Molecular evidence for selection on female color polymorphism in the damselfly Ischnura graellsii

The significance of female color polymorphism in Odonata remains controvers ial despite many field studies. The importance of random factors (founder e ffects, genetic drift and migration) versus selective forces for the mainte nance of this polymorphism is still discussed. In this study, we specifical ly test whether the female color polymorphism of Ischnura graellsii (Odonat a, Coenagrionidae) is under selection in the wild. We compared the degree o f genetic differentiation based on RAPD markers (assumed to be neutral) wit h the degree of differentiation based on color alleles. Weir and Cockerham' s theta values showed a significant degree of population differentiation fo r both sets of loci (RAPD and color alleles) but the estimated degree of po pulation differentiation (theta) was significantly greater for the set of R APD loci. This result shows that some sort of selection contributes to the maintenance of similar color morph frequencies across the studied populatio ns. Our results combined with those of previous held studies suggest that a t least in some I. graellsii populations, density-dependent mechanisms migh t help to prevent the loss of this polymorphism but cannot explain the simi larity in morph frequencies among populations.