Adaptive value of PGM polymorphism in laboratory populations of Drosophila melanogaster

FREQUENCY-DEPENDENT SELECTION IN DROSOPHILA: ESTIMATION OF NET FITNESS IN PSEUDOHAPLOID POPULATIONS

There is much evidence that the viability of Drosophila larvae depends on the genotypes of other larvae with which they develop. There is, however, little evidence concerning frequency dependence of net fitness. This report documents variation in net fitness among male D. melanogaster from 17 lines studied in 49 discrete-generation populations. The experimental design, which uses attached-X females, eliminates effects of dominance, heterosis, recombination, genetic background, and selection in females. Selection operates only on patroclinously inherited X-chromosomes in males, causing frequency dynamics to mimic those of haploid populations. Analysis of one-generation transitions shows a general pattern of frequency dependence of net fitnesses. Analysis of multigeneration trajectories suggests frequency dependence in 12 populations, but trajectories tend to be heterogeneous among replicates. For a deterministic model in which fitness w(p) is a function of allelic frequency (p), oscillatory dynamics can occur where |dw(p)/dp| > w(p̂) (Curtsinger, 1984a). The derivative measures the strength of frequency-dependent selection, and divides models into two categories: dynamically "well-behaved" and "complex." For the pooled data, this criterion is violated in about half of the frequency space, though several factors probably cause strength to be overestimated. The strength criterion is a useful predictor of evolutionary dynamics: observed oscillations tend to occur most often where frequency dependence is strong, and less often where it is weak.

The functional impact of Pgm amino acid polymorphism on glycogen content in Drosophila melanogaster

Earlier studies of the common PGM allozymes in Drosophila melanogaster reported no in vitro activity differences. However, our study of nucleotide variation observed that PGM allozymes are a heterogeneous mixture of amino acid polymorphisms. In this study, we analyze 10 PGM protein haplotypes with respect to PGM activity, thermostability, and adult glycogen content. We find a twofold difference in activity among PGM protein haplotypes that is associated with a threefold difference in glycogen content. The latitudinal clines for several Pgm amino acid polymorphisms show that high PGM activity, and apparently higher flux to glycogen synthesis, parallel the low activity clines at G6PD for reduced pentose shunt flux in northern latitudes. This suggests that amino acid polymorphism is under selection at this branch point and may be favored for increased metabolic storage associated with stress resistance and adaptation to temperate regions.

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.

Frequency-dependent selection and competition: empirical approaches

When Darwin and Wallace first formulated the theory of evolution by natural selection, they were greatly influenced by the idea that populations tend to increase geometrically and rapidly outgrow the resources available to them. They argued that the ensuing competition among individuals would be a major agent of natural selection. Since their day, competition has become almost synonymous with the idea of natural selection or survival of the fittest. In this paper we examine the relation between competition and selection by using simple competition models, consider the interaction of density and frequency in determining competitive outcome, and review the literature on frequency-dependent competitive interactions among genotypes within populations.

Effects of various metabolites on two phosphoglucomutase allozyme activities from Drosophila melanogaster

The effects of various metabolites on the two most common phosphoglucomutase allozymes (PGMA and PGMB) in Drosophila melanogaster have been investigated in vitro. 2,3-Diphosphoglycerate (2,3DPG) inhibited PGMA and PGMB to the same degree in the presence of 25 microM glucose-1,6-diphosphate (G1,6P2). However a higher concentration of G1,6P2 partially reversed the inhibition of PGMA exerted by 2,3DPG, so that in the presence of 150 microM G1,6P2 the inhibition of PGMA was half that of PGMB at pH 6.0. Glycerol-3-phosphate (G3P) had no significant effect at pH 7.4 but exerted an activating effect at pH 6.0 which was more pronounced in the case of PGMB. ATP, citrate, and fructose-1, 6-diphosphate (F1,6P2) inhibited both PGMA and PGMB. The differences found in vitro between these two allozymes can have a significant impact on in vivo function and, therefore, on the maintenance of PGM polymorphism in experimental populations of D. melanogaster studied in the laboratory.

Properties of allelic variants of phosphoglucomutase from the sea anemone Metridium senile

The phosphoglucomutase (Pgm) locus from populations of the sea anemone Metridium senile has three alleles in natural populations from the northeastern coast of North America. Two of the alleles exhibit clinal variation north of Cape Cod, suggesting a possible association of allele frequency with environmental temperature. This clinal pattern is reproducible and stable over at least brief periods of time. The allozymes encoded by each of the six Pgm genotypes have been partially purified and characterized. The symmetrical pH optimum for Vmax is pH 7.5; the apparent Km (Kmapp) of glucose-1-phosphate declines monotonically as the pH increases from 6.5 to 8.5. There are no pronounced differences in heat stabilities of PGM produced by various genotypes, nor are there significant differences in specific activities. There are no differences in the sensitivity of Vmax to temperature. Kmapp values are very low for all genotypes, ranging from about 2 to 12 microM, depending upon the temperature. Kmapp of glucose-1-phosphate declines as the temperature is raised for all genotypes, whether the pH is held constant or allowed to vary with the temperature. Under certain conditions, there are small significant differences among genotypes in Kappm values, but there is no systematic pattern to these differences. The present data provide no biochemical explanation for the maintenance of the Pgm cline by selection for functional differences under different thermal regimes.

Latitudinal relationships of esterase-6 and phosphoglucomutase gene frequencies in Drosophila melanogaster

Geographic variation in Esterase-6 (Est-6) and Phosphoglucomutase (Pgm) gene frequencies in Australasian populations of Drosophila melanogaster are compared with analogous data collated from 16 previous reports for North America and Europe/Asia. A large-scale latitudinal cline is found on all three zoogeographic zones for Est-6 and overall, Est-61.00 frequency increases from about 20 per cent around 20 degrees latitude to about 80 per cent approaching 50 degrees latitude. In contrast, there is no consistent evidence for the latitudinal cline in Pgm gene frequencies in any of the three zones with Pgm1.00 frequency generally about 85 per cent and Pgm1.20 and Pgm0.70 frequencies each between 5 per cent and 10 per cent. The consistent Est-6 clines are attributed to latitudinal selection gradients but not consistent correlations are found between Est-6 gene frequencies and maximum or minimum temperature or rainfall which might be associated with these gradients. The directions of the Est-6 clines in fact run counter to expectations based on the in vitro thermostabilities of the respective allozymes.