Did historical events shape current geographic variation in morph frequencies of a polymorphic damselfly?

Abstract

In several animal species, discrete, heritable phenotypic morphs occur in one sex only. This phenomenon is commonly observed in damselfly species where the coexistence of different female colour morphs is often explained in the context of sexual conflict. However, theories based on sexual conflict alone appear to be insufficient for explaining the inter-population variation in morph frequencies. A case in point is the widespread North American damselfly Nehalennia irene, in which one female morph occurs predominantly in populations in Western Canada, while another morph is more common in Eastern Canada. Given its large distribution range, historical events may be of particular relevance in explaining the observed spatial variation in morph frequencies in this species. In order to relate the distribution of female morph frequencies with the population genetic structure, we studied sequence variation in five mtDNA gene fragments. Moreover, we compared the population genetic structure of N. irene with its sister species Nehalennia gracilis, which lacks female polymorphism. Remarkably, our results indicate that the overall genetic variability is three times lower in N. irene than in N. gracilis, which might be related to the availability of the species' preferred habitat. Furthermore, haplotype and nucleotide diversity of N. irene differed considerably among sampled sites and appears to be related to the spatial distribution in female morph frequencies. In addition to previously studied selective agents, we suggest that the species' evolutionary history, such as random genetic drift during recolonization, may also be important in explaining the current geographical distribution of female morph frequencies.