HETEROCHROMATIN ACCUMULATION AND KARYOTYPIC EVOLUTION IN SOME DIPTERAN INSECTS

Evolutionary divergence among eukaryotes always involves genetic chang es at different levels of the genome. At the chromosomal level, hetero chromatin differentiation resulting in karyotypic evolution provides a useful tool for cytotaxonomy of many groups of animals including the dipteran insects. In our studies, detectable differences in the amount and distribution of heterochromatin have been observed in several gro ups of closely related species and some sibling species complexes of D rosophila, Anopheles, and Bactrocera, for example, the D. kikkawai com plex and the montium subgroup, the An. dirus complex and the maculatus group, and the B. dorsalis complex and the Zeugodacus group, respecti vely. Most cases, ii not all, of our studies point to the fact that in ter-and intraspecific differences in mitotic chromosomes are due to th e acquisition of major block(s) of constitutive heterochromatin in the sex chromosome(s) and/or autosome(s), particularly at the pericentric region. Further, quantitative differences in heterochromatin of mitot ic chromosomes can be successfully employed as genetic markers for sep aration of cryptic or isomorphic species in these groups of insects. A lthough the functional role and implications of heterochromatin in spe cies differentiation is an unsolved problem, heterochromatin accumulat ion in the genome is clearly involved in genetic differentiation and k aryotypic evolution of dipteran insects as demonstrated in the present study.