Genetics of diapause in the comma butterfly Polygonia c-album

The processes of local adaptation and ecological speciation can be better understood by studying the genetic background of life-history decisions. The sex chromosomes host genes for many population differences in the Lepidoptera and therefore the inheritance of diapause determination in the butterfly Polygonia c-album may be hypothesized to be sex-linked. In the present study, Polygonia c-album (L.) from Spain and Sweden and hybrid offspring are raised under an LD 17 : 7 h photocycle that induces most pure Swedish individuals to develop into the diapausing dark morph and most pure Spanish individuals into the light and directly-developing morph. If inheritance of the daylength threshold for diapause is X-linked, as is known to be the case for host-plant preferences, females should follow the developmental path of their male parents' populations. However, female hybrids instead have a diapause propensity intermediate to that of their parental stocks and, consequentially, diapause determination is not X-linked. However, male hybrids eclose as the diapausing morph to a higher extent than females and, moreover, this pattern is more pronounced in the Spanish female x Swedish male cross than in the reciprocal cross. Hence, it is concluded that the genetic determination of the critical daylength for diapause is mainly autosomal but with some influence of sex-linked genes and/or parental effects, possibly as an effect of the importance of protandry for males. Such sex effects could provide a starting point for the evolution of population differences inherited on the sex chromosomes.