KARYOTYPIC DIFFERENTIATION IN THE PHYTOPHAGOUS LADYBIRD BEETLES EPILACHNA-VIGINTIOCTOMACULATA COMPLEX AND ITS POSSIBLE RELEVANCE TO THE REPRODUCTIVE ISOLATION, WITH A NOTE ON SUPERNUMERARY Y CHROMOSOMES FOUND IN E-PUSTULOSA

Chromosomes of phytophagous ladybird beetles belonging to the Epilachn a vigintioctomaculata complex (Coccinellidae) were investigated in 18 populations. All the populations basically showed a diploid number of 20 with Xy(p) sex chromosome associations in male meiosis. Detailed co mparison of karyotypes using cluster and principal component analyses revealed a considerable divergence between the two groups of this spec ies complex, the group A containing E. vigintioctomaculata and the gro up B including three other ''species'', E. pustulosa, E. niponica, and E. yasutomii. The divergence comes mainly from the addition of a larg e amount of heterochromatic segments on short arms of chromosomes Nos. 3 to 9 in karyotypes of the group B. This change in chromosomal struc ture confers the so-called diphasic state, which stands for a conditio n of chromosomes with one arm euchromatic and the other heterochromati c, in karyotypes of the group B species. The chromosome configurations observed suggested the late replication of heterochromatic arms in th ose diphasics. A new hypothetical model that accounts for the postmati ng reproductive isolation between the groups A and B is proposed on th e basis of the karyotypic difference between the groups. Karyotypic di fferentiation within each group is also briefly mentioned. Significant differences were found among karyotypes of three populations of E. vi gintioctomaculata, each of which represents one of three different geo graphic forms. Karyotypes of populations belonging to E. yasutomii dev iated slightly from other species of the group B. Supernumerary Y chro mosomes were found in males of some populations of E. pustulosa with c onsiderable frequencies.