. adaptation to brackish waters) and endogenous pre-and post-zygotic factors (e.g. selection against hybrids). The overwhelming majority of new SNPs markers showed a larger representation of M. trossulus than M. edulis genes in the nuclear DNA of Baltic Mytilus species. Finally, we identified a few markers with an elevated level of introgression of M. edulis alleles in the Baltic Sea M. trossulus populations in comparison to the reference M. trossulus population of the Pacific.

Gaining new knowledge of the native distributions of species (phylogeography)

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Human-mediated biological transfers of species have substantially modified many ecosystems with profound environmental and economic consequences. However, in many cases, invasion events are very hard to identify because of the absence of an appropriate baseline of information for receiving sites/regions. In this study, use of high-resolution genetic markers (single nucleotide polymorphisms - SNPs) highlights the threat of introduced Northern Hemisphere blue mussels (Mytilus galloprovincialis) at a regional scale to Southern Hemisphere lineages of blue mussels via hybridization and introgression. Analysis of a multispecies SNP dataset reveals hotspots of invasive Northern Hemisphere blue mussels in some mainland New Zealand locations, as well as the existence of unique native lineages of blue mussels on remote oceanic islands in the Southern Ocean that are now threatened by invasive mussels. Samples collected from an oil rig that has moved between South Africa, Australia, and New Zealand were identified as invasive Northern Hemisphere mussels, revealing the relative ease with which such non-native species may be moved from region to region. In combination, our results highlight the existence of unique lineages of mussels (and by extension, presumably of other taxa) on remote offshore islands in the Southern Ocean, the need for more baseline data to help identify bioinvasion events, the ongoing threat of hybridization and introgression posed by invasive species, and the need for greater protection of some of the world's last great remote areas.

Semi-sessile Mytilus mussels are used as indicators of climate changes, but their geographic distribution is not sufficiently known in the Arctic. The aim of this study was to investigate the taxonomic status and genetic differentiation of Mytilus populations in a Northwest Greenlandic fjord at Maarmorilik, impacted by contaminations from a former mine. In this study, mussels were collected at three sites differing in exposure to environmental factors. A total of 54 polymorphic SNPs found in the Mytilus EST and DNA sequences analyzed were successfully applied to 256 individuals. The results provided the first evidence for the existence of M. trossulus in Greenland. The mussel from M. trossulus and M. edulis taxa are shown to coexist and hybridize in the fjord. The three studied sites were found to differ significantly in the distribution of taxa with a higher prevalence of M. trossulus in the inner fjord. The identified M. edulis 9 M. trossulus hybrids mostly had a hybrid index score of about 0.5, indicating a similar number of alleles characteristic for M. trossulus and M. edulis. There was a low number of backcrosses between 'pure' taxa and hybrids. This newly discovered hybrid zone between the two taxa is unique in comparison with the Canadian populations. As Mytilus mussels in Greenland hitherto have been regarded as the one taxon M. edulis, the results have importance for biogeography and future monitoring and environmental studies.

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