Clinal variation in Drosophila serrata for stress resistance and body size

Clines for size and stress resistance traits have been described for several Drosophila species and replicable clines across different species may indicate climatic selection. Here we consider clines in stress resistance traits in an Australian endemic species, D. serrata, by comparing levels of variation within and among isofemale lines initiated with flies collected from the eastern coast of Australia. We also consider clinical variation in chill coma recovery, a trait that has recently been shown to exhibit high levels of variation among Drosophila species. Patterns were compared with those in the cosmopolitan species D. melanogaster from the same area. Both desiccation and starvation resistance showed no clinical pattern despite heritable variation among isofemale lines. In contrast chill coma resistance exhibited a linear cline in the anticipated direction, resistance increasing with latitude. Body size was measured as wing length and body weight. Both traits showed geographic variation and strong non-linear clines with a sharp reduction in size in the tropics. These results are discussed in the context of climatic selection and evolutionary processes limiting species borders.     

Latitudinal Variation in Starvation Resistance is Explained by Lipid Content in Natural Populations of Drosophila melanogaster

One of the most common environmental stressors is a shortage or suboptimal quality of food, thus all animals deal with periods of starvation. In the present study we examine variation in starvation resistance, longevity and body lipid content and the correlations between traits along an environmental gradient using isofemale lines recently derived from natural populations of Drosophila melanogaster from South America. The use of isofemale lines and controlled rearing laboratory conditions allows us to investigate within and among population components of genetic variation and the potential associations among starvation resistance, longevity and body lipid content. All these traits were analyzed separately in females and males, improving our understanding of sexual dimorphism. Our results revealed significant differences among populations in starvation resistance and longevity. Actually, the opposing latitudinal cline detected for starvation resistance suggests that natural selection played an essential role in shaping the pattern of geographic variation in this trait. Moreover, we also detected a positive relationship between starvation resistance and body lipid content in both sexes, providing evidence for a physiological and/or evolutionary association between these traits. Conversely, starvation resistance was not correlated with longevity indicating that these traits might be enabled to evolve independently. Finally, our study reveals that there is abundant within population genetic variation for all traits that may be maintained by sex-specific effects.     

Divergence of larval resource acquisition for water conservation and starvation resistance in Drosophila melanogaster

Laboratory selection experiments have evidenced storage of energy metabolites in adult flies of desiccation and starvation resistant strains of D. melanogaster but resource acquisition during larval stages has received lesser attention. For wild populations of D. melanogaster, it is not clear whether larvae acquire similar or different energy metabolites for desiccation and starvation resistance. We tested the hypothesis whether larval acquisition of energy metabolites is consistent with divergence of desiccation and starvation resistance in darker and lighter isofemale lines of D. melanogaster. Our results are interesting in several respects. First, we found contrasting patterns of larval resource acquisition, i.e., accumulation of higher carbohydrates during 3rd instar larval stage of darker flies versus higher levels of triglycerides in 1st and 2nd larval instars of lighter flies. Second, 3rd instar larvae of darker flies showed ~40?h longer duration of development at 21°C; and greater accumulation of carbohydrates (trehalose and glycogen) in fed larvae as compared with larvae non-fed after 150?h of egg laying. Third, darker isofemale lines have shown significant increase in total water content (18%); hemolymph (86%) and dehydration tolerance (11%) as compared to lighter isofemale lines. Loss of hemolymph water under desiccation stress until death was significantly higher in darker as compared to lighter isofemale lines but tissue water loss was similar. Fourth, for larvae of darker flies, about 65% energy content is contributed by carbohydrates for conferring greater desiccation resistance while the larvae of lighter flies acquire 2/3 energy from lipids for sustaining starvation resistance; and such energy differences persist in the newly eclosed flies. Thus, larval stages of wild-caught darker and lighter flies have evolved independent physiological processes for the accumulation of energy metabolites to cope with desiccation or starvation stress.     

Changes in cuticular lipids, water loss and desiccation resistance in a tropical drosophilid: analysis of variation between and within populations

We investigated within as well as between population variability in desiccation resistance, cuticular lipid mass per fly and cuticular water loss in nine geographical populations of a tropical drosophilid, Zaprionus indianus. Interestingly, the amount of cuticular lipids and desiccation resistance in this non-melanic species are significantly higher as compared with melanic Drosophila melanogaster. On the basis of isofemale line analysis, within population trait variability in cuticular lipid mass per fly is positively correlated with desiccation resistance and negatively correlated with cuticular water loss but show lack of correlation with body size. We observed geographical variation in the amount of cuticular lipid mass per fly in Zaprionus indianus but no such divergence was found in D.melanogaster. In both the species, geographical variations in desiccation resistance are negatively correlated with cuticular water loss but the underlying mechanisms for changes in cuticular permeability are quite different. Thus, we may suggest that body melanisation and cuticular lipids may represent alternative strategies for coping with dehydration stress in melanic versus non-melanic drosophilids. For both the species, desiccation resistance and cuticular water loss are correlated with regular increase in aridity in the northern subtropical localities as compared with southern peninsular humid tropical localities. The role of climatic selection is evident from multiple regression analysis with seasonal changes in temperature and humidity (Tcv and RHcv) of the sites of origin of populations of Zaprionus indianus along latitude.     

Desiccation tolerance and starvation resistance exhibit opposite latitudinal clines in Indian geographical populations of Drosophila kikkawai

Abstract. 1. Desiccation tolerance and starvation resistance demonstrated significant differentiation among seven Indian geographical populations of Drosophila kikkawai, collected along a latitudinal range of 12.6–32.7 °N. Lack of significant differences in two successive generations suggested that these physiological traits were genetically controlled. 2. North Indian populations of D. kikkawai displayed significantly higher desiccation tolerance than southern populations, whereas there was a reverse trend for starvation tolerance (r > 0.90). Regression slope values indicated an increase of 0.61 h for desiccation and a decrease of 1.71 h per degree latitude for starvation tolerance at 17 °C. The traits evidenced opposite latitudinal clines, and such data also matched thermal climatic conditions on the Indian subcontinent. The survival duration for such traits was significantly higher at 17 than at 25 °C. 3. Significantly higher starvation tolerance in south Indian populations might be due to large population size, species interactions, and higher metabolic rates in the humid tropical environments. In contrast, prolonged unfavourable colder climatic conditions are known to favour starvation tolerance in temperate regions. Thus, the causes of desiccation and starvation tolerance seem quite different under tropical and temperate conditions. 4. Starvation tolerance was correlated negatively with body weight and ovariole number, which might be due to a trade-off in favour of greater allocation to non-lipidic reserves for sustaining starvation tolerance in the tropics. Reduction in metabolic rate may not be applicable for observed higher starvation tolerance in the tropical populations. 5. Multiple regression analysis demonstrated a major effect of coefficient of variation of mean monthly temperature for both the traits of ecological significance. Thus, Indian geographical populations of D. kikkawai provided evidence of independent genetic divergence for starvation and desiccation tolerance under natural conditions.     

Seasonal changes in humidity level in the tropics impact body color polymorphism and desiccation resistance in Drosophila jambulina—Evidence for melanism-desiccation hypothesis

Drosophila jambulina exhibits color dimorphism controlled by a single locus but its ecological significance is not clear. Dark and light morphs differ significantly in body melanisation, desiccation resistance, rate of water loss, mating activity and fecundity. Interestingly, this species lacks clinal variation for body size, desiccation resistance and life history traits. For body melanisation, lack of geographical variation as well as plastic effects is not in agreement with a thermal melanism hypothesis. However, based on field data, there are seasonal changes in phenotypic frequencies of dark and light body color morphs which correlate significantly with variation in humidity levels. Under short-term (8 h) desiccation stress, we observed higher number of assortative matings, longer copulation period and increased fecundity for dark strains as compared with light strains. By contrast, both the morphs when exposed to high humid conditions exhibited higher assortative matings and fecundity for light strains as compared with dark strains. In tropical populations of D. jambulina, body color polymorphism seems to be maintained through humidity changes as opposed to thermal melanism. Thus, seasonal changes in the frequency of body color morphs in this tropical species supports melanism-desiccation hypothesis.     

Desiccation resistance along an aridity gradient in the cactophilic fly Drosophila buzzatii: sex-specific responses to stress

Stress resistance characters are valuable tools for the study of acclimation potential, adaptive strategies and biogeographic patterns in species exposed to environmental variability. Water stress is a challenge to terrestrial arthropods because of their small size and relatively high area: volume ratio. Fruit flies have been investigated to record adaptive morphological and physiological traits, as well as to test their responses to stressful factors. In this study, we investigate the ability to cope with water stress, by examining variation in desiccation resistance in a species that lives mainly in desert lands. Specifically, we explored the genetic and ecological basis of desiccation resistance in populations of Drosophila buzzatii from Northern Argentina. We used a common garden experiment with desiccation treatments on a number of isofemale lines from four populations along an aridity gradient. Our results revealed significant among-population differentiation and substantial amounts of genetic variation for desiccation resistance. We also detected significant genotype-by-environment and genotype-by-sex interactions indicative that desiccation resistance responses of the lines assayed were environment- and sex-specific. In addition, we observed clinal variation in female desiccation resistance along gradients of altitude, temperature and humidity; that desiccation resistance is a sexually dimorphic trait, and that sexual dimorphism increased along the aridity and altitudinal gradients. Based on current evidence, we propose that the observed sex-specific responses may reflect different life history traits, and survival and reproductive strategies in different ecological scenarios.     

Temperature and photoperiod affect stress resistance traits in Drosophila melanogaster

The long‐term survival of species and populations depends on their ability to adjust phenotypic values to environmental conditions. In particular, the capability of dealing with environmental stress to buffer detrimental effects on fitness is considered to be of pivotal importance. Resistance traits are readily modulated by a wide range of environmental factors. In the present study, Drosophila melanogaster Meigen is used to investigate plastic responses to temperature and photoperiod in stress resistance traits. The results reveal that stress resistance traits (cold, heat, starvation and desiccation resistance) are affected by the factors temperature and sex predominantly. Cooler temperatures compared with warmer temperatures increase cold tolerance, desiccation and starvation resistance, whereas they reduce heat tolerance. Except for heat resistance, females are more stress‐resistant than males. Stress resistance traits are also affected by photoperiod. Shorter photoperiods decrease cold tolerance, whereas longer photoperiods enhance desiccation resistance. Overall, thermal effects are pervasive throughout all measured resistance traits, whereas photoperiodic effects are of limited importance in the directly developing (i.e. nondiapausing) flies used here, suggesting that pronounced photoperiodic effects on stress resistance traits may be largely limited to, and triggered by, diapause‐inducing effects.     

Behavioural response to combined insecticide and temperature stress in natural populations of Drosophila melanogaster

Insecticide resistance evolves extremely rapidly, providing an illuminating model for the study of adaptation. With climate change reshaping species distribution, pest and disease vector control needs rethinking to include the effects of environmental variation and insect stress physiology. Here, we assessed how both long‐term adaptation of populations to temperature and immediate temperature variation affect the genetic architecture of DDT insecticide response in Drosophila melanogaster. Mortality assays and behavioural assays based on continuous activity monitoring were used to assess the interaction between DDT and temperature on three field‐derived populations from climate extremes (Raleigh for warm temperate, Tasmania for cold oceanic and Queensland for hot tropical). The Raleigh population showed the highest mortality to DDT, whereas the Queensland population, epicentre for derived alleles of the resistance gene Cyp6g1, showed the lowest. Interaction between insecticide and temperature strongly affected mortality, particularly for the Tasmanian population. Activity profiles analysed using self‐organizing maps show that the insecticide promoted an early response, whereas elevated temperature promoted a later response. These distinctive early or later activity phases revealed similar responses to temperature and DDT dose alone but with more or less genetic variance depending on the population. This change in genetic variance among populations suggests that selection particularly depleted genetic variance for DDT response in the Queensland population. Finally, despite similar (co)variation between traits in benign conditions, the genetic responses across population differed under stressful conditions. This showed how stress‐responsive genetic variation only reveals itself in specific conditions and thereby escapes potential trade‐offs in benign environments.     

Life span variation in 13 Drosophila species: a comparative study on life span,environmental variables and stress resistance

Recently, heterogeneity of the environment has been suggested as an important player in the evolution of life span variation. Established ageing theories propose that life span variation is the result of coevolution with other traits, such as stress resistance. This study aimed to compare these alternative hypotheses by examining the relationship between four environmental variables and different types of stress resistance traits with life span in 13 Drosophila species originating from tropical, subtropical and temperate environments (ecotypes). Average life span was found to differ significantly both between species and sexes, but only male life span correlated with the environment and cold resistance. While controlling for phylogeny, the environmental variable precipitation seasonality and resistance against cold‐induced stress explained most variation in male life span. Furthermore, male life span varied between species in a manner represented by environmental variables linked to the different ecotypes, such that tropical species lived longer and were less cold resistant. The current results suggest that general mechanisms underlying stress resistance and life span are unlikely. In addition, our results point to the environment independently shaping variation in life span and cold resistance rather than genetic interactions.     

Resistance to temperature extremes between and within life cycle stages in Drosophila serrata, D. birchii and their hybrids: intraspecific and interspecific comparisons

Intraspecific Drosophila studies suggest that resistance to heat and cold stresses are largely independent and that correlations across life cycle stages are low whereas comparisons of Drosophila species indicate correlations between heat and cold resistance as well as between resistance levels in different life cycle stages. These inconsistent results may reflect differences in associations among traits at the interspecific and intraspecific levels or interspecific correlations arising because of correlated selection pressures. These alternatives were tested using Drosophila serrata, D. birchii and hybrids derived from these species. Variation among hybrid lines and families was used to test associations at the interspecific level while intraspecific variation was examined using isofemale lines ol'D. serrata. There was a significant association between adult heat knockdown time at 38^oC and adult cold resistance in one set of hybrid lines. An association between female knockdown resistance to heat and larval heat resistance was also evident in one set of hybrids. Resistance to heat and cold at the larval stage were not correlated at either the intraspecific or interspecific levels. At the intraspecific level, larval heat resistance and two measures of adult heat resistance were uncorrected. Moreover, adult and larval cold resistance measures were not correlated at either the intraspecific or interspecific levels. These results suggest that there arc no associations between resistance to heat and cold extremes and that extreme temperature resistance is largely independent across life cycle stages at both the intraspecific and interspecific levels. Species associations may therefore arise from correlated selection pressures rather than trait correlations.     

Evidence for a robust sex-specific trade-off between cold resistance and starvation resistance in Drosophila melanogaster

In insects changes in lipid metabolism may underlie a trade-off between cold resistance and starvation resistance. To test this we examined correlated responses in independent sets of Drosophila melanogaster lines selected for increased cold resistance and increased starvation resistance. The starvation lines showed correlated patterns found in other D. melanogaster populations selected for this trait, including higher lipid levels and increased resistance to desiccation, although the selected lines did not show a longer development time as found in some other studies. Consistent with the trade-off hypothesis, selected lines with increased starvation resistance showed decreased resistance to a cold stress as measured by mortality, whereas selected lines with increased cold resistance showed a decrease in starvation resistance. To counter the possibility of inadvertent selection accounting for these patterns, selected and control lines from both selection regimes were crossed to form mass bred populations, which were left for four generations prior to establishing isofemale lines. By scoring starvation and cold resistance in these lines derived from both sets of selection regimes, we confirmed the negative association between resistance to these stresses in females but not in males. Potential implications of this trade-off for surviving cold conditions when food resources are limiting are discussed.     

Partitioning variance in a physiological trait: desiccation resistance in keratin beetles (Coleoptera, Trogidae)

1. Although variation in physiological traits forms the substance of evolutionary physiology, the way that this variation is partitioned among different hierarchical levels (e.g. population, species, genus) is not well known.
2. In this study variance partitioning is examined in body size, water content, lipid content, rate of water loss, maximum tolerable water loss and survival time at four levels (individual, population, species, genus) in southern African keratin beetles.
3. It is found that most variance in body size, and the physiological traits that are strongly influenced by body size (water and lipid content, maximum tolerable water loss, rate of water loss), is partitioned at the generic level (50–70%), then at the species level (20–50%) and finally at the population (1–9%) and individual (2–18%) levels.
4. On the other hand, variance in survival time, and variance in rate of maximum water loss once the effects of body size have been taken into account, are partitioned mostly at the species level (40–70%), whereas maximum tolerable water loss and lipid and water content show greatest variance at the individual level (63–75%). This is largely a consequence of differences in the extent of scaling of these traits.
5. The results suggest that where possible the effects of phylogeny should be controlled for when examining desiccation resistance in insects using comparative methods, but that confidence in conclusions from previous studies, which have used mass-specific data, is not unwarranted.     

Geographic variation for climatic stress resistance traits in the springtail Orchesella cincta

Multiple traits of stress resistance were investigated in the epedaphic springtail Orchesella cincta. Second generation adults from five laboratory populations were compared with respect to resistance to extreme temperatures and desiccation, and traits relevant to climatic adaptation. Populations were collected along a 2000-km latitudinal gradient ranging from Denmark to southern Italy and reared under the same standard laboratory conditions. Traits investigated were resistance to high and low temperature, desiccation resistance, body size and water loss rate (WLR). Results showed genetically based differences in resistance to high and low temperature, desiccation, WLR, water pool and body size between populations. Individuals from the most northern population had the highest desiccation-and cold shock resistance, and the lowest heat shock resistance. Females were significantly more desiccation resistant than males. The results of cold shock resistance showed a positive increase with lowest environmental temperature recorded at the sites of population origin, whereas heat shock resistance showed a positive increase with highest recorded temperature at the sites of population origin. Desiccation resistance increased towards the most southern and northern population, suggesting that both low and high temperature extremes affect desiccation resistance. Body mass, water pool and WLR showed interpopulation as well as sex specific variation. This provides evidence for geographical variation in stress resistance of springtails related to climatic conditions.     

Clinal variation and laboratory adaptation in the rainforest species Drosophila birchii for stress resistance, wing size, wing shape and development time

Clinal variation has been described in many invertebrates including drosophilids but usually over broad geographical gradients. Here we describe clinal variation in the rainforest species Drosophila birchii from Queensland, Australia, and potential confounding effects of laboratory adaptation. Clinal variation was detected for starvation and development time, but not for size or resistance to temperature extremes. Starvation resistance was higher at southern locations. Wing shape components were not associated with latitude although they did differ among populations. Time in laboratory culture did not influence wing size or heat knockdown resistance, but increased starvation resistance and decreased recovery time following a cold shock. Laboratory culture also increased development time and altered wing shape. The results indicate that clinal patterns can be detected in Drosophila over a relatively narrow geographical area. Laboratory adaptation is unlikely to have confounded the detection of geographical patterns.     

Absence of clinal variation in virgin retention capacity in Australian Drosophila melanogaster

The ability of virgin Drosophila melanogaster adults to retain eggs is thought to be an adaptation to persisting in temperate areas, based on differences in this trait between European and African populations, and based on seasonal changes in this trait in France. By retaining eggs in the absence of males and under conditions of poorer nutrition (conditions common in temperate areas during colder months), females reduce the wastage of resources and increase their probability of surviving spring into summer, enabling them to initiate summer population expansions. To test for variation in virgin egg retention along a climatic gradient, we characterized clinal variation in strains collected from eastern Australia extending from temperate Tasmania to tropical northern Queensland. Despite testing a large number of strains and repeated testing of the cline ends, we did not detect any evidence for clinal variation in virgin egg retention. Therefore although D. melanogaster in temperate Australia overwinter at the adult stage, there is no evidence for selection on virgin retention capacity producing clinal patterns. This contrasts with other evidence for clinal variation in egg production patterns over winter.     

Seasonal trade-off between starvation resistance and cold resistance in temperate wild-caught Drosophila simulans

Abstract  Selection experiments with Drosophila have revealed a possible evolutionary trade-off between cold resistance and starvation resistance that may be controlled by lipid metabolism. To test this trade-off in naturally occurring Drosophila simulans populations, flies were simultaneously collected from two temperate locales experiencing contrasting seasons. Flies from a tropical locale served as a control. Cold coma recovery, starvation resistance and lipid proportion were assayed on adult males and females from each locale. Compared with the summer-collected flies from Canberra, the winter-collected flies from San Diego recovered from cold coma more quickly, were less starvation resistant and had lower lipid levels. These results support an evolutionary trade-off between cold resistance and starvation resistance. Combined, these data also suggest that differences in lipid metabolism may be an underlying mechanism for this trade-off.     

Altitudinal variation for stress resistance traits and thermal adaptation in adult Drosophila buzzatii from the New World

Multiple stress resistance traits were investigated in the cactophilic fly Drosophila buzzatii. Adults from seven populations derived from North-Western Argentina were compared with respect to traits relevant for thermal stress resistance and for resistance to other forms of environmental stress. The populations were collected along an altitudinal gradient spanning more than 2000 m in height, showing large climatic differences. The results suggest that knock-down resistance to heat stress, desiccation resistance and Hsp70 expression at a relatively severe stressful temperature best reflect thermal adaptation in this species. Furthermore, cold resistance seemed to be of less importance than heat resistance, at least for the adult life stage, in these populations. Clinal variation in thermal resistance traits over short geographical distances suggests relatively strong adaptive differentiation of the populations. This study provides the first evidence for altitudinal differentiation in stress-related traits, and suggests that Hsp70 expression level can be related to altitudinal clines of heat-stress resistance.     

LIMITS TO THE SOUTHERN BORDER OF DROSOPHILA SERRATA: COLD RESISTANCE,HERITABLE VARIATION,AND TRADE-OFFS

There are a number of evolutionary hypotheses about why species distributions are limited, but very little empirical information to test them. We present data examining whether the southern distribution of Drosophila serrata is limited by cold responses. Species comparisons were undertaken for cold resistance, development time, and viability at 15°C and 25°C for D. serrata and other species with a more southerly distribution (D. melanogaster, D. simulans, and D. immigrans). Relative to the other species, D. serrata had a long development time at both temperatures and a low level of cold resistance. Using isofemale lines collected in different seasons, central and marginal populations were compared for cold resistance, as well as development time and viability at 14°C. The border population had a relatively higher resistance to cold shock in postwinter collections, but there was no population differentiation for prewinter collections or for the other traits. The presence of variation among isofemale lines within the border populations suggests that genetic variation as measured in the laboratory is unlikely to limit range expansion. Population cages were used in the field to determine if D. serrata persisted over winter at borders. Although all cages yielded adult offspring at northern sites, only a few produced offspring at or just south of the border. In contrast, all cages with D. simulans produced adult offspring, suggesting that climatic factors limited D. serrata numbers. Offspring from surviving adults showed a phenotypic trade-off between fecundity and cold resistance. Comparisons of the cold resistance of field males and females with their laboratory-reared offspring provided evidence for heritable variation in field-reared flies. Overall, the results suggest that cold stress is important in limiting the southern distribution of D. serrata, but it seems unlikely that a lack of genetic variation restricts range expansion.     

Desiccation resistance in interspecific Drosophila crosses. Genetic interactions and trait correlations

We used crosses between two closely related Drosophila species, Drosophila serrata and D. birchii, to examine the genetic basis of desiccation resistance and correlations between resistance, physiological traits, and life-history traits. D. serrata is more resistant to desiccation than D. birchii, and this may help to explain the broader geographical range of the former species. A comparison of F2's from reciprocal crosses indicated higher resistance levels when F2's originated from D. birchii mothers compared to D. serrata mothers. However, backcrosses had a resistance level similar to that of the parental species, suggesting an interaction between X-linked effects in D. serrata that reduce resistance and autosomal effects that increase resistance. Reciprocal differences persisted in hybrid lines set up from the different reciprocal crosses and tested at later generations. Increased desiccation resistance was associated with an increased body size in two sets of hybrid lines and in half-sib groups set up from the F4's after crossing the two species, but size associations were inconsistent in the F2's. None of the crosses provided evidence for a positive association between desiccation resistance and glycogen levels, or evidence for a tradeoff between desiccation resistance and early fecundity. However, fecundity was positively correlated with body size at both the genetic and phenotypic levels. This study illustrates how interspecific crosses may provide information on genetic interactions between traits following adaptive divergence, as well as on the genetic basis of the traits.