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 ope ration of several different forces of sexual selection. The overall pattern of mate choice seen in natural populations is not consistent with the pred ictions 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 be haviour needs to be studied. Previous studies have revealed associations be tween chromosomal inversion karyotype and both general female willingness t o mate and mate choice for large male size, however these associations were lost after several generations of laboratory culture. Here several isokary otypic lines from wild collections of flies were derived. The willingness t o 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 behav iour and inversion karyotype of the F-2 progeny (all F-1 progeny are hetero karyotypes) were determined. Clear differences in the general levels of fem ale willingness to mate were found between the two inversion homokaryotypes in several families, whereas variation in the strength of female choice fo r large male size were not revealed. It is suggested that mate choice in th is species occurs as a pleiotropic effect of selection acting on female wil lingness to mate.