Hybridization between Ischnura graellsii (Vander linder) and I. elegans (Rambur) (Odonata: Coenagrionidae): are they different species?

Two closely related damselflies, Ischnura graellsii and I. elegans, were analyzed for morphological differences and reproductive isolation in the north coast of Galicia (NW Spain). We compared animals from sympatric and allopatric localities, including I. elegans from Belgium and I. graellsii from southern Spain as pure allopatric populations. A set of morphometric characters was studied by means of multivariate discriminant analysis to determine if these two species can be unambiguously distinguished. Discriminant analysis revealed that I. graellsii and I. elegans are well differentiated on the first two axis (86% and 11%, respectively). I. graellsii individuals are distinguished from I. elegans by their smaller size and, specifically, by their narrower and shorter wings and shorter tibiae. In addition, I. elegans has a narrower space between the branches of each cercus, and greater distance between the branches of each paraproct. Sympatric individuals are morphologically intermediate, suggesting hybridization. When the species were put together in the laboratory, they showed partial temporal separation in mating behavior, but males of I. elegans readily mated with females of I. graellsii, and hybrid individuals were obtained. The opposite heterospecific cross was almost impossible, apparently because of mechanical problems with the tandem linkage. Laboratory-reared hybrids (from male I. elegans x female I. graellsii) are morphologically intermediate, mainly resembling the maternal phenotype. Although hybridization between both taxa is common, we suggest maintaining the specific status for both phenotypes because they show incipient reproductive isolation, as it is reported in the literature.