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Latitudinal clines are considered a powerful means of investigating evolutionary responses to climatic selection in nature. However, most clinal studies of climatic adaptation in Drosophila have involved species that contain cosmopolitan inversion polymorphisms that show clinal patterns themselves, making it difficult to determine whether the traits or inversions are under selection. Further, although climatic selection is unlikely to act on only one life stage in metamorphic organisms, a few studies have examined clinal patterns across life stages. Finally, clinal patterns of heat tolerance may also depend on the assay used. To unravel these potentially confounding effects on clinal patterns of thermal tolerance, we examined adult and larval heat tolerance traits in populations of Drosophila simulans from eastern Australia using static and dynamic (ramping 0.06 °C min−1) assays. We also used microsatellites markers to clarify whether demographic factors or selection are responsible for population differentiation along clines. Significant cubic clinal patterns were observed for adult static basal, hardened and dynamic heat knockdown time and static basal heat survival in larvae. In contrast, static, hardened larval heat survival increased linearly with latitude whereas no clinal association was found for larval ramping survival. Significant associations between adult and larval traits and climatic variables, and low population differentiation at microsatellite loci, suggest a role for climatic selection, rather than demographic processes, in generating these clinal patterns. Our results suggest that adaptation to thermal stress may be species and life-stage specific, complicating our efforts to understand the evolutionary responses to selection for increasing thermotolerance. [ABSTRACT FROM AUTHOR]

Body size and thermal tolerance clines in Drosophila melanogaster occur along the east coast of Australia. However the extent to which temperature affects the genetic architecture underlying the observed clinal divergence remains unknown. Clinal variation in these traits is associated with cosmopolitan chromosome inversions that cline in D. melanogaster. Whether this association influences the genetic architecture for these traits in D. melanogaster is unclear. Drosophila simulans shows linear clines in body size, but nonlinear clines in cold resistance. Clinally varying inversions are absent in D. simulans. Line-cross and clinal analyses were performed between tropical and temperate populations of D. melanogaster and D. simulans from the east coast of Australia to investigate whether clinal patterns and genetic effects contributing to clinal divergence in wing centroid size, thorax length, wing-to-thorax ratio, cold and heat resistance differed under different developmental temperatures (18°C, 25°C, and 29°C). Developmental temperature influenced the genetic architecture in both species. Similarities between D. melanogaster and D. simulans suggest clinally varying inversion polymorphisms have little influence on the genetic architecture underlying clinal divergence in size in D. melanogaster. Differing genetic architectures across different temperatures highlight the need to consider different environments in future evolutionary and molecular studies of phenotypic divergence. [ABSTRACT FROM AUTHOR]

Latitudinal clines have been demonstrated for many quantitative traits in Drosophila and are assumed to be due to climatic selection. However, clinal studies are often performed in species of Drosophila that contain common cosmopolitan inversion polymorphisms that also show clinal patterns. These inversion polymorphisms may be responsible for much of the observed clinal variation. Here, we consider latitudinal clines for quantitative traits in Drosophila simulans from eastern Australia. Drosophila simulans does not contain cosmopolitan inversion polymorphisms, so allows the study of clinal selection on quantitative traits that are not confounded by associations with inversions. Body size showed a strong linear cline for both females and males. Starvation resistance exhibited a weak linear cline in females, whereas chill-coma recovery exhibited a significant nonlinear cline in females only. No clinal pattern was evident for development time, male chill-coma recovery, desiccation or heat resistance. We discuss these results with reference to the role inversion polymorphisms play in generating clines in quantitative traits of Drosophila. [ABSTRACT FROM AUTHOR]