Female mating behaviour, sexual selection and chromosome I inversion karyotype in the seaweed fly, coelopa frigida.

Previous studies of the seaweed fly, Coelopa frigida, have revealed the operation of several different forces of sexual selection. The overall pattern of mate choice seen in natural populations is not consistent with the predictions of indirect sexual selection as females do not express preferences that maximize the fitness of their offspring, even though the benefits from such choice are relatively large in this species. Thus, the maintenance of female mate choice for large male size must instead either be a result of a direct benefit to the female of mating with large males, or a side-effect of the evolution of another character, in other words pleiotropy. In order to separate these two alternatives the genetical basis of female mating behaviour needs to be studied. Previous studies have revealed associations between chromosomal inversion karyotype and both general female willingness to mate and mate choice for large male size, however these associations were lost after several generations of laboratory culture. Here several isokaryotypic lines from wild collections of flies were derived. The willingness to mate and mate choice of females from each line were determined. Pairs of lines of opposite inversion karyotype that significantly differed in either or both willingness to mate and mate choice were crossed. The mating behaviour and inversion karyotype of the F2 progeny (all F1 progeny are heterokaryotypes) were determined. Clear differences in the general levels of female willingness to mate were found between the two inversion homokaryotypes in several families, whereas variation in the strength of female choice for large male size were not revealed. It is suggested that mate choice in this species occurs as a pleiotropic effect of selection acting on female willingness to mate.