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Deleterious mutations in a hybrid zone: can mutational load decrease the barrier to gene flow?

Published online by Cambridge University Press:  17 February 2003

NICOLAS BIERNE
Affiliation:
Laboratoire Génome, Populations, Interactions, CNRS-UMR5000 – SMEL, 1 Quai de la daurade, 34200 Sète, France Present address: Centre for the Study of Evolution & School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK. Tel: +44 (0)1273 606755, ext 2954. Fax: +44 (0)1273 678433. e-mail: n-bierne@univ-montp2.fr
THOMAS LENORMAND
Affiliation:
CEFE – CNRS, 34293 Montpellier Cedex 5, France
FRANÇOIS BONHOMME
Affiliation:
Laboratoire Génome, Populations, Interactions, CNRS-UMR5000 – SMEL, 1 Quai de la daurade, 34200 Sète, France
PATRICE DAVID
Affiliation:
CEFE – CNRS, 34293 Montpellier Cedex 5, France
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Abstract

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The aim of this paper is to investigate the effect of deleterious mutations in a hybrid zone maintained by selection against hybrids. In such zones, linkage disequilibria among hybrid depression loci, resulting from a balance between migration and selection, are crucial in maintaining the barrier because they allow each locus, in addition to its own selection coefficient, to cumulate indirect selective effects from other loci. Deleterious alleles produce heterosis and increase by this means the effective migration rate in structured populations. In a hybrid zone, they therefore contribute to decrease linkage disequilibria as well as the barrier to gene flow imposed by hybrid depression. However, deleterious mutations have no effect: (i) when selection against hybrids is weak, because linkage disequilibria are small even without heterosis in this case, or (ii) when selection against hybrids is so strong that it overwhelms heterosis. On the other hand, with moderate selection against hybrids, the decrease in the strength of the barrier due to heterosis may reach detectable levels, although it requires relatively small population sizes and/or migration rates. The effect is expected to be small and only within small genomes where loci are tightly linked can it become strong. Nevertheless, neglecting mutational load may to some extent obscure the estimations of selective parameters based either on artificial F1 crosses or on cline characteristics.

Type
Research Article
Copyright
© 2002 Cambridge University Press