• Calopteryx splendens ;
  • Calopteryx virgo ;
  • density dependence ;
  • introgression ;
  • microsatellite marker ;
  • operational sex ratio ;
  • relative abundance

Hybridization is a well-known phenomenon, but there are still relatively few studies addressing the question of reproductive isolation between related sympatric animal species with largely overlapping ranges. Population density, relative abundance, and operational sex ratio (OSR) are among the factors known to have an influence on the frequency of heterospecific matings in sympatric populations. Here we had two aims. First, we used microsatellite markers and field observations to study the frequency of hybrids, and backcrosses, and the rate of heterospecific matings between two sympatric damselfly species Calopteryx splendens (Harris, 1780) and Calopteryx virgo (Linné, 1758). Second, we investigated the role of population densities, relative abundances, and OSRs on conspecific and heterospecific mating rates. Altogether we genotyped 2104 individuals from both species and found four hybrids (0.19%), one of which was a backcross. Of all the 272 matings observed, 17 (6%) were between heterospecifics, and all of these were between a C. splendens male and a C. virgo female. In addition, all of the hybrids contained mitochondrial DNA (mtDNA) of C. virgo. We show that the population density of C. virgo, which was the maternal species of all the heterospecific matings and hybrid individuals, was the only significant factor covarying with the rate of the heterospecific matings. The OSRs did not correlate with the rate of con- or heterospecific matings. Studies on interspecific interactions in sympatric species can give information about the maintenance of reproductive isolation, and thus speciation. © 2012 The Linnean Society of London