CHROMOSOME REARRANGEMENTS IN BELOSTOMA-PLEBEJUM (STAL) (BELOSTOMATIDAE, HETEROPTERA)

Cytogenetic analysis of different populations of Belostoma plebejum (2 n = 14 + XY, male) has revealed the presence of two naturally occurrin g chromosome mutants: one heterozygous for an autosomal fusion, and th e other with a sex chromosome fragmentation. Meiosis in both individua ls is very regular. The fusion heterozygote (2n = 13 + XY) presented a n autosomal trivalent at meiosis I, which divided reductionally at fir st anaphase; cells at meiosis II had 6 or 7 autosomes plus the XY pseu do-bivalent. The other chromosome mutant presented three sex chromosom es, which probably resulted from the fragmentation of the original X c hromosome (2n = 14 + X1X2Y). The three sex chromosomes divided equatio nally at anaphase I and associated in a chain at metaphase II. Many au thors are of the opinion that karyotype evolution in Heteroptera has p roceeded principally by fusions and fragmentations. Particularly in Be lostoma, it has been proposed that evolution from the ancestral number 2n = 26 + XY has occurred through autosomal fusions and an X chromoso me fragmentation. The facts that these rearrangements have been found in natural populations of B. plebejum and that they show a regular mei otic behaviour, seem to support the suggested hypothesis.