Geographical variation of chromosomal structure in Drosophila gasici

Summary Drosophila gasici Brncic 1957, is a neotropical species found in several parts of the Andes Mountain System. By means of the analysis of their external characteristics, chromosomes and hybridization test they have been included in the mesophragmatica group of species of the sub-genus Drosophila.The present paper describes the mitotic and polytene chromosomes of D. gasici from samples of natural populations collected at Bogotá (Colombia), Cochabamba (Bolivia), Arica (Chile) and San Luis (Argentina). The comparative study of all these populations has disclosed that the species has split in well defined geographic races. The Colombian and Chilean flies differ from those living in Bolivia and Argentina by three independent inversions in chromosome I (the sexual pair). The only polymorphic populations seem to be the Chilean ones which exhibit two inversions in the second chromosome, besides the Standard gene arrangement. All the other populations are homozygous for all their chromosomal sequences. Studies on reproductive isolation have demonstrated that there is some sexual discrimination between the Colombian and Chilean flies in respect to the Bolivian and Argentinean ones.The populational structure of D. gasici is in contrast to that observed in the other six species belonging to the mesophragmatica group in which there are no good evidences of geographical variations at the chromosomal level.This article is dedicated to Professor Hans Bauer on the occasion of his 60th birthday.