The Drosophila serido speciation puzzle: putting new pieces together

The D. serido superspecies is a complex mosaic of populations distributed o ver a vast part of South America and showing various degrees of genetical d ivergence. We have analyzed its chromosomal constitution in 16 new localiti es of southeastern and southern Brazil. Both the metaphase and salivary gla nd chromosomes show a sharp split of these populations in two groups. Four populations, fixed for inversion 2e(8) and showing the type I karyotype, re present the southwestern limit of D. serido type B, which inhabits the Cerr ado in central-western Brazil. The remaining populations are homozygous for 2x(7), an inversion also fixed in the Caatinga populations of northeastern Brazil. However, their karyotype, in those populations analyzed, belong to a different type (V) from that of the Caatinga populations. Populations in this second group are polymorphic for five inversions on chromosome 2 plus another on chromosome 5 and show considerable interpopulation differentiat ion. The breakpoints of chromosome 2 inversions are described and the inver sion loops of sever heterokaryotypes are presented. Biogeographical informa tion suggests that there are clear ecological differences between the two g roups of populations as well as among the populations within the second gro up. The possible role of host plants in promoting the genetic divergence am ong the D. serido populations is discussed.