ALCOHOL DEHYDROGENASE AND GLYCEROL-3-PHOSPHATE DEHYDROGENASE CLINES IN DROSOPHILA MELANOGASTER ON DIFFERENT CONTINENTS.

Abstract

Some of the most persuasive evidence that natural selection can maintain allozymic variation derives from work on polymorphisms for electrophoretically and enzymes of alcohol dehydrogenase (ADH, EC 1.1.1.1.) and glycerol-3-phosphate dehydrogenase (GPDH, EC 1.1.1.8.) in Drosophila melanogaster. The ADH-F and ADH-S variants are encoded by the AdhF and Adhs alleles (11-50. 1; Grell et al., 1965) and the GPDH-F and GPDH-S variants by the GpdhF and Gpdhs alleles (11-20.5; Grell, 1967). Almost every wild population of D. melanogaster which has been investigated has been found to segregate for all four genes (e.g., Voelker et al., 1978). The frequencies of all the genes also show latitudinal clines in eastern United States and Mexico; Adhs and GpdhF decrease, and AdhF and Gpdhs increase in frequency with increasing distance from the equator (Berger, 1971; Johnson and Schaffer, 1973; Pipkin et al., 1973; Vigue and Johnson, 1973). The clines in North America are extensive, covering about 250 of latitude, and it has been suggested that they are maintained by latitude-related selection pressures (e.g., Pipkin et al., 1973; Vigue and Johnson, 1973). However, either stochastic processes or transient stages in the dynamics between migration and latitudinally invariant selection pressures can also produce gene frequency clines (Endler, 1973; Jacquard, 1974). Therefore, we have tested the hypothesis that latitudinally varying selection maintains the Adh and Gpdh clines in North America by testing for the presence of complementary latitudinal clines in Asia, Europe and Australasia. Our rationale is that if the North American clines are simply due to stochastic processes, then they are unlikely to recur in the southern hemisphere or elsewhere in the northern hemisphere. A second hypothesis which we have investigated is based on evidence for thermostability differences between the fast and slow allozymes at the two loci. GPDHF is more thermostable than GPDH-S at 39 C (Alahiotis et al., 1977) and ADH-S is generally more thermostable than ADHF at a variety of temperatures up to 44 C (Gibson, 1970; Vigue and Johnson, 1973; Day et al., 1974; but see also Oakeshott, 1976). Furthermore, in some laboratory tests, flies homozygous for Adhs are more likely to survive heat shock than AdhF homozygotes (Bijlsma-Meeles and van Delden, 1974; Johnson and Powell, 1974; but see Milkman, 1977). These results have led to the hypothesis that positive relationships between maximum temperature and Adhs and GpdhF gene frequencies underlie the latitudinal clines (Johnson and Powell, 1974; Alahiotis et al., 1977). We have tested this hypothesis by examining these relationships on the continents of Asia, Europe, Australasia and North America.