Selective effects of temperature on some enzyme polymorphisms in laboratory populations of Drosophila melanogaster

Extensive amino acid polymorphism at the pgm locus is consistent with adaptive protein evolution in Drosophila melanogaster

PGM plays a central role in the glycolytic pathway at the branch point leading to glycogen metabolism and is highly polymorphic in allozyme studies of many species. We have characterized the nucleotide diversity across the Pgm gene in Drosophila melanogaster and D. simulans to investigate the role that protein polymorphism plays at this crucial metabolic branch point shared with several other enzymes. Although D. melanogaster and D. simulans share common allozyme mobility alleles, we find these allozymes are the result of many different amino acid changes at the nucleotide level. In addition, specific allozyme classes within species contain several amino acid changes, which may explain the absence of latitudinal clines for PGM allozyme alleles, the lack of association of PGM allozymes with the cosmopolitan In(3L)P inversion, and the failure to detect differences between PGM allozymes in functional studies. We find a significant excess of amino acid polymorphisms within D. melanogaster when compared to the complete absence of fixed replacements with D. simulans. There is also strong linkage disequilibrium across the 2354 bp of the Pgm locus, which may be explained by a specific amino acid haplotype that is high in frequency yet contains an excess of singleton polymorphisms. Like G6pd, Pgm shows strong evidence for a branch point enzyme that exhibits adaptive protein evolution.

Interaction between the Adh and alpha Gpdh loci in Drosophila melanogaster: adult survival at high temperature

The role of high temperature resistance in the world-wide cline of Adh and alpha Gpdh allele frequencies of Drosophila melanogaster was investigated. Experimental strains were used with different combinations of Adh and alpha Gpdh alleles but with similar genetic background. The survival time of adult males, reared at different pre-adult temperatures, was measured at 35 degrees C. To investigate the relationship between survival and protein content, triglycerides content and ADH and alpha GPDH enzyme activity, the latter characters were measured before and after 12 h exposure to 35 degrees C. Three-way ANOVA of survival at 35 degrees C showed significant effects of rearing temperature and Adh genotype, interaction between Adh and alpha Gpdh, interaction between Adh and rearing temperature and interaction between Adh, alpha Gpdh and rearing temperature, alpha GPDH enzyme activity did not change significantly. Although protein content, triglycerides content and ADH enzyme activity decreased significantly at 35 degrees C, no correlation was observed between survival and these traits. It is concluded that the world-wide cline of Adh and alpha Gpdh allele frequencies cannot simply be explained by the genotypic differences in resistance to high temperature of adult Drosophila melanogaster.

Polymorphism at the Adh and alpha Gpdh loci in Drosophila melanogaster: effects of rearing temperature on developmental rate, body weight, and some biochemical parameters

The role of developmental time in the world-wide cline of Adh and alpha Gpdh allele frequencies of Drosophila melanogaster, and the relationship with weight and some biochemical characters, were investigated. Experimental strains were constructed with different combinations of Adh and alpha Gpdh alleles but with similar genetic background. Developmental time, adult weight, protein-and triglyceride-content, and ADH and alpha GPDH enzyme activity were measured at a rearing temperature of 20, 25 and 29 degrees C. Genotype effects were found in all studied characters. In general the developmental times of genotypes were: AdhFF less than AdhFS less than AdhSS and alpha GpdhFF greater than alpha GpdhFS = alpha GpdhSS. Developmental time and adult weight were strongly affected by rearing temperature. Triglyceride content and ADH and alpha GPDH enzyme activity were slightly affected by temperature. Interactions between genotype and temperature effects were found for developmental rate, adult weight and protein content. No trade off was observed between developmental time on the one hand and adult weight, protein- and triglyceride-content, and ADH and alpha GPDH enzyme activity on the other hand. It is argued that developmental rate differences might be one of the underlying mechanisms of the world-wide cline of the Adh and alpha GPdh allele frequencies.

Allozyme polymorphism at the alpha Gpdh and Adh loci and fitness in Drosophila melanogaster

We have studied in Drosophila melanogaster the effects of allozyme variation at the alpha Gpdh and Adh loci on the following fitness components: female fecundity, egg hatchability, egg-to-adult survival under near-optimal and competitive conditions, rate of development under near-optimal and competitive conditions, and mating capacity of males. Significant effects of the alpha Gpdh locus on rate of development under competitive conditions (SS greater than FS greater than SS), and of the Adh locus on egg hatchability (FF = SS greater than FS) and egg-to-adult survival under competitive conditions (FF greater than SS = FS) were revealed. Possible natural selection mechanisms involved in the maintenance of allozyme polymorphisms at the alpha Gpdh and Adh loci are described. These mechanisms and the fitness of In(2L)t may account for the persistence and clinal distribution of the two allozyme polymorphisms in nature.

Observations on the extent and temporal stability of latitudinal clines for alcohol dehydrogenase allozymes and four chromosome inversions in Drosophila melanogaster

Previously we have presented evidence of large-scale latitudinal clines in the frequencies of four chromosome inversions and alleles at six enzyme loci in populations of D. melanogaster in Australasia, Asia and North America. Subsequent sampling by others in Japan and western U.S.A. has failed to repeat this observation for the steepest of the clines (alcohol dehydrogenase and the four chromosome inversions). We argue that this failure reflects the few populations and small latitudinal range sampled in these later studies. From extensive sampling over a long latitudinal transect in Australasia we here document Adh and inversion clines which are virtually identical to those originally obtained in different Australian populations four years earlier. We also repeat our observation that the Adh cline is largely independent of the cline in the linked inversion In(2L)t. We therefore retain our original conclusion that these polymorphisms are subject to natural selection. However the new Australasian data do not indicate an association between Adh and maximum rainfall which had been evident in the earlier data for Australasia, Asia and North America. We therefore retract our claim that the selective agent on Adh is related to rainfall.

Frequency and temperature dependent selection at the alcohol dehydrogenase-1 locus of Drosophila buzzatii

Genotype frequencies at the alcohol dehydrogenase-1 (Adh-1) locus of D. buzzatii were analysed for deviations from Hardy-Weinberg equilibria in the progeny of laboratory populations established at five initial Adh-1b allele frequencies and kept at either 18 degrees C, 25 degrees C or 30 degrees C. At 25 degrees C, no observed genotype frequencies were significantly different from Hardy-Weinberg expectations. Observed frequencies of heterozygotes were generally less than expected for populations at 18 degrees C and 30 degrees C. Fitness differences among genotypes were greater in males than in females, with Adh-1b homozygotes having highest fitness at 18 degrees C and Adh-1c homozygotes having highest fitness at 30 degrees C. The results are discussed in relation to previous field and laboratory studies on D. buzzatii and to the Adh polymorphism of D. melanogaster.