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 ... -  《-》

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 ... -  《-》

Sensitivity to thermal extremes in Australian Drosophila implies similar impacts of climate change on the distribution of widespread and tropical species.

Climatic factors influence the distribution of ectotherms, raising the possibility that distributions of many species will shift rapidly under climate change and/or that species will become locally extinct. Recent studies have compared performance curves of species from different climate zones and suggested that tropical species may be more susceptible to climate change than those from temperate environments. However, in other comparisons involving responses to thermal extremes it has been suggested that mid-latitude populations are more susceptible. Using a group of 10 closely related Drosophila species with known tropical or widespread distribution, we undertake a detailed investigation of their growth performance curves and their tolerance to thermal extremes. Thermal sensitivity of life history traits (fecundity, developmental success, and developmental time) and adult heat resistance were similar in tropical and widespread species groups, while widespread species had higher adult cold tolerance under all acclimation regimes. Laboratory measurements of either population growth capacity or acute tolerance to heat and cold extremes were compared to daily air temperature under current (2002-2007) and future (2100) conditions to investigate if these traits could explain current distributions and, therefore, also forecast future effects of climate change. Life history traits examining the thermal sensitivity of population growth proved to be a poor predictor of current species distributions. In contrast, we validate that adult tolerance to thermal extremes provides a good correlate of current distributions. Thus, in their current distribution range, most of the examined species experience heat exposure close to, but rarely above, the functional heat resistance limit. Similarly, adult functional cold resistance proved a good predictor of species distribution in cooler climates. When using the species' functional tolerance limits under a global warming scenario, we find that both tropical and widespread Drosophila species will face a similar proportional reduction in distribution range under future warming.

Overgaard J ，Kearney MR ，Hoffmann AA 《-》

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 ... -  《-》

Thermal tolerance in widespread and tropical Drosophila species: does phenotypic plasticity increase with latitude?

Overgaard J ，Kristensen TN ，Mitchell KA ，Hoffmann AA ... -  《-》