Abstract
Three closely related forms of flightless leaf beetles belonging to the Chrysolina angusticollis species complex are parapatrically distributed in the vicinity of Sapporo, Japan. We studied sexual isolation between the three forms by no choice and multiple choice designs of crossing experiments. Sexual isolation was strong, the isolation index according to Malogolowkin-Cohen et al. (1965) being 0.350-0.951, but not enough to completely prevent gene flow between the forms. Furthermore, sperms kept by females of different forms were viable and active, although the quantity was smaller in some heterogamic combinations. The results suggest that a strong postzygotic isolation operates in the contact zones of these strictly parapatric forms.
