In Drosophila melanogaster, exposure of females to low temperature and shortened photoperiod can induce the expression of reproductive quiescence or diapause. Diapause expression is highly variable within and among natural populations and has significant effects on life-history profiles, including patterns of longevity, fecundity, and stress resistance. We hypothesized that if diapause expression is associated with overwintering mechanisms and adaptation to temperate environments, the frequency of diapause incidence would exhibit a latitudinal cline among natural populations. Because stress resistance and reproductive traits are also clinal in this species, we also examined how patterns of fecundity and longevity varied with geography and how stress resistance and associated traits differed constitutively between diapause and nondiapause lines. Diapause incidence was shown to vary predictably with latitude, ranging from 35% to 90% among natural populations in the eastern United States Survivorship under starvation stress differed between diapause and nondiapause lines; diapause phenotypes were also distinct for total body triglyceride content and the developmental distribution of oocytes in the ovary following stress exposure. Patterns of longevity, fecundity, and ovariole number also varied with geography. The data suggest that, for North American populations, diapause expression is functionally associated with overwintering mechanisms and may be an integral life-history component in natural populations.

▪ Abstract  Allozyme polymorphisms have been the focus of studies of selection at single enzyme loci, and most involve the enzymes of central metabolism. DNA sequencing of enzyme loci has shown numerous examples of multiple amino acid polymorphisms segregating within electromorphs. The amino acid heterogeneity underlying many allozyme polymorphisms should confound analysis of functional differences and selection. Metabolic control theory proposed that pathways will be insensitive to functional changes in allozymes; however, there is evidence that many polymorphisms modulate fluxes. Studies of model systems have provided detailed evidence for selection acting on enzyme polymorphisms in metabolic genes. There is also evidence that regulatory changes are superimposed on structural changes. Codon bias implies that the functional differences encountered in allozyme studies should be detectable by natural selection; however, amino acid polymorphisms may also represent weakly deleterious mutations. Future studies should connect structural changes with effects on function and stability, and they should emphasize the multilocus nature of responses to selection.

Diapause is the classic adaptation to seasonality in arthropods, and its expression can result in extreme lifespan extension as well as enhanced resistance to environmental challenges. Little is known about the underlying evolutionary genetic architecture of diapause in any organism. Drosophila melanogaster exhibits a reproductive diapause that is variable within and among populations; the incidence of diapause increases with more temperate climates and has significant pleiotropic effects on a number of life history traits. Using quantitative trait mapping, we identified the RNAbinding protein encoding gene couch potato (cpo) as a major genetic locus determining diapause phenotype in D. melanogaster and independently confirmed this ability to impact diapause expression through genetic complementation mapping. By sequencing this gene in samples from natural populations we demonstrated through linkage association that variation for the diapause phenotype is caused by a single Lys/Ile substitution in one of the six cpo transcripts. Complementation analyses confirmed that the identified amino acid variants are functionally distinct with respect to diapause expression, and the polymorphism also shows geographic variation that closely mirrors the known latitudinal cline in diapause incidence. Our results suggest that a naturally occurring amino acid polymorphism results in the variable expression of a diapause syndrome that is associated with the seasonal persistence of this model organism in temperate habitats.cline ͉ diapause ͉ life history ͉ mapping ͉ tradeoff N atural populations encounter environmental stresses that diminish individual fitness, and these stresses are variable across space and time. It is predicted that the resulting natural selection often leads to a situation in which no genotype has the highest fitness across all environments, and polymorphism is maintained. This concept is pervasive in arguments about the evolution of life history variation and associated genetic tradeoffs (1). However, the expected molecular polymorphism associated with life history tradeoffs and adaptation remains elusive. Although we would expect these phenomena to be universal, the complexities of genetic dissection of such variation suggest that the best opportunity to identify the genetic basis for life history variation lies in the study of the genetic models in their natural populations (2).Drosophila melanogaster is a human commensal that has spread from areas of Sub-Saharan Africa to Europe and Asia, possibly over the last 5,000 to 16,000 years, and into the Western Hemisphere and Australia in the past several hundred years (3-5). This worldwide expansion from the tropics has required adaptation to the pronounced seasonality present in temperate habitats, and there are many examples of both single-gene polymorphism and quantitative trait variation that show geographic patterns associated with the transition from tropical to temperate climates in this species (6, 7). There also is good evidence that D. melanogaster ov...

We report a study in Drosophila melanogaster of latitudinal clines for 23 SNPs embedded in 13 genes (Pgi, Gapdh1, UGPase, Pglym78, Pglym87, Eno, Men, Gdh, Sod, Pgk, Mdh1, TreS, Treh) representing various metabolic enzymes. Our samples are from 10 populations spanning latitude from southern Florida to northern Vermont. Three new clines with latitude were detected. These are the amino acid polymorphisms in the NAD-dependent glutamate dehydrogenase (Gdh) and trehalase (Treh) genes, and a silent site polymorphism in the UDP-glucose pyrophosphorylase gene (UGPase). The result, when combined with the overall incidence and pattern of reports for six other genes (Adh, Gpdh, Pgm, G6pd, 6Pgd,, presents a picture of latitudinal clines in metabolic genes prevalent around the branch point of competing pathways. For six of the seven amino acid polymorphisms showing significant latitudinal clines in North America, the derived allele is the one increasing with latitude, suggesting temperate adaptation. This is consistent with a model of an Afrotropical ancestral species adapting to temperate climates through selection favoring new mutations.