Stage-specific effects of candidate heterochronic genes on variation in developmental time along an altitudinal cline of Drosophila melanogaster

Abstract

Background: Previously, we have shown there is clinal variation for egg-to-adult developmental time along geographic gradients in Drosophila melanogaster. Further, we also have identified mutations in genes involved in metabolic and neurogenic pathways that affect development time (heterochronic genes). However, we do not know whether these loci affect variation in developmental time in natural populations.

Methodology/principal findings: Here, we constructed second chromosome substitution lines from natural populations of Drosophila melanogaster from an altitudinal cline, and measured egg-adult development time for each line. We found not only a large amount of genetic variation for developmental time, but also positive associations of the development time with thermal amplitude and altitude. We performed genetic complementation tests using substitution lines with the longest and shortest developmental times and heterochronic mutations. We identified segregating variation for neurogenic and metabolic genes that largely affected the duration of the larval stages but had no impact on the timing of metamorphosis.

Conclusions/significance: Altitudinal clinal variation in developmental time for natural chromosome substitution lines provides a unique opportunity to dissect the response of heterochronic genes to environmental gradients. Ontogenetic stage-specific variation in invected, mastermind, cricklet and CG14591 may affect natural variation in development time and thermal evolution.

Figure 1. Crosses generating second chromosome substitution lines.

Canton S B chromosomes are colored in white. Natural chromosomes are colored in blue. Small bars in chromosomes represent dominant phenotypic markers utilized in the isogenization of chromosomes: Curly (red), Stubble (green), Sternopleural (pink) and white eyes (blue). See Materials and Methods for more information.

Figure 2. Variation in developmental time among second chromosome substitution lines.

Distribution of genetic variation among the 50 second chromosome substitution lines. Arrows indicate the most divergent lines: USP S (Uspallata slow), SB S (San Blas slow), LAV F (Lavalle fast) and GUE F (Güemes fast).

Figure 3. Regression of egg-to-adult developmental time on altitude.

Positive association between egg-to-adult developmental time and altitude for the 50 second chromosome substitution lines studied.

Figure 4. Regression of egg-to-adult developmental time on thermal amplitude.

Positive association between egg-to-adult developmental time and thermal amplitude for the 50 second chromosome substitution lines studied.

Figure 5. Quantitative complementation tests.

Developmental time ratios for each line over a heterochronic mutant allele relative to that line over a Canton S B allele. The heterogeneity in the magnitude and direction of the DT ratios represents the genetic variation for each heterochronic gene. A) invected. B) mastermind. C) cricklet. D) CG1491. Blue bars represent larval developmental time and red bars refer to egg-to-adult developmental time.

Figure 6. Contrasting pattern of altitudinal genetic variation among heterochronic genes.

Black circles represent Lavalle fast alleles and white circles refer to Uspallata slow alleles. m refers to a mutated allele of a candidate gene. A) invected/Uspallata slow genotype exhibited a longer larval developmental time than the rest of the genotypes analyzed in the quantitative complementation test, indicative of genetic variation for this gene. B–D) In contrast, m/Lavalle fast (mastermind, cricklet and CG14591) genotypes showed a longer larval developmental time than the rest of the genotypes in the other quantitative complementation tests.