MEPS 245:157-170 (2002)  -  doi:10.3354/meps245157

Can habitat specialisation maintain a mosaic hybrid zone in marine bivalves?

Nicolas Bierne1,*, Patrice David2, Aimé Langlade3, François Bonhomme1

1Laboratoire Génome, Populations, Interactions, CNRS UMR 5000, SMEL, 1 Quai de la Daurade, 34200 Sète, France
2CEFE - CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France
3Laboratoire Conchylicole de Bretagne IFREMER, BP 26, 56470 La Trinité-sur-Mer, France
*Present address: Centre for the Study of Evolution and School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom. Email:

ABSTRACT: Maintaining the integrity of differentiated genomes in marine organisms needs efficient isolation mechanisms, because planktonic larval dispersion provides contacts between taxa. Habitat specialisation is interesting in this respect, because it can both prevent interspecific crosses (each taxon reproduces in its own habitat) and eliminate hybrids (typically less fit than a parental taxon in each habitat). The contact zone between smooth-shelled mussels Mytilus edulis and M. galloprovincialis in Europe is a good example, as allozyme genotypes typical of both taxa seem to segregate into different habitats. However, allozymes may be selected directly and it is not known whether the same pattern can be extended to the whole genome. Here, we used 6 presumably neutral PCR markers to investigate habitat specialisation, focussing on the Bay of Quiberon, a small region in the midst of the contact zone between the 2 taxa. Confirming allozyme findings, our results indicate that habitat specialisation is apparent at the genomic scale, as M. edulis-like genotypes are found in sheltered or open-sea sites under freshwater influence, whereas M. galloprovincialis-like genotypes occupy exposed sites. Hybrid (or mixed) populations are found in open-sea or sheltered areas without freshwater influence. Therefore, habitat specialisation does contribute to the interspecific barrier. However, this mechanism seems insufficient to completely prevent the mixing of the 2 genomes, as mixed populations exist and provide opportunity for further hybridisation. Large gametic disequilibria within hybrid populations indicate the existence of restrictions to genetic exchange between the 2 taxa, even within a single habitat. Habitat-independent isolation mechanisms must, therefore, exist in addition.

KEY WORDS: Habitat specialisation · Hybrid zone · Introgression · Mytilus edulis · Mytilus galloprovincialis

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