Cytogenetic analysis of a hybrid, Glyptotendipes pallens Mg. × Glyptotendipes glaucus Mg. (Diptera, Chironomidae): evolutionary considerations

The karyotype of experimentally obtained hybrids between the two closely related species Glyptotendipes pallens and Glyptotendipes glaucus is described. Hybridization was successful in one direction only ( G. pallens ♂ x G. glaucus ♀). The polytene chromosomes AB and EF of the hybrid show a more or less intimate pairing throughout their length. In the chromosomes CD in which an inversion occurs the characteristic loop is formed. The homologues of chromosome G are almost completely asynaptic. The localization of centromere heterochromatin was also studied. Centromere heterochromatin as well as intercalary heterochromatin could be observed in all chromosomes. By C banding analyses it could be shown that G. pallens has a telomeric chromosome G while in G. glaucus it is acrocentric. According to karyotype similarity it can be assumed that these two species have quite recently derived from a common ancestor since they still share much of their genomic organization. On the Black Sea coast (southeast part of Bulgaria) a natural hybridization zone between the sympatric species G. pallens and G. glaucus has been detected. The idea that hybridization between the two species might finally proceed to the formation of a new species by hybrid origin and introgression is discussed.