Latitudinal variation for two enzyme loci and an inversion polymorphism inDrosophila melanogaster from Central and South America

Many organisms show latitudinal variation for various genetically determine d traits. Such dines may involve neutral variation and originate from histo rical events or their maintenance may be explained by selection. For Drosop hila melanogaster, latitudinal variation for allozymes, inversions, and qua ntitative traits has been found on several continents. We sampled D. melano gaster populations in Panama and along a transect of 40 latitudinal degrees on the west coast of South America. Negative correlations with latitude we re found for Adh(S) and alpha Gpdh(F) allele frequencies and for the freque ncy of the cosmopolitan inversion In(2L)t in Adh(S) alpha Gpdh(F) chromosom es. A positive correlation existed between wing length and latitude. Signif icant correlations were found between these traits and climatic variables l ike temperature and rainfall. The observed dines show considerable resembla nce to those found on other continents. Gametic disequilibrium between Adh( S) and alpha Gpdh(F) occurred predominantly at higher latitudes and was cau sed by the presence of In(2L)t. The reasons for the clinaI. distributions a re discussed and it is argued that selection is the most likely explanation . However, the exact nature of the selective force and the interactions of allozymes with each other and with In(2L)t are complex and not fully unders tood. In tropical regions In(2L)t-containing genotypes have higher fitness than ST/ST and Adh and alpha Gpdh hitchhike with the inversion, but there i s also evidence for balancing selection at the Adh locus.