Rate of development and inversion polymorphism in Drosophila pavani.

DROSOPHZLA PAVANI BRNCIC 1957 is a neotropical species belonging to the mesophragmatica group which is very abundant in central Chile and on the eastern slope 01 the Andes in Argentine. The natural populations of this species are polymorphic for the gene arrangements in two of its chromosomes. Chromosome 2 has two inversions, one included within the other, which have up to now never been found separately, that is, no individuals have been found in nature or in the laboratory which differ from the arbitrarily designed “Standard” by only one of these arrangements. In the right arm of the fourth chromosome, besides “Standard” there is an arrangement constituted by three overlapping inversions, which are always found together, and in the left arm there is another complex rearrangement which also differs from “Standard” by three overlapping inversions. Just as occurs with the gene orders in the right arm, in the left one no intermediate steps have been found between the actual complex arrangements and the “Standard” sequence (BRNCIC 1957). There is a tendency for nonrandom association between the arrangements in both anns of this chromosome (BRNCIC 1961). After 15 years of cytogenetic studies, both in natural populations as well as in laboratory stocks of D. pauani, it has been possible to establish that there are no appreciable geographic, seasonal, altitudinal or other environmental fluctuations of the different chromosomal variants. In all the samples analyzed, the frequency of the structural heterozygotes for the genetic arrangements in the fourth chromosome is always around 50%. In the stocks maintained in the laboratory, the different chromosomal arrangements conserve in general the same frequencies found in nature. Summarizing, D. paumi is a good example of a species bearing a “rigid” or “stable” polymorphism, according to the definition given by DOBZHANSKY (1962), in contrast to other species in which polymorphism is of the “flexible” type, where the frequencies of the different arrangements are modified by environmental changes both in nature and under laboratory conditions (ref. in DOBZHANSKY 1961 ; CARSON 1965; SPERLICH 1967). These facts have led to the conclusion that the chromosomal polymorphism actually present in D. pauani represents the final product of a long and continuous selective process, which has