Morphometric variation in a hybrid zone of two subspecies of Gerris costae (Heteroptera: Gerridae) in the Maritime Alps

Two morphologically distinct subspecies of Gerris costae form a contact zone extending from the Maritime to the Western Alps. Within this area canonical trend surface analysis revealed a geographic pattern of morphometric variation consistent with topography. From North to South, and from high to low elevation there was a transition from G. c. costae-like phenotypes to phenotypes resembling pure G. c. fieberi. The same pattern was confirmed with canonical variate analysis not taking geographic location into account; it is therefore not an artifact of trend surface analysis. Comparisons of the pattern of morphometric variation of laboratory-reared offspring with the pattern of their parents sampled from natural populations show that geographic variation is mostly determined genetically. Intermediate individuals from field populations probably are natural hybrids between the two subspecies, because laboratory-reared hybrids were intermediate between the offspring of pure strains. We did not find increased morphometric variation within the contact zone. This suggests unimpeded introgression and is in contrast with an increase in size variability that is predicted to be associated with a transition between uni- and bivoltine forms.