Male sterility thermal thresholds in Drosophila: D. simulans appears more cold-adapted than its sibling D. melanogaster

Numerous different criteria may be used for analysing species thermal adaptation. We compared male sterility thresholds in the two most investigated cosmopolitan siblings, D. melanogaster and D. simulans. A survey of various populations from Europe and North Africa evidenced consistent differences between the two species, and a detailed analysis was made on flies from Marrakech. Sharp sterility thresholds were observed in both species but at different temperatures: D. simulans appeared more tolerant to cold than its sibling (difference 1°C) but more sensitive to heat (difference 1.5°C). When transferred to an optimum temperature of 21°C, D. simulans males, sterilized by a low temperature, recovered more rapidly than males of D. melanogaster; the reverse was true on the high temperature side. The analysis of progeny number also revealed the better tolerance of D. simulans males to cold but a lesser tolerance to heat. From these observations, we might expect that D. simulans should be more successful in cold temperate countries than its sibling, while ecological observations point to the contrary. Our data clearly show the difficulty of comparing ecophysiological data to field observations, and also the need of extensive comparative life history studies in closely related species.     

Divergence for tolerance to thermal-stress related traits in two Drosophila species of immigrans group

We investigated associations of the species specific divergence of thermal adaptations of Drosophila nasuta and Drosophila immigrans with seasonal as well as geographical changes in the relative abundance. In D. immigrans, cold resistance and egg-to-adult viability after a cold-shock have shown a positive cline with latitude, whereas no such clinal patterns were evident for heat resistance as well as egg-to-adult viability after heat stress. In contrast, basal heat tolerance and egg-to-adult viability after a heat-shock increased towards the equator i.e. from north to south in D. nasuta. Further, we also found species-specific divergence in seasonal climatic selection responses for heat as well as cold resistance i.e. warmer and higher humidity conditions of the rainy season have evidenced ∼2-fold higher heat tolerance of D. nasuta, but lower temperature as well as lower humidity conditions of the autumn significantly increased (∼3-fold) cold tolerance of D. immigrans. Geographical variations were significant for the absolute and relative hardening capacity for cold tolerance in D. immigrans, but for heat resistance of D. nasuta. Results of multiple regression analysis of changes in thermal tolerance level in latitudinal populations as a simultaneous function of climatic variables (T_ave and RH) have shown that average yearly relative humidity levels significantly affect heat tolerance of D. nasuta, but both T_ave and RH contribute to changes in the cold tolerance of D. immigrans. Thus, our results suggest that climatic selection responses for basal and hardened thermo-tolerance levels affect relative abundance of D. nasuta and D. immigrans across a latitudinal transect as well as seasons.     

Life history adaptations and stress tolerance of four domestic species of Drosophila

Life history traits and stress tolerance were studied in four domestic species of Drosophila–D. melanogaster, D. simulans, D. auraria and D. immigrans– to understand how they adapt to their environments. In all species, larval weight approximately doubled in 1 day. The relative egg weight (egg weight : pupal weight) was smaller and the larval period was longer in D. immigrans than in the other three species. The pupal period was the longest in D. auraria. However, the adaptive significance of these differences in larval and pupal periods was not clear. The pupal case was generally thicker in the larger species, probably to support the larger pupal body. The start of oviposition was earliest and reproductive effort was greatest in female D. simulans, followed by female D. melanogaster. In contrast, starvation tolerance and the increase in bodyweight after eclosion was greater in D. immigrans and D. auraria than in the other two species. Pupal desiccation tolerance was greatest in D. melanogaster and lowest in D. auraria, and the less tolerant species seemed to select more humid sites for pupation. Adult tolerance to desiccation was greatest in D. melanogaster and lowest in D. simulans. In contrast, adult cold tolerance was greater in D. auraria and adult heat tolerance was lower in D. immigrans than in the other species. These differences in life history traits and stress tolerance represent the Drosophila species differential adaptations, and are assumed to allow coexistence of the species.     

The Importance of Acoustic Signals in Multimodal Courtship Behavior in Drosophila virilis,D. lummei and D. littoralis

The courtship rituals of Drosophila include an exchange of several signals with different modalities, chemical, visual, acoustic and tactile stimuli, between sexes. Using a video recording method, we studied the role of acoustic communication in courtship behavior in three species of the Drosophila virilis group, D. virilis, D. lummei and two populations of D. littoralis. Five series of experiments were performed: tests with intact flies (control), tests with mute flies (wingless males or females), and tests with deaf flies (aristaless males or females). We distinguished the two situations: either a female did not hear a male or vice versa, males did not hear females. When females did not hear males, the reduction in the copulation number was found in D. virilis and both populations of D. littoralis, but not in D. lummei. When males did not hear females, the reduction in the copulation number was only found in D. littoralis. The ablation of the male aristae in D. virilis and D. lummei even increased the mating success as compared to the control, which may be explained by the ‘sensory overload’ hypothesis. The changes in courtship temporal structure usually included the delayed onset of the main courtship elements (tapping, licking, and singing), and the variation in their duration and the total time of courtship. These effects were, however, more substantial in D. virilis and both populations of D. littoralis than in D. lummei. Thus, the effect of blocking the acoustic channel was different in the three species regardless of their phylogenetic relationship, and the role of acoustic communication in courtship behavior seemed to increase in the order D. lummei – D. virilis – D. littoralis.

     

No trade-off between high and low temperature tolerance in a winter acclimatized Danish Drosophila subobscura population

Coping with cold winter conditions is a major challenge for many insects.In early spring we observed newly emerged Drosophila subobscura, which had overwintered as larvae and pupae. As temperatures increase during spring these flies are faced with higher minimum and maximum temperatures in their natural microhabitat. Thus, there is a potential costly mismatch between winter and early spring acclimatization and the increased ambient temperatures later in adult life.We obtained individuals from a natural Danish population of D. subobscura and acclimated them in the laboratory to 20 °C for one generation, and compared critical thermal maximum (CTmax) and minimum (CTmin) to that of individuals collected directly from their natural microhabitat. The two populations (laboratory and field) were subsequently both held in the laboratory at 20 °C and tested for their CTmax and CTmin every third day for 28 days.At the first day of testing, field acclimatized D. subobscura had both higher heat and cold resistance compared to laboratory flies, and thereby a considerable larger thermal scope. Following transfer to the laboratory, cold and heat resistance of the field flies decreased over time relative to the laboratory flies. Despite the substantial decrease in thermal tolerances the thermal scope remained larger for field acclimatized individuals for the duration of the experiment.We conclude that flies acclimatized to their natural microhabitat had increased cold resistance, without a loss in heat tolerance. Thus while a negative correlation between cold and heat tolerance is typically observed in laboratory studies in Drosophila sp., this was not observed for field acclimatized D. subobscura in this study. We suggest that this is an adaptation to juvenile overwintering in temperate cold environments, where developmental (winter) temperatures can be much lower than temperatures experienced by reproducing adults after emergence (spring). The ability to gain cold tolerance through acclimatization without a parallel loss of heat tolerance affects thermal scope and suggests that high and low thermal tolerance act through mechanisms with different dynamics and reversibility.     

Plastic responses of survival and fertility following heat stress in pupal and adult Drosophila virilis

The impact of rising global temperatures on survival and reproduction is putting many species at risk of extinction. In particular, it has recently been shown that thermal effects on reproduction, especially limits to male fertility, can underpin species distributions in insects. However, the physiological factors influencing fertility at high temperatures are poorly understood. Key factors that affect somatic thermal tolerance such as hardening, the ability to phenotypically increase thermal tolerance after a mild heat shock, and the differential impact of temperature on different life stages are largely unexplored for thermal fertility tolerance. Here, we examine the impact of high temperatures on male fertility in the cosmopolitan fruit fly Drosophila virilis. We first determined whether temperature stress at either the pupal or adult life history stage impacts fertility. We then tested the capacity for heat‐hardening to mitigate heat‐induced sterility. We found that thermal stress reduces fertility in different ways in pupae and adults. Pupal heat stress delays sexual maturity, whereas males heated as adults can reproduce initially following heat stress, but become sterile within seven days. We also found evidence that while heat‐hardening in D. virilis can improve high temperature survival, there is no significant protective impact of this same hardening treatment on fertility. These results suggest that males may be unable to prevent the costs of high temperature stress on fertility through heat‐hardening, which limits a species’ ability to quickly and effectively reduce fertility loss in the face of short‐term high temperature events.     

Basal cold but not heat tolerance constrains plasticity among Drosophila species (Diptera: Drosophilidae)

Thermal tolerance and its plasticity are important for understanding ectotherm responses to climate change. However, it is unclear whether plasticity is traded‐off at the expense of basal thermal tolerance and whether plasticity is subject to phylogenetic constraints. Here, we investigated associations between basal thermal tolerance and acute plasticity thereof in laboratory‐reared adult males of eighteen Drosophila species at low and high temperatures. We determined the high and low temperatures where 90% of flies are killed (ULT₉₀ and LLT₉₀, respectively) and also the magnitude of plasticity of acute thermal pretreatments (i.e. rapid cold‐ and heat‐hardening) using a standardized, species‐specific approach for the induction of hardening responses. Regression analyses of survival variation were conducted in ordinary and phylogenetically informed approaches. Low‐temperature pretreatments significantly improved LLT₉₀ in all species tested except for D. pseudoobscura, D. mojavensis and D. borealis. High‐temperature pretreatment only significantly increased ULT₉₀ in D. melanogaster, D. simulans, D. pseudoobscura and D. persimilis. LLT₉₀ was negatively correlated with low‐temperature plasticity even after phylogeny was accounted for. No correlations were found between ULT₉₀ and LLT₉₀ or between ULT₉₀ and rapid heat‐hardening (RHH) in ordinary regression approaches. However, after phylogenetic adjustment, there was a positive correlation between ULT₉₀ and RHH. These results suggest a trade‐off between basal low‐temperature tolerance and acute low‐temperature plasticity, but at high temperatures, increased basal tolerance was accompanied by increased plasticity. Furthermore, high‐ and low‐temperature tolerances and their plasticity are clearly decoupled. These results are of broad significance to understanding how organisms respond to changes in habitat temperature and the degree to which they can adjust thermal sensitivity.     

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.     

Indirect selection of thermal tolerance during experimental evolution of Drosophila melanogaster

Natural selection alters the distribution of a trait in a population and indirectly alters the distribution of genetically correlated traits. Long‐standing models of thermal adaptation assume that trade‐offs exist between fitness at different temperatures; however, experimental evolution often fails to reveal such trade‐offs. Here, we show that adaptation to benign temperatures in experimental populations of Drosophila melanogaster resulted in correlated responses at the boundaries of the thermal niche. Specifically, adaptation to fluctuating temperatures (16–25°C) decreased tolerance of extreme heat. Surprisingly, flies adapted to a constant temperature of 25°C had greater cold tolerance than did flies adapted to other thermal conditions, including a constant temperature of 16°C. As our populations were never exposed to extreme temperatures during selection, divergence of thermal tolerance likely reflects indirect selection of standing genetic variation via linkage or pleiotropy. We found no relationship between heat and cold tolerances in these populations. Our results show that the thermal niche evolves by direct and indirect selection, in ways that are more complicated than assumed by theoretical models.     

Assessing population and environmental effects on thermal resistance in Drosophila melanogaster using ecologically relevant assays

To make laboratory studies of thermal resistance in ectotherms more ecologically relevant, temperature changes that reflect conditions experienced by individuals in nature should be used. Here we describe an assay that is useful for quantifying multiple measures of thermal resistance of individual adult flies. We use this approach to assess upper and lower thermal limits and functional thermal scope for Drosophila melanogaster and also show that the method can be used to (1) detect a previously described latitudinal cline for cold tolerance in D. melanogaster populations collected along the east coast of Australia, (2) demonstrate that acclimation at variable temperatures during development increases tolerance to both low and high thermal stresses and therefore increases thermal scope compared to acclimation at a constant temperature, (3) show that temperate populations adapted to variable thermal environments have wider thermal limits compared to those from the less variable tropics, at least when flies were reared under constant temperature conditions and (4) demonstrate that different measures of cold resistance are often not strongly correlated. Based on our findings, we suggest that the method could be routinely used in evaluating thermal responses potentially linked to ecological processes and evolutionary adaptation.     

Inheritance of cold shock tolerance in hybrids of Drosophila virilis and Drosophila lummei

The genetic basis of the difference in cold shock tolerance between the southern temperate Drosophila virilis and its boreal relative D. lummei is studied. After adult eclosion, the parental stocks, reciprocal F₁ and backcross hybrids were pretreated for eight days at 18°C or at 6°C. The cold shock used consisted of fast cooling to-10°C and exposure to this temperature for varying lengths of time. D. lummei tolerated such exposure for 40–50% longer than did D. virilis (100–135% after acclimation). Reciprocal F₁ females, differing only in their maternal cytoplasm deviated significantly from each other, and the reciprocal F₁ males even more so, the contribution of the X chromosome being three to four times that of the cytoplasm. The cold resistance scores of the hybrid males were more extreme than those of the parental stocks. Autosomally heterozygous males with the X chromosome and cytoplasm of virilis were the weakest flies studied. The reciprocal males (X chromosome and cytoplasm of lummei) survived better than the parental lummei stock. The reciprocal differences decreased after cold temperature acclimation. The roles of the four major autosomes were analyzed by backcrossing the reciprocal F₁ males with females of the virilis marker stock. The third chromosome of lummei as heterozygous contributed most to cold tolerance, while the other autosomes had a rather weak effect in the opposite direction (virilis homozygotes survived better), which disappeared after acclimation at 6°C. Some of the cold susceptibility of F₁ hybrids disappeared in chromosomally identical backcross flies, indicating complex cytoplasmchromosomal interactions.     

Strong responses of Drosophila melanogaster microbiota to developmental temperature

Physiological responses to changes in environmental conditions such as temperature may partly arise from the resident microbial community that integrates a wide range of bio-physiological aspects of the host. In the present study, we assessed the effect of developmental temperature on the thermal tolerance and microbial community of Drosophila melanogaster. We also developed a bacterial transplantation protocol in order to examine the possibility of reshaping the host bacterial composition and assessed its influence on the thermotolerance phenotype. We found that the temperature during development affected thermal tolerance and the microbial composition of male D. melanogaster. Flies that developed at low temperature (13°C) were the most cold resistant and showed the highest abundance of Wolbachia, while flies that developed at high temperature (31°C) were the most heat tolerant and had the highest abundance of Acetobacter. In addition, feeding newly eclosed flies with bacterial suspensions from intestines of flies developed at low temperatures changed the heat tolerance of recipient flies. However, we were not able to link this directly to a change in the host bacterial composition.     

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.     

Intraspecific variation in thermal acclimation and tolerance between populations of the winter ant,Prenolepis imparis

Thermal phenotypic plasticity, otherwise known as acclimation, plays an essential role in how organisms respond to short‐term temperature changes. Plasticity buffers the impact of harmful temperature changes; therefore, understanding variation in plasticity in natural populations is crucial for understanding how species will respond to the changing climate. However, very few studies have examined patterns of phenotypic plasticity among populations, especially among ant populations. Considering that this intraspecies variation can provide insight into adaptive variation in populations, the goal of this study was to quantify the short‐term acclimation ability and thermal tolerance of several populations of the winter ant, Prenolepis imparis. We tested for correlations between thermal plasticity and thermal tolerance, elevation, and body size. We characterized the thermal environment both above and below ground for several populations distributed across different elevations within California, USA. In addition, we measured the short‐term acclimation ability and thermal tolerance of those populations. To measure thermal tolerance, we used chill‐coma recovery time (CCRT) and knockdown time as indicators of cold and heat tolerance, respectively. Short‐term phenotypic plasticity was assessed by calculating acclimation capacity using CCRT and knockdown time after exposure to both high and low temperatures. We found that several populations displayed different chill‐coma recovery times and a few displayed different heat knockdown times, and that the acclimation capacities of cold and heat tolerance differed among most populations. The high‐elevation populations displayed increased tolerance to the cold (faster CCRT) and greater plasticity. For high‐temperature tolerance, we found heat tolerance was not associated with altitude; instead, greater tolerance to the heat was correlated with increased plasticity at higher temperatures. These current findings provide insight into thermal adaptation and factors that contribute to phenotypic diversity by revealing physiological variance among populations.     

The effects of acclimation and rearing conditions on the response of tropical and temperate populations ofDrosophila melanogaster andD. simulans to a temperature gradient (Diptera: Drosophilidae)

The effects of rearing and acclimation on the response of adultDrosophila to temperature were investigated in a gradient.D. melanogaster flies preferred a higher mean temperature and were distributed over a wider range of temperatures thanD. simulans flies. Acclimating adults at different temperatures for a week did not influence the response of either species. Adults reared at 28°C as immatures had a lower mean preference than those reared at cooler temperatures, suggesting that flies compensated for the effects of rearing conditions. Adults from tropical and temperate populations ofD. melanogaster andD. simulans did not differ in the mean temperature they preferred in a gradient, suggesting little genetic divergence for this trait within species. The species differences and environmental responses may be related to changes in optimal physiological conditions for the flies.     

Measuring thermal behavior in smaller insects: A case study in Drosophila melanogaster demonstrates effects of sex,geographic origin,and rearing temperature on adult behavior

Measuring thermal behavior in smaller insects is particularly challenging. In this study, we describe a new horizontal thermal gradient apparatus designed to study adult thermal behavior in small insects and apply it using D. melanogaster as a model and case study. Specifically, we used this apparatus and associated methodology to examine the effects of sex, geographic origin, and developmental rearing temperature on temperature preferences exhibited by adults in a controlled laboratory environment. The thermal gradient established by the apparatus was stable over diurnal and calendar time. Furthermore, the distribution of adult flies across thermal habitats within the apparatus remained stable following the period of acclimation, as evidenced by the high degree of repeatability across both biological and technical replicates. Our data demonstrate significant and predictable variation in temperature preference for all 3 assayed variables. Behaviorally, females were more sensitive than males to higher temperatures. Flies originating from high latitude, temperate populations exhibited a greater preference for cooler temperatures; conversely, flies originating from low latitude, tropical habitats demonstrated a relative preference for higher temperatures. Similarly, larval rearing temperature was positively associated with adult thermal behavior: low culture temperatures increased the relative adult preference for cooler temperatures, and this response was distinct between the sexes and for flies from the temperate and subtropical geographic regions. Together, these results demonstrate that the temperature chamber apparatus elicits robust, predictable, and quantifiable thermal preference behavior that could readily be applied to other taxa to examine the role of temperature-mediated behavior in a variety of contexts.     

Strong Costs and Benefits of Winter Acclimatization in Drosophila melanogaster

Studies on thermal acclimation in insects are often performed on animals acclimated in the laboratory under conditions that are not ecologically relevant. Costs and benefits of acclimation responses under such conditions may not reflect costs and benefits in natural populations subjected to daily and seasonal temperature fluctuations. Here we estimated costs and benefits in thermal tolerance limits in relation to winter acclimatization of Drosophila melanogaster. We sampled flies from a natural habitat during winter in Denmark (field flies) and compared heat and cold tolerance of these to that of flies collected from the same natural population, but acclimated to 25 °C or 13 °C in the laboratory (laboratory flies). We further obtained thermal performance curves for egg-to-adult viability of field and laboratory (25 °C) flies, to estimate possible cross-generational effects of acclimation. We found much higher cold tolerance and a lowered heat tolerance in field flies compared to laboratory flies reared at 25 °C. Flies reared in the laboratory at 13 °C exhibited the same thermal cost-benefit relations as the winter acclimatized flies. We also found a cost of winter acclimatization in terms of decreased egg-to-adult viability at high temperatures of eggs laid by winter acclimatized flies. Based on our findings we suggest that winter acclimatization in nature can induce strong benefits in terms of increased cold tolerance. These benefits can be reproduced in the laboratory under ecologically relevant rearing and testing conditions, and should be incorporated in species distribution modelling. Winter acclimatization also leads to decreased heat tolerance. This may create a mismatch between acclimation responses and the thermal environment, e.g. if temperatures suddenly increase during spring, under current and expected more variable future climatic conditions.     

A comparison of behavioural change in Drosophila during exposure to thermal stress

In order to understand how adaptive tolerance to stress has evolved, we compared related species and populations of Drosophila for a variety of fitness relevant traits while flies directly experienced the stress. Two main questions were addressed. First, how much variation exists in the frequency of both courtship and mating among D. melanogaster, D. simulans, and D. mojavensis when each are exposed to a range of temperatures? Second, how does variation in these same behaviours compare among four geographically isolated populations of D. mojavensis, a desert species with a well defined ecology? Our hierarchical study demonstrated that mating success under stress can vary as much between related species, such as D. melanogaster and D. simulans, as between the ecologically disparate pair, D. melanogaster and D. mojavensis. Strains of this latter desert species likewise varied in tolerance, with differences approaching the levels observed among species. The consequences of stress on male courtship differed markedly from those on female receptivity to courtship, as mating behaviours among species and among strains of D. mojavensis varied in subtle but significant ways. Finally, a comparison of variation in thermotolerance of F₁ hybrids between the two most extreme D. mojavensis populations confirmed that genetic variation underlying traits such as survival or the ability to fly after heat stress is completely different. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83 , 197–205.     

Genetics of esterases in Drosophila of the virilis group. I. Characteristics of esterase patterns in D. virilis,D. texana,D. littoralis,and their hybrids

Starch and polyacrylamide gel electrophoreses have detected six esterase fractions in Drosophila of the virilis group. These esterases have been characterized in detail using a series of substrates and inhibitors and also thermal treatment. Differences in esterase patterns have been found between D. virilis, D. texana, and D. litoralis as well as between D. virilis stocks. An interstock polymorphism for different esterase patterns has been established with respect to the electrophoretic mobilities of a number of esterase fractions. In rare instances, it has been observed within some D. virilis stocks, too. There is specificity in organ distribution of esterase fractions in Drosophila. Monogenic control of the electrophoretic mobilities of esterase-2 and esterase-4 has been demonstrated in D. virilis, and a dimer structure has been found in esterase-2. Genes controlling esterase-2 and esterase-4 are located on the second chromosome (209.3 for esterase-2 and 192.0 for esterase-4). In interstock and interspecific hybrids, esterases usually manifest codominance. In interstock hybrids, esterase-2 forms a hybrid band not observed in interspecific hybrids. In third instar larvae of interspecific hybrids, differential expression of certain esterase isozymes has been noted. These observations are in agreement with data from histochemical studies of organs of different hybrids.     

Variation in adult life history and stress resistance across five species of<Emphasis Type="Italic">Drosophila</Emphasis>

Dry weight at eclosion, adult lifespan, lifetime fecundity, lipid and carbohydrate content at eclosion, and starvation and desiccation resistance at eclosion were assayed on a long-term laboratory population ofDrosophila melanogaster, and one recently wild-caught population each of four other species ofDrosophila, two from themelanogaster and two from theimmigrans species group. The relationships among trait means across the five species did not conform to expectations based on correlations among these traits inferred from selection studies onD. melanogaster. In particular, the expected positive relationships between fecundity and size/lipid content, lipid content and starvation resistance, carbohydrate (glycogen) content and desiccation resistance, and the expected negative relationship between lifespan and fecundity were not observed. Most traits were strongly positively correlated between sexes across species, except for fractional lipid content and starvation resistance per microgram lipid. For most traits, there was evidence for significant sexual dimorphism but the degree of dimorphism did not vary across species except in the case of adult lifespan, starvation resistance per microgram lipid, and desiccation resistance per microgram carbohydrate. Overall,D. nasuta nasuta andD. sulfurigaster neonasuta (immigrans group) were heavier at eclosion than themelanogaster group species, and tended to have somewhat higher absolute lipid content and starvation resistance. Yet, these twoimmigrans group species were shorter-lived and had lower average daily fecundity than themelanogaster group species. The smallest species,D. malerkotliana (melanogaster group), had relatively high daily fecundity, intermediate lifespan and high fractional lipid content, especially in females.D. ananassae (melanogaster group) had the highest absolute and fractional carbohydrate content, but its desiccation resistance per microgram carbohydrate was the lowest among the five species. In terms of overall performance, the laboratory population ofD. melanogaster was clearly superior, under laboratory conditions, to the other four species if adult lifespan, lifetime fecundity, average daily fecundity, and absolute starvation and desiccation resistance are considered. This finding is contrary to several recent reports of substantially higher adult lifespan and stress resistance in recently wild-caught flies, relative to flies maintained for a long time in discretegeneration laboratory cultures. Possible explanations for these apparent anomalies are discussed in the context of the differing selection pressures likely to be experienced byDrosophila populations in laboratory versus wild environments. This paper is dedicated to the memory of our friend and former colleague Dr Hans Raj Negi, who tragically passed away at a very young age in a road accident in November 2003.