INTEGRATION OF THE GENOTYPE IN GEOGRAPHIC POPULATIONS OF DROSOPHILA PAVANI

Drosophila pavani, Brncic 1957, is a common species in central Chile, and in Argentina along the eastern slope of the Andes Mountains. Its populations are polymorphic for the gene arrangements in the second and fourth chromosomes (Brncic, 1957, 1958). In chromosome II there is a gene order which differs from Standard by two constantly associated inversions (A, B) in its middle part. In the right arm of chromosome IV there exists, besides Standard, a second gene sequence due to the presence of three overlapping inversions (IV-R; A + B + C). Finally, in the left arm of the same chromosome, there is a gene arrangement which differs from Standard also by a complex of three inversions (IV-L; A + B + C). The heterozygotes for these gene orders occur in most of the populations studied with frequencies close to 50 per cent (Brncic, 1957, 1958). This suggests that the inversion heterozygotes are adaptively superior to the corresponding homozygotes. In Drosophila pseudoobscura (Dobzhansky, 1951), D. willistoni (da Cunha and Dobzhansky, 1954) and in D. robusta (Carson and Stalker, 1947) the frequencies of inversions may show geographic and seasonal changes. When populations of these species are maintained in the laboratory under certain experimental conditions, substantial changes may occur in the frequencies of the different gene orders. The situation in D. pavani seems to be different. There is no good evidence of geographic or seasonal fluctuations (Brncic, 1957, 1958, and unpublished data). Moreover, in some of the stocks maintained for more than six years under the usual laboratory conditions, it has been observed that, although the frequencies of inversions suffer some variations, they usually remain high, as in the corresponding natural populations. This seemed to indicate that D. pavani represents a single large polymorphic population, not subdivided into geographic races differing in chromosome structure. However, the adaptive function of the chromosomal polymorphism need not depend on the inversions themselves, but on the polygene complexes which they maintain (Dobzhansky and Pavlovsky, 1958). Polygenes may exhibit two types of balance (Mather, 1943; Lerner, 1958), namely internal balance, by means of which certain blocks of genes are adapted to produce superior fitness, and relational balance, established between the genes in a pair of homologous chromosomes. The chromosomal inversions are a mechanism for maintaining the integrity of such coadapted genetic systems in the population. The relational balance is responsible for the high adaptive values of inversion heterozygotes and for establishment of balanced polymorphism (Dobzhansky, 1955). There is, however, evidence that, in at least some species of Drosophila, the same inversions may carry different polygene complexes in different populations. The high fitness may be lost in the heterozygotes for chromosomes with different gene arrangements coming from distant geographic regions; the genes of one region may not give high adaptive values with the genes from other regions. This has been found in D. pseudoobscura (Dobzhansky, 1957), D. willistoni, and D. paulistorum (Dobzhansky and Pavlovsky, 1958). Thus, natural populations of these and probably of some other species, show not only qualitative and 1 The work reported in this article has been carried out under Contract Number A.T. (30-1) 2465, U. S. Atomic Energy Commission. 2 Instituto de Biologia "Juan Noe. " Zaniartu 1042. Santiago, Chile.