Divergence of reaction norms of size characters between tropical and temperate populations of Drosophila melanogaster and D. simulans

Reaction norms to growth temperature of two size-related traits, wing and thorax length, were compared in tropical (West Indies) and temperate (France) populations of the two sibling species, Drosophila melanogaster and D. simulans. A major body size difference was found in D. melanogaster, with much smaller Caribbean flies, while D. simulans exhibited little size variation between geographical populations. The concave norms of reaction were adjusted to second- or third-degree polynomials, and characteristic points calculated i.e. maximum value (MV) and temperature of maximum value (TMV). TMVs were confirmed to be higher for thorax than for wing length, higher in D. melanogaster than in D. simulans, and higher in females than in males. For both traits Caribbean populations exhibited higher TMVs in the two species, strongly suggesting an adaptive shift of the reaction norms toward higher temperature in warm-adapted populations. The wing/thorax ratio was also analysed, and found to be significantly lower in tropical populations of both species. This ratio, which is related to wing loading and flight capacity, might evolve independently of body weight itself.     

VARIATION IN THERMAL PERFORMANCE AND REACTION NORMS AMONG POPULATIONS OF DROSOPHILA MELANOGASTER

The major goal of evolutionary thermal biology is to understand how variation in temperature shapes phenotypic evolution. Comparing thermal reaction norms among populations from different thermal environments allows us to gain insights into the evolutionary mechanisms underlying thermal adaptation. Here, we have examined thermal adaptation in six wild populations of the fruit fly (Drosophila melanogaster) from markedly different natural environments by analyzing thermal reaction norms for fecundity, thorax length, wing area, and ovariole number under ecologically realistic fluctuating temperature regimes in the laboratory. Contrary to expectation, we found only minor differences in the thermal optima for fecundity among populations. Differentiation among populations was mainly due to differences in absolute (and partly also relative) thermal fecundity performance. Despite significant variation among populations in the absolute values of morphological traits, we observed only minor differentiation in their reaction norms. Overall, the thermal reaction norms for all traits examined were remarkably similar among different populations. Our results therefore suggest that thermal adaptation in D. melanogaster predominantly involves evolutionary changes in absolute trait values rather than in aspects of thermal reaction norms.     

REACTION NORMS OF MORPHOLOGICAL TRAITS IN DROSOPHILA: ADAPTIVE SHAPE CHANGES IN A STENOTHERM CIRCUMTROPICAL SPECIES?

Reaction norms of wing length, thorax length, and ovariole number were studied according to growth temperature in the circumtropical Drosophila ananassae, and compared to similar data from the cosmopolitan D. melanogaster. In the two species convex reaction norms were observed, but they were not parallel and sometimes exhibited intersections either at high (wing) or at low (thorax) temperature. On average, D. ananassae may be considered as a species with a bigger thorax but shorter wings than D. melanogaster. The shapes of reaction norms were analyzed and compared after quadratic polynomial adjustments. Significant differences were observed, in several cases between polynomial parameters, and in all cases between characteristic points that is, Maximum Value (MV) and Temperature of Maximum Value (TMV). The wing/thorax ratio may also be considered as a specific trait related to wing loading. Major differences were observed between the two species for the mean value and the shape of the response curves of this trait. The main observation of this work was however a shift of TMVs for wing and thorax length and ovariole number in D. ananassae toward higher temperatures. These variations in the reaction norms corresponded to a shift in the species thermal range, suggesting that temperature adaptation was accompanied by a modification of the shape of the response curves.     

Comparative Analysis of Morphological Traits Among Drosophila Melanogaster and D. Simulans: Genetic Variability, Clines and Phenotypic Plasticity

The two sibling cosmopolitan species, Drosophila melanogaster and D. simulans, are able to proliferate under very different climatic conditions. This has resulted in local adaptations, which are often arranged in latitudinal clines. Such clines are documented for body weight, wing and thorax length, sternopleural and abdominal bristle number, ovariole number and thoracic pigmentation. The overall magnitude of geographical differentiation is, however, much less in D. simulans than in D. melanogaster, and latitudinal clines are less pronounced. The fact that natural populations live under different climates raises the problem of interaction between temperature and phenotype. The reaction norms of morphometrical traits have been investigated as a function of growth temperature. The shapes of the response curves vary according to the investigated trait. They are generally curvilinear and can be described by calculating characteristic values after polynomial adjustments. For a given trait, the reaction norms of the two species are similar in their shape, although some significant differences may be observed. Within each species, significant differences are also observed between geographic populations: reaction norms are not parallel and the divergence is better marked when more distant populations (e.g., temperate and tropical) are compared. It thus appears that besides mean trait value, phenotypic plasticity is also a target of natural selection. A specific analysis of wing shape variation according to growth temperature was also undertaken. Reaction norms with different shapes may be observed in various parts of the wing: the major effect is found between the basis and the tip of the wing, but in a similar way in the two species. By contrast, some ratios, called wing indices by taxonomists, may exhibit completely different reaction norms in the two species. For a single developmental temperature (25 degrees C) the phenotypic variability of morphometrical traits is generally similar in the two species, and also the genetic variability, estimated by the intraclass correlation. A difference exists, however, for the ovariole number which is less variable in D. simulans. Variance parameters may vary according to growth temperature, and a detailed analysis was made on wing dimensions. An increase of environmental variability at extreme, heat or cold temperatures, has been found in both species. Opposite trends were, however, observed for the genetic variability: a maximum heritability in D. simulans at middle temperatures, corresponding to a minimum heritability in D. melanogaster. Whether such a difference exists for other traits and in other populations deserves further investigations. In conclusion, morphometrical analyses reveal a large amount of significant differences which may be related to speciation and to the divergence of ecological niches. Within each species, numerous geographic variations are also observed which, in most cases, reflect some kinds of climatic adaptation.     

Growth temperature and phenotypic plasticity in two Drosophila sibling species: probable adaptive changes in flight capacities

In the sibling species Drosophila melanogaster and D. simulans, growth and development at constant temperatures, from 12 to 30 °C, resulted in extensive variations of adult size and flight parameters with significant differences between species. Changes in body weight, thorax length and wing length were nonlinear, with maximum values of each trait at lower temperatures for D. simulans than for its sibling species. By contrast, the wing/thorax ratio and the wing loading varied monotonically with growth temperature. These traits were negatively correlated, the wing/thorax ratio decreasing with growth temperature while the wing loading increased. Wing/thorax ratio, which is easier to measure, thus appears as a convenient predictor of wing loading. During tethered flight at the same ambient temperature, the wingbeat frequency changed linearly as a function of the wing moment of inertia. More interestingly, the beat rate was strongly correlated with the increase of wing loading at growth temperature above 13 °C. The likely adaptive significance of these morphometrical changes for flight efficiency is discussed.     

Morphometrical evolution in a Drosophila clade: the Drosophila obscura group

Five morphometrical traits (wing and thorax length, ovariole number, and thoracic and female abdomen pigmentation) were investigated in laboratory stocks of 20 species belonging to the Drosophila obscura group (subgenus Sophophora). These species originated from four biogeographical regions and represent all five of the presently recognized, taxonomic subgroups. Size‐related traits (wing and thorax length) were highly variable across species, and interspecific variation explained more than 90% of total variability. In both traditional and phylogenetic analyses, wing size was positively correlated with latitude of origin. These interspecific correlations were however notably weaker than those for intraspecific correlations. Wing/thorax ratio, which may be related to flight capacity, showed little variation. Ovariole number was highly variable (range 27–53) both within and between species, and was positively correlated with the wing/thorax ratio, suggesting that species with relatively large ovaries have relatively low wing loading. Although many species are completely dark, 11 had some regions of light coloration. A light thorax with a median darkening was observed in six species. A variable pigmentation of abdominal tergites, in females only, was found in nine species, belonging to three subgroups only. With respect to both molecular phylogeny and morphometrical evolution, the D. obscura subgroup is probably now the best investigated clade in Drosophila.     

Phenotypic plasticity of body size in a temperate population ofDrosophila melanogaster: When the temperature—size rule does not apply

A natural population ofDrosophila melanogaster in southern France was sampled in three different years and 10 isofemale lines were investigated from each sample. Two size-related traits, wing and thorax length, were measured and the wing/thorax ratio was also calculated. Phenotypic plasticity was analysed after development at seven different constant temperatures, ranging from 12‡C to 31‡C. The three year samples exhibited similar reaction norms, suggesting a stable genetic architecture in the natural population. The whole sample (30 lines) was used to determine precisely the shape of each reaction norm, using a derivative analysis. The practical conclusion was that polynomial adjustments could be used in all cases, but with different degrees: linear for the wing/thorax ratio, quadratic for thorax length, and cubic for wing length. Both wing and thorax length exhibited concave reaction norms, with a maximum within the viable thermal range. The temperatures of the maxima were, however, quite different, around 15‡C for the wing and 19.5‡C for the thorax. Assuming that thorax length is a better estimate of body size, it is not possible to state that increasing the temperature results in monotonically decreasing size (the temperature-size rule), although this is often seen to be the case for genetic variations in latitudinal clines. The variability of the traits was investigated at two levels—within and between lines—and expressed as a coefficient of variation. The within-line (environmental) variability revealed a regular, quadratic convex reaction norm for the three traits, with a minimum around 21‡C. This temperature of minimum variability may be considered as a physiological optimum, while extreme temperatures are stressful. The between-line (genetic) variability could also be adjusted to quadratic polynomials, but the curvature parameters were not significant. Our results show that the mean values of the traits and their variance are both plastic, but react in different ways along a temperature gradient. Extreme low or high temperatures decrease the size but increase the variability. These effects may be considered as a functional response to environmental stress.     

Geographical Clines for Quantitative Traits in Natural Populations of a Tropical Drosophilid: Zaprionus Indianus

We analyzed natural populations of Zaprionus indianusin 10 Indian localities along a south-north transect (latitude: 10–31°3 N). Size traits (body weight, wing length and thorax length) as well as a reproductive trait (ovariole number) followed a pattern of clinal variation, that is, trait value increased with latitude. Wing/thorax ratio, which is inversely related to wing loading, also had a positive, but non-significant correlation with latitude. By contrast, bristle numbers (sternopleural and abdominal) exhibited a non-significant but negative correlation with latitude. Sex dimorphism, estimated as the female/male ratio, was very low in Z. indianus, contrasting with results already published in other species. Genetic variations among populations were also analyzed according to other geographic parameters (altitude and longitude) and to climatic conditions from each locality. A significant effect of altitude was found for size traits. For abdominal bristles, a multiple regression technique evidenced a significant effect of both latitude and altitude, but in opposite directions. Genetic variations were also correlated to climate, and mainly with average year temperature. Taking seasonal variations into account failed however to improve the predictability of morphometrical variations. The geographic differentiation of Z.indianusfor quantitative traits suggests adaptive response to local conditions, especially to temperature, but also reveals a complex situation according to traits investigated and to environmental parameters, which does not match results on other drosophilid species.     

Phenotypic variability of quantitative traits in Indian populations of Drosophila kikkawai

Drosophila kikkawai, which has colonized the Indian subcontinent in the recent past, exhibits geographical variations for five quantitative traits among eight Indian populations (8.29–32.7°N). Body weight, wing length, thorax length, abdominal bristles and ovariole number exhibit significant clinal variation with increase in latitude, while sternopleural bristles do not demonstrate such a trend. For the female sex, the slope values for body weight (2.25) and wing length (2.40) are higher but they are lower for thorax length (0.64) and ovariole number (0.51 per degree latitude). There is significant sexual dimorphism for the slope values only for body weight and thorax length suggesting simultaneous action of latitudinal selection pressure on these traits. However, the two sexes do not differ statistically in the latitudinal slope values for the wing length. A regression analysis of different traits on body weight implies correlated selection response on wing length and wing/thorax ratio while thorax length corresponds to changes in body size and does not differ in the two sexes. Regression analysis, on the basis of temperature-related climatic variables, evidence significantly higher association between all the five size-related traits and coefficient of variation of mean annual temperature (seasonal thermal amplitude; T _cv), T _min and relative humidity. Thus, genetic differentiation for quantitative traits in D. kikkawai are due to selective pressure from variable climatic conditions occurring on the Indian subcontinent.     

Microspatial structure of Drosophila melanogaster populations in Brazzaville: evidence of natural selection acting on morphometrical traits

Two genetically distinct habitat races of Drosophila melanogaster coexist in Brazzaville (Congo). One is the typical field type of Afrotropical populations, the other mainly breeds in beer residues in breweries. These two populations differ in their ethanol tolerance, in their allelic frequencies at several enzyme and microsatellite loci and in the composition of their cuticular hydrocarbons. The brewery population is quite similar to European temperate populations with regard to all these traits. Previous investigations of two morphological traits (ovariole number and sternopleural bristle number) failed to detect any difference between the two habitat races. Here we investigated other morphological traits (wing and thorax length, thorax pigmentation and female abdomen pigmentation). The reaction norms of these traits according to growth temperature were compared in the two Afrotropical habitat races and in a French temperate population. As expected, the French population was very different from the field African population: as a general rule, the brewery population (Kronenbourg) was intermediate in several aspects between the other two. We conclude that the strong selective forces that maintain the genetic divergence between the two habitat races also act on morphometrical traits, and the possible selective mechanisms are discussed.     

Genetic variability of sexual size dimorphism in a natural population of<Emphasis Type="Italic">Drosophila melanogaster</Emphasis>: An isofemale-line approach

Most animal species exhibit sexual size dimorphism (SSD). SSD is a trait difficult to quantify for genetical purposes since it must be simultaneously measured on two kinds of individuals, and it is generally expressed either as a difference or as a ratio between sexes. Here we ask two related questions: What is the best way to describe SSD, and is it possible to conveniently demonstrate its genetic variability in a natural population? We show that a simple experimental design, the isofemale-line technique (full-sib families), may provide an estimate of genetic variability, using the coefficient of intraclass correlation. We consider two SSD indices, the female-male difference and the female/male ratio. For two size-related traits, wing and thorax length, we found that both SSD indices were normally distributed. Within each family, the variability of SSD was estimated by considering individual values in one sex (the female) with respect to the mean value in the other sex (the male). In a homogeneous sample of 30 lines ofDrosophila melanogaster, both indices provided similar intraclass correlations, on average 0.21, significantly greater than zero but lower than those for the traits themselves: 0.50 and 0.36 for wing and thorax length respectively. Wing and thorax length were strongly positively correlated within each sex. SSD indices of wing and thorax length were also positively correlated, but to a lesser degree than for the traits themselves. For comparative evolutionary studies, the ratio between sexes seems a better index of SSD since it avoids scaling effects among populations or species, permits comparisons between different traits, and has an unambiguous biological significance. In the case ofD. melanogaster grown at 25?C, the average female/male ratios are very similar for the wing (1.16) and the thorax (1.15), and indicate that, on average, these size traits are 15–16% longer in females.     

Variation of life-history and morphometrical traits in Drosophila buzzatii and Drosophila simulans collected along an altitudinal gradient from a Canary island

Variation in three life‐history traits (developmental time, preadult viability and daily female productivity) and five morphometrical traits (thorax length, wing length, wing width, wing/thorax ratio and wing‐aspect ratio) was studied at three developmental temperatures (20, 25 and 30 °C) in Drosophila buzzatii and Drosophila simulans collected on the island of La Gomera (Canary Archipelago). The flies originated from five closely situated localities, representing different altitudes (from 20 to 886 m above sea level) and a range of climatic conditions. We found statistically significant population effects for all traits in D. buzzatii and for most of the traits in D. simulans. Although no correlations of trait values with altitude were detected, geographical patterns for three life‐history traits and body size in D. buzzatii indicated that short‐range geographical variation in this species could be maintained by local climatic selection. Five of eight traits showed population‐by‐temperature interactions either in D. buzzatii or in D. simulans, but in all cases except wing width in D. buzzatii this could not be interpreted as adaptive responses to thermal conditions in the localities. The range of plastic changes across temperatures for particular traits differed between species, indicating a possibility for different levels of environmental stress experienced by the natural populations. The reaction norm curves and the response of within‐population variability to thermal treatments suggested better adaptations to higher and lower temperatures for D. buzzatii and D. simulans, respectively. The levels of among‐population differentiation depended on developmental temperature, implying environmental effects on the expression of the genetic variance. At 20 and 25 °C, interpopulation variability in D. buzzatii was higher than in D. simulans, while at 30 °C the opposite trend was observed. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84 , 119–136.     

Reaction norms across and genetic parameters at different temperatures for thorax and wing size traits in Drosophila aldrichi and D. buzzatii

Reaction norms across seven constant and one fluctuating temperature of development were measured for thorax length and several wing size traits for up to 10 isofemale lines of each of the cactophilic Drosophila species, D. aldrichi and D. buzzatii, originating from the same locality. Maximum thorax length was reached at different low to intermediate temperatures for the two species, whereas wing length was highest at the lowest temperature in both species. Various ratio parameters showed pronounced species differences. The reaction norm for the wing loading index (wing length/thorax length) decreased monotonically with temperature in both species, but was much steeper and spanned a wider range in D. aldrichi than in D. buzzatii, suggesting either that wing loading is not a good characterization of flight capacity or, more likely, that flight optimization does not occur in the same manner in both species. The vein ratio (distal length/proximal length of the third vein) increased with temperature in D. buzzatii but decreased in D. aldrichi. Wing development in the two species thus is very different, with the proximal part of the wing in D. buzzatii more closely allied to the thorax than to the distal part. Among line variation was significant for all traits in both species, and most pronounced for thorax length and the ratio parameters. Coefficients of variation were significantly different between the species for all traits, with those in D. aldrichi higher than in D. buzzatii. Genetic variance in plasticity was significant for all traits in D. buzzatii, but only for seven out of 12 in D. aldrichi. Additive genetic variances for all traits in both species were significantly larger than zero. Genetic correlations between thorax length and several wing length parameters, and between these and wing area, were positive and generally significant in both species. The genetic correlation between the distal and the proximal length of the third vein was not significantly different from zero in D. aldrichi, but negative and significant in D. buzzatii. Heritabilites varied significantly among temperatures for almost all traits in both species. Phenotypic variances were generally higher in D. aldrichi than in D. buzzatii, and commonly highest at the extreme temperatures in the former species. At the high temperature the genetic variances also were usually highest in D. aldrichi. The data clearly suggest that the process of thermal adaptation is species specific and caution against generalizations based on the study of single species.     

Evolutionary genetics of Drosophila ananassae III. Effect of temperature on certain fitness traits in two natural populations

Ten isofemale lines of two natural populations of Drosophila ananassae were compared at four different temperatures for body size, W/T ratio, ovariole number and different life history traits. Three-factor nested ANOVA for thorax length and W/T ratio shows significant differences for temperature and sex, while for wing length it shows significant sex difference only. Two-factor nested ANOVA for ovariole number shows a significant difference between populations only, and not for different temperatures. Tests of correlations among different life history traits show novel trade-offs between LDT and pupal period and between pupal period and egg–pupa DT at different temperatures.     

Thermal phenotypic plasticity of body size in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>: sexual dimorphism and genetic correlations

Thirty isofemale lines collected in three different years from the same wild French population were grown at seven different temperatures (12–31°C). Two linear measures, wing and thorax length, were taken on 10 females and 10 males of each line at each temperature, also enabling the calculation of the wing/thorax (W/T) ratio, a shape index related to wing loading. Genetic correlations were calculated using family means. The W–T correlation was independent of temperature and on average, 0.75. For each line, characteristic values of the temperature reaction norm were calculated, i.e. maximum value, temperature of maximum value and curvature. Significant negative correlations were found between curvature and maximum value or temperature of maximum value. Sexual dimorphism was analysed by considering either the correlation between sexes or the female/male ratio. Female–male correlation was on average 0.75 at the within line, within temperature level but increased up to 0.90 when all temperatures were averaged for each line. The female/male ratio was genetically variable among lines but without any temperature effect. For the female/male ratio, heritability (intraclass correlation) was about 0.20 and evolvability (genetic coefficient of variation) close to 1. Although significant, these values are much less than for the traits themselves. Phenotypic plasticity of sexual dimorphism revealed very similar reaction norms for wing and thorax length, i.e. a monotonically increasing sigmoid curve from about 1.11 up to 1.17. This shows that the males are more sensitive to a thermal increase than females. In contrast, the W/T ratio was almost identical in both sexes, with only a very slight temperature effect.     

Quantitative morphometrical analysis of a North African population of <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>: sexual dimorphism,and comparison with European populations

Genetic variability of quantitative traits was investigated in aMoroccan population of Drosophila melanogaster, with an isofemale line design. Results were compared with data previously obtained from French populations. Although the environmental and thermal conditions are very different in France and Morocco, only two significant differences were observed: a shorter wing and a lighter abdomen pigmentation in Morocco. It is, therefore, concluded that Moroccan D. melanogaster are quite typical temperate flies, belonging to the Palaearctic region, and very different from the ancestral Afrotropical populations. Almost all traits were genetically variable, as shown by significant intraclass correlations among lines. Genetic correlations were highly significant among three size-related traits, while much lower between size and bristle numbers. Fluctuating asymmetry was greater for abdominal bristles than for sternopleural bristles. Sex dimorphism, analysed as a female/male ratio, was identical in French and Moroccan populations. Examination of the thorax length/thorax width ratio showed that the thorax is more elongated in females. Sexual dimorphism of wing length was significantly more correlated to thorax width than to thorax length. The results illustrate the value of measuring numerous quantitative traits on the same flies for characterizing the genetic architecture of a natural population. In several cases, and especially for genetic correlations, some interesting suggestions could be made, which should be confirmed, or invalidated, by more extensive investigations.     

Genetic Architecture of Two Fitness-related Traits in Drosophila melanogaster: Ovariole Number and Thorax Length

In Drosophila melanogaster, ovariole number and thorax length are morphological characters thought to be associated with fitness. Maximum daily egg production in females is positively correlated with ovariole number, while thorax length is correlated with male reproductive success and female fecundity. Though both traits are related to fitness, ovariole number is likely to be under stabilizing selection, while thorax length appears to be under directional selection. Current research has focused on examining the sources of variation for ovariole number in relation to fitness, with a view towards elucidating how segregating variation is maintained in natural populations. Here, we utilize a diallel design to explore the genetic architecture of ovariole number and thorax length in nine isogenic lines derived from a natural population. The full diallel design allows the estimation of general combining ability (GCA), specific combining ability (SCA), and also describes variation due to reciprocal effects (RGCA and RSCA). Ovariole number and thorax length differed with respect to their genetic architecture, reflective of the independent selective forces acting on the traits. For ovariole number, GCA accounted for the majority (67.3%) of variation segregating between the lines, with no evidence of reciprocal effects or inbreeding depression; SCA accounted for a small percentage (3.9%) of the variance, suggesting dominance variation; no reciprocal effects were observed. In contrast, for thorax length, the majority of the non-error variance was accounted for by SCA (17.9%), with only one third as much variance (6.2%) due to GCA. Interestingly, RSCA (nuclear–extranuclear interactions) accounted for slightly more variation (7.5%) than GCA in these data. Thus, genetic variation for thorax length is largely in accord with predictions for a fitness trait under directional selection: little additive genetic variation and substantial dominance variation (including a suggestion of inbreeding depression); while the mechanisms underlying the maintenance of variation for ovariole number are more complex.     

PLASTICITY VERSUS ENVIRONMENTAL CANALIZATION: POPULATION DIFFERENCES IN THERMAL RESPONSES ALONG A LATITUDINAL GRADIENT IN DROSOPHILA SERRATA

The phenotypic plasticity of traits, defined as the ability of a genotype to express different phenotypic values of the trait across a range of environments, can vary between habitats depending on levels of temporal and spatial heterogeneity. Other traits can be insensitive to environmental perturbations and show environmental canalization. We tested levels of phenotypic plasticity in diverse Drosophila serrata populations along a latitudinal cline ranging from a temperate, variable climate to a tropical, stable climate by measuring developmental rate and size-related traits at three temperatures (16°C, 22°C, and 28°C). We then compared the slopes of the thermal reaction norms among populations. The 16–22°C part of the reaction norms for developmental rate was flatter (more canalized) for the temperate populations than for the tropical populations. However, slopes for the reaction norms of the two morphological traits (wing size, wing:thorax ratio), were steeper (more plastic) in the temperate versus the tropical populations over the entire thermal range. The different latitudinal patterns in plasticity for developmental rate and the morphological traits may reflect contrasting selection pressures along the tropical–temperate thermal gradient.     

Effects of body-size variation on flight-related traits in latitudinal populations of Drosophila melanogaster

In the present study, we tested the hypothesis whether flight-related traits such as wing area, flight-muscle ratio, wing loading and dispersal yield evidence of geographical variation in nine wild-collected as well as laboratory-reared (at 21°C) latitudinal populations of Drosophila melanogaster from the Indian subcontinent. We observed positive clinal variation in the wing–thorax ratio, wing aspect ratio and wing area, along a latitudinal gradient for both the sexes. In contrast, geographical changes in three parameters of flight ability, i.e. flight-muscle ratio, wing loading and dispersal, showed negative correlation withlatitude. On the basis of isofemale line variability, we observed positive correlation of wing loading with flight-muscle ratio as well as dispersal behaviour in both the sexes. We also found positive correlation between duration of development and wing area. Interestingly, southern populations of D. melanogaster from warm and humid habitats exhibited higher flight-muscle ratio as well as the higher wing loading than northern populations which occur in cooler and drier climatic conditions. Laboratory tests for dispersal-related walking behaviour showed significantly higher values for southern populations compared with northern populations of D. melanogaster. Multiple regression analysis of geographical changes in flight-muscle ratio, wing loading as well as walking behaviour as a function of average temperature and relative humidity of the origin of populations in wild-collected flies have suggested adaptive changes in flight-related traits in response to steeper gradients of climatic factors in the Indian subcontinent. Finally, adaptive latitudinal variations in flight-related traits in D. melanogaster are consistent with results of other studies from different continents despite differences due to specific climatic conditions in the Indian subontinent.     

Genetic variation and plasticity of thorax length and wing length in Drosophila aldrichi and D. buzzatii

Reaction norms across three temperatures of development were measured for thorax length, wing length and wing length/thorax length ratio for ten isofemale lines from each of two populations of Drosophila aldrichi and D. buzzatii. Means for thorax and wing length in both species were larger at 24 °C than at either 18 °C or 31 °C, with the reduction in size at 18 °C most likely due to a nutritional constraint. Although females were larger than males, the sexes were not different for wing length/thorax length ratio. The plasticity of the traits differed between species and between populations of each species, with genetic variation in plasticity similar for the two species from one locality, but much higher for D. aldrichi from the other. Estimates of heritabilities for D. aldrichi generally were higher at 18 °C and 24 °C than at 31 °C, but for D. buzzatii they were highest at 31 °C, although heritabilities were not significantly different between species at any temperature. Additive genetic variances for D. aldrichi showed trends similar to that for heritability, being highest at 18 °C and decreasing as temperature increased. For D. buzzatii, however, additive genetic variances were lowest at 24 °C. These results are suggestive that genetic variation for body size characters is increased in more stressful environments. Thorax and wing lengths showed significant genetic correlations that were not different between the species, but the genetic correlations between each of these traits and their ratio were significantly different. For D. aldrichi, genetic variation in the wing length/thorax length ratio was due primarily to variation in thorax length, while for D. buzzatii, it was due primarily to variation in wing length. The wing length/thorax length ratio, which is the inverse of wing loading, decreased linearly as temperature increased, and it is suggested that this ratio may be of greater adaptive significance than either of its components.