Hybridization in Calopteryx damselflies: the role of males

Females are often considered responsible for hybridization between two species because usually they are the choosier sex and their cooperation is needed for successful copulation. However, males can also be responsible for hybridization, for example in species in which males are able to force copulation. We studied the pattern of hybridization in two congeneric damselfly species, Calopteryx splendens and Calopteryx virgo, and provide evidence that F1 hybrids between the two damselfly species occur in the wild. According to mitochondrial DNA analysis, hybridization is reciprocal: five of seven hybrids were sired by C. splendens and two by C. virgo males. We conducted an experiment that revealed that males of both species have surprisingly poor premating reproductive isolation in that they accept heterospecific females, but C. splendens males were less discriminating against con- and heterospecific females than were C. virgo males. Moreover, our data on the number of hybrids sired by either species in the wild are congruent with the results of the discrimination experiment, supporting the conclusion that males may be responsible for the hybridization. Our results suggest that the males' role in hybridization studies should no longer be neglected.