Early View (EV): 1-EV geographic distribution limits and climatic conditions across species (Gaston 2000, Gaston andBlackburn 2000), several climate-based hypotheses have been proposed. Th ese include the climate variability hypothesis (CVH), which states that terrestrial organisms distributed in highly variable environments (e.g. high latitudes or altitudes) are adapted to withstand a broader range of climatic conditions than organisms in less variable environments (e.g. low latitudes or altitudes). Th erefore, species at high latitudes (or altitudes) develop a broader range of tolerance to temperatures, hereinafter thermo-tolerance range, thus becoming more extensively distributed than species at low latitudes (or altitudes) (Snyder and Weathers 1975, Osovitz and Hofmann 2005, Compton et al. 2007.Th e thermo-tolerance range is a critical trait that determines the physiological niche of insect species (Spicer and Gaston 1999) and it could be delimited by critical temperatures (minimum and maximum) within which the individuals are generally active (Chown and Nicolson 2004). Th e critical temperatures are defi ned as those beyond which the insects cannot respond to any further change in temperature, and therefore they will become vulnerable

The range of thermal tolerance is one of the main factors influencing the geographic distribution of species. Climate change projections predict increases in average and extreme temperatures over the coming decades; hence, the ability of living beings to resist these changes will depend on physiological and adaptive responses. On an evolutionary scale, changes will occur as the result of selective pressures on individual heritable differences. In this work, we studied the genetic basis of tolerance to high temperatures in the fly Drosophila melanogaster and whether this species presents sufficient genetic variability to allow expansion of its upper thermo‐tolerance limit. To do so, we used adult flies derived from a natural population belonging to the Drosophila Genetic Reference Panel, for which genomic sequencing data are available. We characterized the phenotypic variation of the upper thermal limit in 34 lines by measuring knockdown temperature (i.e., critical thermal maximum [CTmax]) by exposing flies to a ramp of increasing temperature (0.25°C/min). Fourteen percent of the variation in CTmax is explained by the genetic variation across lines, without a significant sexual dimorphism. Through a genomewide association study, 12 single nucleotide polymorphisms associated with the CTmax were identified. In most of these SNPs, the less frequent allele increased the upper thermal limit suggesting that this population harbors raw genetic variation capable of expanding its heat tolerance. This potential upper thermal tolerance increase has implications under the global warming scenario. Past climatic records show a very low incidence of days above CTmax (10 days over 25 years); however, future climate scenarios predict 243 days with extreme high temperature above CTmax from 2045 to 2070. Thus, in the context of the future climate warming, rising temperatures might drive the evolution of heat tolerance in this population by increasing the frequency of the alleles associated with higher CTmax.

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Long-term exposure to low temperatures during adult maturation might decrease fertility after cold recovery as a consequence of carryover effects on reproductive tissues. This pattern should be more pronounced in tropical than in temperate species as protective mechanisms against chilling injuries are expected to be more effective in the latter. We initially determined the lower thermal thresholds to induce ovarian maturation in four closely related Drosophila species, two inhabiting temperate regions and the other two tropical areas of South America. As expected, only temperate species regularly experience cold-inducing conditions for reproductive arrest during winter in their natural environment. Subsequently, we exposed reproductively arrested and mature females to cold-inducing conditions for reproductive arrest over a long period. Following cold exposure, tropical species exhibited a dramatic fertility decline, irrespective of reproductive status. In contrast, not only were temperate females fecund and fertile but also fertility was superior in females that underwent cold-induced reproductive arrest, suggesting that it might act as a protecting mechanism ensuring fertility after cold recovery. Based on these findings, we decided to evaluate the extent to which reproductive status affects cold tolerance and energy metabolism at low temperature. We found a lower metabolic rate and a higher cold tolerance in reproductively arrested females, although only temperate species attained high levels of chill tolerance. These findings highlight the role of cold-induced reproductive arrest as part of an integrated mechanism of cold adaptation that could potentially contribute to the spread of temperate species into higher latitudes or altitudes.