THE EVOLUTIONARY HISTORY OF DROSOPHILA-BUZZATTI .26. MACROGEOGRAPHIC PATTERNS OF INVERSION POLYMORPHISM IN NEW-WORLD POPULATIONS

Inversion polymorphisms in the second and fourth chromosomes of the ca ctophilic Drosophila buzzatti in the native distribution range of the species are described. Over 5,000 flies from 26 localities were scored revealing interesting geographic structuring of arrangement frequenci es. Multiple regression and partial correlation approaches showed that the frequencies of second and fourth chromosome arrangements vary cli nically along latitudinal and altitudinal gradients and to a lesser ex tent with longitude. Although many non selective explanations can acco unt for this pattern, its resemblance to the clinal pattern described in recently established Australian populations of Drosophila buzzatii, strongly suggests a selective explanation. Additionally, the correlat ed variation observed between the frequencies of arrangements 2St on t he second chromosome and 4St on the fourth suggests a pattern of inter chromosomal association, which, when considering the vast area surveye d, might be explained as the result of epistatic interactions. The ana lysis of population structure revealed a significant regional pattern, concordant with previously described phytogeographic regions. F-stati stics showed that the patterns of variation were different not only be tween the second and fourth chromosomes, but also between second chrom osome arrangements, suggesting that selective differentiation might ha ve contributed to population structure. Since D. buzzatii breeds and f eeds on the decaying tissues of diverse cactus species present in diff erent phytogeographic regions, and given that latitude and altitude ar e strong determinants of phytogeography, it is difficult to distinguis h the underlying causes of the geographic patterns observed. However, inversion heterozygosity is not correlated with the diversity of poten tial cactus hosts. The evidence presented suggests that differential s election may be the main cause for the population structure. It is als o possible to conclude that the inversion polymorphism of D. buzzatti is flexible rather than rigid.