A highly pleiotropic amino acid polymorphism in the Drosophila insulin receptor contributes to life-history adaptation

Abstract

Finding the specific nucleotides that underlie adaptive variation is a major goal in evolutionary biology, but polygenic traits pose a challenge because the complex genotype-phenotype relationship can obscure the effects of individual alleles. However, natural selection working in large wild populations can shift allele frequencies and indicate functional regions of the genome. Previously, we showed that the two most common alleles of a complex amino acid insertion-deletion polymorphism in the Drosophila insulin receptor show independent, parallel clines in frequency across the North American and Australian continents. Here, we report that the cline is stable over at least a five-year period and that the polymorphism also demonstrates temporal shifts in allele frequency concurrent with seasonal change. We tested the alleles for effects on levels of insulin signaling, fecundity, development time, body size, stress tolerance, and life span. We find that the alleles are associated with predictable differences in these traits, consistent with patterns of Drosophila life-history variation across geography that likely reflect adaptation to the heterogeneous climatic environment. These results implicate insulin signaling as a major mediator of life-history adaptation in Drosophila, and suggest that life-history trade-offs can be explained by extensive pleiotropy at a single locus.

Keywords: Adaptation; InR; cline; pleiotropy; seasonality.

Figure 1

From pooled sequencing data, we identified eight discrete polymorphisms associated with the complex indel in the first exon of InR (A). Each polymorphism is labeled according to position on chromosome 3R relative to the published D. melanogaster reference genome (version 5.48). The discrete polymorphisms describe the complex indel haplotypes we previously characterized, which include two alleles, InR^short and InR^long, that are common at high and low latitudes, respectively (B). Polymorphisms that unambiguously discriminate between the InR^short and InR^long alleles are denoted with a large dot; in each case, the allele present in the reference genome identifies InR^short. (See Methods for relationships between the other discrete polymorphisms and InR^short and InR^long.) All eight of the discrete polymorphisms demonstrate some degree of seasonality, but four exhibit especially strong patterns in which the plotted allele increases in frequency by approximately 20% over the winter for three consecutive years (C). Alleles assigned to the InR^short allele class (large dots) generally show higher frequency in the spring and lower frequency in the fall, suggesting that overwintering may impose selection pressures consistent with high latitudes. As in our earlier report (Paaby et al. 2010), alleles assigned to the InR^short allele class also increase in frequency with latitude (D). The plotted alleles are all reference genome alleles.

Figure 2

Relative abundance of seven transcriptional targets of dFOXO, a central transcription factor in the IIS pathway that is repressed by InR activity. Increased abundance of these targets indicates reduced IIS; transcript abundance for l(2)eft, ac76e, dLip4, and InR T1 were significantly greater (P < 0.05) in InR^short samples compared to InR^long. These results are consistent with the expectation that InR^short is associated lower IIS than InR^long. Error bars show 95% confidence intervals.

Figure 3

Average eggs laid per female over lifetime and in the first 12 h after mating. Lifetime fecundity was not significantly different between any genotypes except between InR^short and the heterozygote (A). However, InR^long females laid many more eggs in the first 12 h after mating than InR^short females, in comparisons from both populations (C). Error bars show 95% confidence intervals.

Figure 4

Average development time, from the time the egg was laid to eclosion of the adult. Flies derived from the Mount Sinai population showed no significant differences in development time by sex, and overall InR^long flies developed faster than InR^short flies (A). However, InR^long females from Bowdoin developed faster than InR^short females (B), whereas male genotypes were not significantly different (C). Error bars show 95% confidence intervals.

Figure 5

Average dry body weight, lipid weight, length of the L2 wing vein, and wing:thorax ratio. InR^short flies had lower dry weight (A) and lower lipid content (B) than InR^long flies. InR^short flies were also smaller, as measured by the L2 wing vein (C), and had a smaller wing:thorax ratio (D). These results are from flies derived from Bowdoin; similar findings, but of lower magnitude, were observed in flies derived from Mount Sinai (Figure S1). Error bars show 95% confidence intervals.

Figure 6

Average time to recovery from chill coma and proportion of flies surviving cold shock, starvation, and heat shock. Although InR^short females and males both recovered faster from chill coma than did InR^long flies, the effect was only significant for males (A). InR^short flies also better survived cold shock (B) and starvation (C), but more InR^long flies survived heat shock (D). Error bars show 95% confidence intervals.

Figure 7

Survivorship curves, for females (A) and males (B). In both sexes, the heterozygote lived significantly longer than either homozygote genotype. In males, the InR^short genotype exhibited a reduced rate of aging late in life to produce a significant lifespan extension relative to InR^long, but there was no difference in longevity between InR^short and InR^long females.