A comprehensive assessment of geographic variation in heat tolerance and hardening capacity in populations of Drosophila melanogaster from eastern Australia.

We examined latitudinal variation in adult and larval heat tolerance in Drosophila melanogaster from eastern Australia. Adults were assessed using static and ramping assays. Basal and hardened static heat knockdown time showed significant linear clines; heat tolerance increased towards the tropics, particularly for hardened flies, suggesting that tropical populations have a greater hardening response. A similar pattern was evident for ramping heat knockdown time at 0.06°C min(-1) increase. There was no cline for ramping heat knockdown temperature (CT(max) ) at 0.1°C min(-1) increase. Acute (static) heat knockdown temperature increased towards temperate latitudes, probably reflecting a greater capacity of temperate flies to withstand sudden temperature increases during summer in temperate Australia. Larval viability showed a quadratic association with latitude under heat stress. Thus, patterns of heat resistance depend on assay methods. Genetic correlations in thermotolerance across life stages and evolutionary potential for critical thermal limits should be the focus of future studies.

Sgrò CM ，Overgaard J ，Kristensen TN ，Mitchell KA ，Cockerell FE ，Hoffmann AA ... -  《-》

Complex patterns of local adaptation in heat tolerance in Drosophila simulans from eastern Australia.

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.

van Heerwaarden B ，Lee RF ，Wegener B ，Weeks AR ，Sgró CM ... -  《-》

Latitudinal clines in heat tolerance, protein synthesis rate and transcript level of a candidate gene in Drosophila melanogaster.

The occurrence of climatic adaptation in Drosophila melanogaster is highlighted by the presence of latitudinal clines in several quantitative traits, particularly clines in adult heat knockdown tolerance that is higher in tropical populations. However the presence of latitudinal patterns in physiological characteristics that may underlie these traits have rarely been assessed. Protein synthesis has been implicated as an important physiological process that influences thermal tolerance, and this has not been examined in a clinal context. Here, we characterise latitudinal variation in D. melanogaster from eastern Australia in both adult heat knockdown tolerance and rates of protein synthesis following rearing at both 25 °C, approximating summer conditions, and 18 °C approximating winter development. We also examined clinal variation in the predominant nuclear transcript of the heat-inducible RNA gene hsr-omega, which has been implicated in regulating protein synthesis. We find significant clines in heat-hardened tolerance when cultured at both 18 and 25 °C - tolerance increased towards the low latitude tropics. Rates of protein synthesis measured in ovarian tissue also associated negatively with latitude, however the presence of the clines depended on rearing temperature and heat stress conditions. Finally, omega-n levels measured without heat stress showed a positive linear cline. When measured after a mild heat stress higher levels of omega-n were detected and the clinal pattern became parabolic - mid-latitude populations had lower levels of the transcript. While congruent latitudinal trends were detected for these three traits, only a low level of positive association was detected between protein synthesis and thermal tolerance providing little evidence that these traits are related at the level of cellular physiology. However the new clinal patterns of protein synthesis and hsr-omega variation suggest that these variables exert important influences on traits involved with latitudinal climatic adaptation.

Cockerell FE ，Sgrò CM ，McKechnie SW 《-》

Climatic selection on genes and traits after a 100 year-old invasion: a critical look at the temperate-tropical clines in Drosophila melanogaster from eastern Australia.

Hoffmann AA ，Weeks AR 《GENETICA》

No patterns in thermal plasticity along a latitudinal gradient in Drosophila simulans from eastern Australia.

Phenotypic plasticity may be an important initial mechanism to counter environmental change, yet we know relatively little about the evolution of plasticity in nature. Species with widespread distributions are expected to have evolved higher levels of plasticity compared with those with more restricted, tropical distributions. At the intraspecific level, temperate populations are expected to have evolved higher levels of plasticity than their tropical counterparts. However, empirical support for these expectations is limited. In addition, no studies have comprehensively examined the evolution of thermal plasticity across life stages. Using populations of Drosophila simulans collected from a latitudinal cline spanning the entire east coast of Australia, we assessed thermal plasticity, measured as hardening capacity (the difference between basal and hardened thermal tolerance) for multiple measures of heat and cold tolerance across both adult and larval stages of development. This allowed us to explicitly ask whether the evolution of thermal plasticity is favoured in more variable, temperate environments. We found no relationship between thermal plasticity and latitude, providing little support for the hypothesis that temperate populations have evolved higher levels of thermal plasticity than their tropical counterparts. With the exception of adult heat survival, we also found no association between plasticity and ten climatic variables, indicating that the evolution of thermal plasticity is not easily predicted by the type of environment that a particular population occupies. We discuss these results in the context of the role of plasticity in a warming climate.

van Heerwaarden B ，Lee RF ，Overgaard J ，Sgrò CM ... -  《-》