Drosophila simulans is more abundant under colder and drier montane habitats in the western Himalayas as compared to its sibling D. melanogaster but the mechanistic bases of such climatic adaptations are largely unknown. Previous studies have described D. simulans as a desiccation sensitive species which is inconsistent with its occurrence in temperate regions. We tested the hypothesis whether developmental plasticity of cuticular traits confers adaptive changes in water balance-related traits in the sibling species D. simulans and D. melanogaster. Our results are interesting in several respects. First, D. simulans grown at 15 °C possesses a high level of desiccation resistance in larvae (~39 h) and in adults (~86 h) whereas the corresponding values are quite low at 25 °C (larvae ~7 h; adults ~13 h). Interestingly, cuticular lipid mass was threefold higher in D. simulans grown at 15 °C as compared with 25 °C while there was no change in cuticular lipid mass in D. melanogaster. Second, developmental plasticity of body melanisation was evident in both species. Drosophila simulans showed higher melanisation at 15 °C as compared with D. melanogaster while the reverse trend was observed at 25 °C. Third, changes in water balance-related traits (bulk water, hemolymph and dehydration tolerance) showed superiority of D. simulans at 15 °C but of D. melanogaster at 25 °C growth temperature. Rate of carbohydrate utilization under desiccation stress did not differ at 15 °C in both the species. Fourth, effects of developmental plasticity on cuticular traits correspond with changes in the cuticular water loss i.e. water loss rates were higher at 25 °C as compared with 15 °C. Thus, D. simulans grown under cooler temperature was more desiccation tolerant than D. melanogaster. Finally, desiccation acclimation capacity of larvae and adults is higher for D. simulans reared at 15 °C but quite low at 25 °C. Thus, D. simulans and D. melanogaster have evolved different strategies of water conservation consistent with their adaptations to dry and wet habitats in the western Himalayas. Our results suggest that D. simulans from lowland localities seems vulnerable due to limited acclimation potential in the context of global climatic change in the western Himalayas. Finally, this is the first report on higher desiccation resistance of D. simulans due to developmental plasticity of both the cuticular traits (body melanisation and epicuticular lipid mass) when grown at 15 °C, which is consistent with its abundance in temperate regions.
Previous studies on two tropical Drosophila species (D. malerkotliana and D. bipectinata) have shown lower resistance to stress-related traits but the rapid colonization of D. malerkotliana in the past few decades is not consistent with its sensitivity to desiccation and cold stress. We tested the hypothesis that developmental acclimation at two growth temperatures (17 and 25°C) can confer adaptations to desiccation and thermal stresses. We found divergence in developmental plastic effects on cuticular traits, i.e. a significant increase of body melanisation (~2-fold) and of cuticular lipid mass (~3-fold) in D. malerkotliana but only 1.5-fold higher cuticular lipid mass in D. bipectinata when grown at 17°C compared with 25°C. A comparison of the water budget of these two species showed significantly higher effects of developmental acclimation on body water content, rate of water loss and dehydration tolerance resulting in higher desiccation resistance in D. malerkotliana than in D. bipectinata. When grown in cooler conditions (17°C), D. malerkotliana had greater resistance to cold as well as desiccation stress. In contrast, heat resistance of D. bipectinata was higher when grown at 25°C. These laboratory observations are supported by data on seasonally varying populations. Furthermore, adult D. malerkotliana acclimated to different stresses showed greater resistance to those stresses than D. bipectinata adults. Thus, significant increase in stress resistance of D. malerkotliana through developmental acclimation may be responsible for its invasion and ecological success on different continents compared with D. bipectinata.
In the Western Himalayas, Drosophila nepalensis is more abundant during the colder and drier winter than the warmer rainy season but the mechanistic bases of such adaptations are largely unknown. We tested effects of developmental plasticity on desiccation-related traits (body size, body melanization and water balance traits) that may be consistent with changes in seasonal abundance of this species. D. nepalensis grown at 15°C has shown twofold higher body size, greater melanization (∼15-fold), higher desiccation resistance (∼55 h), hemolymph as well as carbohydrate content (twofold higher) as compared with corresponding values at 25°C. Water loss before succumbing to death was much higher (∼16%) at 15°C than 25°C. Developmental plastic effects on body size are associated with changes in water balance-related traits (bulk water, hemolymph and dehydration tolerance). The role of body melanization was evident from the analysis of assorted darker and lighter flies (from a mass culture of D. nepalensis reared at 21°C) which lacked differences in dry mass but showed differences in desiccation survival hours and rate of water loss. For adult acclimation, we found a slight increase in desiccation resistance of flies reared at lower growth temperature, whereas in flies reared at 25°C such a response was lacking. In D. nepalensis, greater developmental plasticity is consistent with its contrasting levels of seasonal abundance. Finally, in the context of global climate change in the Western Himalayas, D. nepalensis seems vulnerable in the warmer season due to lower adult as well as developmental acclimation potential at higher growth temperature (25°C).
Mating speed and copulation duration respond rapidly to laboratory selection in Drosophila melanogaster Meigen (Diptera: Drosophilidae), but there is a lack of data on the evolutionary response to natural selection in the wild. Further, it is not clear whether body melanization and mating behavior are correlated traits. Accordingly, we tested whether variation in body color impacts on mating latency, copulation duration, and fecundity in latitudinal populations of D. melanogaster. We observed geographical variation (cline) for mating propensity, i.e., mating speed as well as copulation duration increased along latitude. Phenotypic plastic responses for body melanization at 17 and 25 °C also showed significant correlations with mating latency and copulation duration. Within‐population analysis based on assorted dark and light flies of five geographical populations showed significant positive correlations of copulation duration and fecundity with body melanization. To assess the role of males and/or females on mating speed and copulation duration, we used atypical body color strains (i.e., dark and light males of D. melanogaster) for no‐choice mating tests. Our data showed a major influence of males for copulation duration and of females for mating speed. Furthermore, a difference in impact of body melanization on mating speed and copulation duration was demonstrated between species, i.e., low melanization in Drosophila ananassae Doleschall is correlated with lower mating speed and shorter copulation duration than in D. melanogaster. Geographical changes in mating propensity were significantly correlated with body melanization at three levels, i.e., within and between populations and between species. Thus, we have shown that a relationship exists between body melanization and mating success. Further, we found seasonal changes in temperature and humidity to confer selection pressures on mating‐related traits.
In the Indian subcontinent, there are significant between-population variations in desiccation resistance in Drosophila melanogaster, but the physiological basis of adult acclimation responses to ecologically relevant humidity conditions is largely unknown. We tested the hypothesis that increased desiccation resistance in acclimated flies is associated with changes in cuticular permeability and/or content of energy metabolites that act as osmolytes. Under an ecologically relevant humidity regime (~50 % relative humidity), both sexes showed desiccation acclimation which persisted for 2-3 days. However, only females responded to acclimation at ~5 % relative humidity (RH). Acclimated flies exhibited no changes in the rate of water loss, which is consistent with a lack of plastic changes in cuticular traits (body melanization, epicuticular lipid). Therefore, changes in cuticular permeability are unlikely in drought-acclimated adult flies of D. melanogaster. In acclimated flies, we found sex differences in changes in the content of osmolytes (trehalose in females versus glycogen in males). These sex-specific changes in osmolytes are rapid and reversible and match to corresponding changes in the increased desiccation resistance levels of acclimated flies. Further, the increased content of trehalose in females and glycogen in males support the bound-water hypothesis for water retention in acclimated flies. Thus, drought acclimation in adult flies of D. melanogaster involves inducible changes in osmolytes (trehalose and glycogen), while there is little support for changes in cuticular permeability.
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