TEMPERATURE-INDUCED SHIFTS IN ASSOCIATIONS OF LONGEVITY WITH BODY SIZE IN DROSOPHILA MELANOGASTER

Temperature-induced shifts in selective pressure at a critical developmental transition

Selective mortality within a population, based on the phenotype of individuals, is the foundation of the theory of natural selection. We examined temperature-induced shifts in the relationships among early life history traits and survivorship over the embryonic and larval stages of a tropical damselfish, Pomacentrus amboinensis. Our experiments show that temperature determines the intensity of selective mortality, and that this changes with ontogeny. The size of energy stores determined survival through to hatching, after which egg size became a good indicator of fitness as predicted by theoretical models. Yet, the benefits associated with egg size were not uniform among test temperatures. Initial egg size positively influenced larval survival at control temperature (29 degrees C). However, this embryonic trait had no effect on post-hatching longevity of individuals reared at the higher (31 degrees C) and lower (25 degrees C) end of the temperature range. Overall, our findings indicate that the outcome of selective mortality is strongly dependent on the interaction between environment conditions and intrinsic developmental schedules.

Heat-induced hormesis in longevity of two sibling Drosophila species

Previous work showed that mild-heat stress induces longevity hormesis in a model organism, D. melanogaster. Here we compared the possible heat-induced hormesis in longevity of other species of Drosophila, D. buzzatii and its sibling species D. koepferae, in a single-sex environment. Possible correlations between longevity and heat-stress resistance were also tested by measuring longevity, heat-knockdown resistance and the heat-induced Hsp70 expression for each species in a common environment. D. buzzatii was longer lived than D. koepferae at benign temperature. Knockdown resistance to heat stress was positively correlated to longevity within species. However, the shorter-lived species was more resistant to knockdown by heat stress than the longer-lived species. The heat-induced Hsp70 expression was similar between species. A heat-shock treatment (37 degrees C for 1 h at 4 days of age) extended mean longevity in the longer lived species but not in the shorter lived species. In D. koepferae, the demographic rate of senescence decreased but the baseline mortality rate increased by heat-shock, resulting in no extension of mean longevity. Sympatric populations of closely related species can be differentially sensitive to temperature and exhibit different patterns of 37 degrees C-induced hormesis in demographic senescence and longevity. The results also show that positive correlations between stress resistance and life span within species can shift in sign across closely related species. Finally, this study shows that heat-induced hormesis in longevity can be found across different Drosophila species, as hormetic effects are not limited to the previously studied D. melanogaster.

Direct and correlated responses to artificial selection on developmental time and wing length in Drosophila buzzatii

Developmental time and body size are two positively correlated traits closely related to fitness in many organisms including Drosophila. Previous work suggested that these two traits are involved in a trade-off that may result from a negative genetic correlation between their effects on pre-adult and adult fitness. Here, we examine the evolution of developmental time and body size (indexed by wing length) under artificial selection applied to one or both traits in replicated D. buzzatii populations. Directional changes in both developmental time and wing length indicate the presence of substantial additive genetic variance for both traits. The strongest response to selection for fast development was found in lines selected simultaneously to reduce both developmental time and wing length, probably as an expected consequence of a synergistic effect of indirect selection. When selection was applied in the direction opposite to the putative genetic correlation, that is, large wing length but fast development, no responses were observed for developmental time. Lines selected to reduce both wing length and developmental time diverged slightly faster from the control than lines selected to increase wing length and reduce developmental time. However, wing length did not diverge from the control in lines selected only for fast development. These results suggest a complex genetic basis of the correlation between developmental time and wing length, but are generally consistent with the hypothesis that both traits are related in a trade-off. However, we found that this trade-off may disappear under uncrowded conditions, with fast-developing lines exhibiting a higher pre-adult viability than other lines when tested at high larval density.

Developmental time, body size and wing loading in Drosophila buzzatii from lowland and highland populations in Argentina

Genotype-by-temperature interaction is a necessary condition for adaptive evolution of fitness traits as a response to temperature. Several fitness-related traits (developmental time, pre-adult survival, thorax and wing lengths, and wing loading) were measured in laboratory-reared D. buzzatii from four populations sampled at different altitudes in north-western Argentina: a lowland population (407 m a.s.l.), two populations from intermediate altitude (780 to 950 m a.s.l.), and a highland population (2380 m a.s.l.). Temperature is the main climatic difference between the collection sites: lowland but not highland populations are exposed to physiologically high temperatures during both spring and summer in nature. Three growth temperatures (20, 25 and 30 degrees C) were used to test for population-by-temperature interactions. Both developmental time and pre-adult survival exhibit highly significant population-by-temperature interaction. Pre-adult survival at 30 degrees C is significantly higher in lowland than in highland populations, but not so at lower growth temperatures (20 and 25 degrees C). Both wing length and wing loading show no population-by-temperature interaction, indicating that these traits are not the direct targets of thermal adaptation in nature. Wing loading is higher in highland than in lowland populations, suggesting that flight performance is subject to stronger selection in the highland population. This hypothesis is consistent with ecological observations in both types of populations. There is no obvious among-population relationship between developmental time and body size, even though both traits are related within populations in a well-known trade-off. Overall, thermal adaptation is evident for developmental time and pre-adult survival but not for size-related traits.