SEXUAL ISOLATION BETWEEN 3 FORMS OF FLIGHTLESS CHRYSOLINA LEAF BEETLES (COLEOPTERA, CHRYSOMELIDAE) PARAPATRICALLY DISTRIBUTED IN THE VICINITY OF SAPPORO, HOKKAIDO, NORTHERN JAPAN

Three closely related forms of flightless leaf beetles belonging to th e Chrysolina angusticollis species complex are parapatrically distribu ted in the vicinity of Sapporo, Japan. We studied sexual isolation bet ween the three forms by no choice and multiple choice designs of cross ing experiments. Sexual isolation was strong, the isolation index acco rding to Malogolowkin-Cohen et al. (1965) being 0.350-0.951, but not e nough to completely prevent gene flow between the forms. Furthermore, sperms kept by females of different forms were viable and active, alth ough the quantity was smaller in some heterogamic combinations. The re sults suggest that a strong postzygotic isolation operates in the cont act zones of these strictly parapatric forms.