Fitness landscapes support the dominance theory of post-zygotic isolation in the musselsMytilus edulisandM. galloprovincialis

Abstract

We studied the genetic basis of post-zygotic isolation in the marine musselsMytilus edulisandMytilus galloprovincialis. Evidence was obtained for a high number of recessive Dobzhansky–Muller substitutions in the genome of these two mussel taxa. We analysed the segregation of unlinked diagnostic markers in the progeny of two backcrosses and an F2cross, 36 h and 200 days after fertilization. Directional selection favouringM. galloprovincialisgenotypes was observed in both kinds of cross. In the F2, epistatic interactions between each pair of chromosome fragments mapped by the markers were identified in addition. Our results imply that homozygous–homozygous interactions are required for breakdown of coadaptation, in accordance with the dominance theory of post-zygotic isolation. Endogenous post-zygotic selection distributed over many loci throughout the genome provides the missing factor explaining the astonishing persistence and strength of barriers to neutral introgression in such a dispersive taxon asMytilus.