Changes in relative fitness with temperature among second chromosome arrangements in Drosophila melanogaster

Abstract

Development time and body weight of In(2L)t, R (a putative short inversion on the left arm of the second chromosome) and ST (standard) karyotypes of Drosophila melanogaster were measured at different temperatures. Frequency changes were followed in populations polymorphic for In(2L)t and ST and kept under different environmental conditions. These experiments were carried out in order to explain the worldwide latitudinal clines for In(2L)t and other inversions. To avoid interactions with the Adh and αGpdh loci, which also have latitudinal clines, all karyotypes were homozygous Adh(s)αGpdh(F). In(2L)t homokaryotypes had a longer development time and a lower weight than the other karyotypes at all temperatures. R/ST heterokaryotypes had the shortest development time and ST/ST had the smallest weight decrease with increasing temperature. The differences among the In(2L)t and ST karyotypes in development time were further analyzed in an experiment where the age at which 50% of the larvae were able to become adults, without further food ingestion, was determined. In polymorphic populations at 20° and 25° a significant decline of In(2L)t frequencies was observed. At 29.5° and 33° there was no change in In(2L)t frequencies but a significant excess of heterokaryotypes occurred. On ethanol-supplemented food the most drastic decline in In(2L)t frequency was observed. Populations transferred at 2- and 3-week intervals at 25° exhibited large differences in final In(2L)t frequencies. The frequency changes could in part be attributed to the differences in development time and to previously observed differences in high temperature resistance. The experiments prove that the karyotypes are under selection. The results are discussed in relation to the geographic distribution of In(2L)t.