Chromosomal polymorphism in natural populations of Drosophila pavani

Summary D. pavani Brncic 1957, is a neotropical species found in Chile and in a part of Argentina along the eastern slope of the Andes. The present paper describes the mitotic and salivary gland chromosomes of this species and gives a composite map of the Standard gene arrangement.Natural populations ofD. pavani are polymorphic with respect to the gene orders in their chromosomes. The observed variations in the gene arrangements are due to complex included and overlapping paracentric inversions. In none of the populations or in the crosses studied were found the hypothetical intermediate steps needed for the establishment of the phylogeny of the gene orders present in nature.The qualitative and quantitative data on the distribution and frequency of inversions show no pronounced geographic variations. The high incidence of inversion heterozygotes and the absence of the intermediate steps between the gene arrangements found in nature, seem to indicate an adaptive nature of the polymorphism observed inD. pavani This work has been supported in part by a grant from the Rockefeller Foundation.