STUDIES ON NATURAL POPULATIONS OF DROSOPHILA. I. HEAT RESISTANCE AND GEOGRAPHICAL VARIATION IN DROSOPHILA MELANOGASTER AND D. SIMULANS

CHANGES IN THE HERITABILITY OF FIVE MORPHOLOGICAL TRAITS UNDER COMBINED ENVIRONMENTAL STRESSES IN DROSOPHILA MELANOGASTER

Heritabilities and evolvabilities for morphological traits were compared between two environments in Drosophila melanogaster using parent-offspring comparisons. One of the environments was favorable. The other stressful environment involved a combination of repeated cold shocks, poor nutrition, and ethanol added to the medium, which markedly decreased viability. For wing traits, heritabilities were relatively lower in the stressful environment, while heritabilities for bristle traits were not influenced by conditions. Heritability changes were largely due to an increase in the environmental variance under stress, whereas levels of additive genetic variance were relatively constant. Evolvabilities were similar between environments except for crossvein length.

Variations in morphological and life-history traits under extreme temperatures in Drosophila ananassae

Using half-sib analysis, we analysed the consequences of extreme rearing temperatures on genetic and phenotypic variations in the morphological and life-history traits of Drosophila ananassae. Paternal half-sib covariance contains a relatively small proportion of the epistatic variance and lacks the dominance variance and variance due to maternal effect, which provides more reliable estimates of additive genetic variance. Experiments were performed on a mass culture population of D. ananassae collected from Kanniyakumari (India). Two extremely stressful temperatures (18 degree C and 32 degree C) and one standard temperature (25 degree C) were used to examine the effect of stressful and non-stressful environments on the morphological and life-history traits in males and females. Mean values of various morphological traits differed signifi cantly among different temperature regimens in both males and females. Rearing at 18 degree C and 32 degree C resulted in decreased thorax length, wing-to-thorax (w/t) ratio, sternopleural bristle number, ovariole number, sex comb-tooth number and testis length. Phenotypic variances increased under stressful temperatures in comparison with non-stressful temperatures. Heritability and evolvability based on among-sires (males), among-dams (females), and the sum of the two components (sire + dam) showed higher values at both the stressful temperatures than at the non-stressful temperature. These differences reflect changes in additive genetic variance. Viability was greater at the high than the low extreme temperature. As viability is an indicator of stress, we can assume that stress was greater at 18 degree C than at 32 degree C in D. ananassae. The genetic variations for all the quantitative and life-history traits were higher at low temperature. Variation in sexual traits was more pronounced as compared with other morphometric traits, which shows that sexual traits are more prone to thermal stress. Our results agree with the hypothesis that genetic variation is increased in stressful environments.

Quantitative morphometrical analysis of a North African population of Drosophila melanogaster: sexual dimorphism, and comparison with European populations

Genetic variability of quantitative traits was investigated in a Moroccan 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.

Investigating latitudinal clines for life history and stress resistance traits in Drosophila simulans from eastern Australia

Latitudinal clines have been demonstrated for many quantitative traits in Drosophila and are assumed to be due to climatic selection. However, clinal studies are often performed in species of Drosophila that contain common cosmopolitan inversion polymorphisms that also show clinal patterns. These inversion polymorphisms may be responsible for much of the observed clinal variation. Here, we consider latitudinal clines for quantitative traits in Drosophila simulans from eastern Australia. Drosophila simulans does not contain cosmopolitan inversion polymorphisms, so allows the study of clinal selection on quantitative traits that are not confounded by associations with inversions. Body size showed a strong linear cline for both females and males. Starvation resistance exhibited a weak linear cline in females, whereas chill-coma recovery exhibited a significant nonlinear cline in females only. No clinal pattern was evident for development time, male chill-coma recovery, desiccation or heat resistance. We discuss these results with reference to the role inversion polymorphisms play in generating clines in quantitative traits of Drosophila.

Associations between environmental stress, selection history, and quantitative genetic variation in Drosophila melanogaster

Stressful environments may increase quantitative genetic variation in populations by promoting the expression of genetic variation that has not previously been eliminated or canalized by natural selection. This "selection history" hypothesis predicts that novel stressors will increase quantitative genetic variation, and that the magnitude of this effect will decrease following continued stress exposure. We tested these predictions using Drosophila melanogaster and sternopleural bristle number as a model system. In particular, we examined the effect of high temperature stress (31 degrees Celsius) on quantitative genetic variation before and after our study population had been reared at 31 degrees Celsius for 15 generations. High temperature stress was found to increase both additive genetic variance and heritability, but contrary to the selection history hypothesis prediction, the magnitude of this effect significantly increased after the study population had been reared for 15 generations under high temperature stress. These results demonstrate that high temperature stress increases quantitative genetic variation for bristle number, but do not support the selection history hypothesis as an explanation for this effect.

Effect of stressful and nonstressful growth temperatures on variation of sternopleural bristle number in Drosophila melanogaster

The effect of stressful (31 degrees C) and nonstressful (25 degrees C) growth temperatures on quantitative variation and developmental stability (fluctuating asymmetry) of Drosophila melanogaster was examined in a short-term selection experiment on sternopleural bristle number. Realized heritabilities based on 10 generations of selection in an upward direction did not differ between the two temperature regimes, which indicated that additive genetic variation was not affected by a high, stressful temperature. Phenotypic variability and fluctuating asymmetry of sternopleural bristles were significantly higher under stressful conditions when averaged over generations, although most pairwise comparisons in separate generations showed nonsignificant differences between temperatures.

Cellular basis of wing size variation in Drosophila melanogaster: a comparison of latitudinal clines on two continents

We investigated the cellular basis of two extensive, continuous, latitudinal, genetic, body size clines of Drosophila melanogaster by measuring wing area and cell size in the wing blade of adult flies reared under standard, laboratory conditions. We report that the contribution of cell size to an Australian cline is much smaller than that to a South American cline. The data suggest that neither cell size nor cell number were the targets of selection, but rather wing area itself, or a trait closely related to it. We hypothesize that the differences between the continents were caused by differences in the initial pattern of genetic variation for the cell traits and/or by the direction of selection on the source populations of the clines. Despite large differences between continents in the cellular basis of the latitudinal variation, multiple regression analysis, using the individual variation within populations, showed that the relationship between cell size and cell number was changed with latitude in the same way in the two clines. The relative contribution of cell number to wing area variation increased with latitude, probably because of compensatory interactions with cell size as a consequence of the latitudinal increase in cell number. Our findings are discussed in relation to the cellular basis of evolutionary change in laboratory thermal selection lines and natural populations along latitudinal clines.

Stress temperatures and quantitative variation in Drosophila melanogaster

Using an isofemale line analysis, we analysed the consequences of extreme rearing temperatures for genetic variation in quantitative characters in Drosophila melanogaster. Three types of characters were used: life history (viability and developmental time), body size (thorax length and wing length) and meristic (number of sternopleural chaetae and number of arista branches). Phenotypic variation significantly increased under stress conditions in all morphological characters studied; for viability, it increased at the low stress temperature. Genetic variation, measured by the coefficient of intraclass correlation, was generally higher at both low and high stress temperatures for thorax length and sternopleural chaeta number. For wing length and viability, genetic variation was higher at the low extreme temperature. No consistent trend was found for genetic variation in arista branch number and developmental time. Our results agree with the hypothesis that genetic variation is increased in stressful environments. A possible mechanism underlying this phenomenon is briefly discussed.

The phenotypic plasticity of wing size in Drosophila melanogaster: the cellular basis of its genetic variation

The reaction norms in Drosophila melanogaster of thorax length, wing length and cell size were determined for 28 isofemale lines from three populations to investigate the role of cell size in determining the response of body size to temperature during the preimaginal stages. Both overall level and plasticity of the reaction norms of thorax length and wing length are highly correlated, leading to a relatively constant wing-thorax ratio between lines. Genetic differences in overall level of wing size reaction norms are mainly caused by differences in cell number. The response of wing size to temperature consists of changes in cell size and, to a lesser extent, cell number. The cellular basis of genetic differences in plasticity shows a transition point at an intermediate level. In steeper reaction norms, genetic differences in plasticity result from differences in the plasticity of cell size, whereas less steep reaction norms only differ in the plasticity of cell number. A significant partial correlation between wing length plasticity and cell size plasticity, correcting for thorax length plasticity, indicates a role of cell size in determining the wing-thorax ratio.

Fitness patterns and phenotypic plasticity in a spatially heterogeneous environment

We analyse patterns of the means and variances of genotypic fitnesses across different niches in a randomly mating haploid population. The population inhabits a spatially heterogeneous environment where it is subject to mutation and weak multilocus additive selection, with different election coefficients in different niches. Approximate analytical expressions are derived for the stationary mean and variance of genotypic fitnesses among the niches in terms of environmental and genetic parameters. As a special case, we analyse an environment described by a variable t, distributed among the niches with mean t(star) and variance D(star) and quadratic decrease in correlation between environments as a function of the difference in values of t. If the niches have the same qualities, the mean and variance of genotypic fitnesses evolve to be quadratic functions of t that achieve their maximum and minimum, respectively, at t(star). With unequal niche qualities, these are non-polynomial functions that attain their extrema at different, usually intermediate values of t, although the coefficient of variation of the genotypic fitnesses still attains its minimum near t(star). The functions involve the total mutation rate, the combination of the loci to genotypic fitnesses, and the frequency and quality distributions of the niches. Thus, for this relatively simple model the norms of reaction may be calculated in terms of the detailed properties of the environmental heterogeneity, and the genetic system.

Genetics and the environment in interspecific competition: a study using the sibling species Drosophila melanogaster and Drosophila simulans

The outcome of interspecific competition of two closely related species may depend upon genetic variation in the two species and the environment in which the experiment is carried out. Interspecific competition in the two sibling species, Drosophila melanogaster and D. simulans, is usually investigated using longterm laboratory stocks that often have mutant markers that distinguish them. To examine competition in flies that genetically more closely resemble flies in nature, we utilized freshly caught wildtype isofemale lines of the two species collected at the same site in San Carlos, Mexico. Under ordinary laboratory conditions, D. melanogaster always won in competition. However, in hotter and drier conditions, D. simulans competed much more effectively. In these environmental conditions, there were genetic differences in competitive ability among lines with the outcome of competition primarily dependent upon the line of D. melanogaster used but in some cases also influenced by the line of D. simulans used. Differences in the measures of productivity and developmental time did not explain the differences in competitive ability among lines. This suggests that the outcome of competition was not due to differences in major fitness components among the isofemale lines but to some other attribute(s) that influenced competitive ability. When lines of flies were combined, the outcome of competition was generally consistent with competitive outcomes between pairs of lines. In several cases, the combination of lines performed better than the best of the constituent lines, suggesting that competitive ability was combined heterotically and that the total amount of genetic variation was important in the outcome of interspecific competition.

Genetic basis of some morphological differences between temperate and equatorial populations of Drosophila melanogaster

The genetic basis of three morphological traits (ovariole number, sternopleural bristle number and wing length) of Drosophila melanogaster has been investigated in natural populations that show great differences in these traits, i.e. Bordeaux (France) and Loua (Congo). F1 and F2 crosses, and chromosome substitutions between these two populations, were analysed. Maternal and/or X chromosome effects were found for sternopleural bristle number and wing length. For all traits, significant effects from each of the three chromosomes were found, but in general only one or two chromosomes had a major effect. Moreover, in all cases significant interactions between chromosomes were observed, suggesting the existence of epistatic effects. Our results are discussed and compared to those obtained from the analysis of selected laboratory strains.

Variation in wing length in Eurasian natural populations of Drosophila melanogaster

A study of 16 natural populations of Drosophila melanogaster from Eastern Europe, the Caucasus and Central Asia has revealed a cline in wing length associated with geographical position of the populations. Wing length was shown to be positively correlated with temperature. The coefficient of variation in wing length was significantly different in town and orchard populations. The existence of a cline in wing length in the northern part of the species range and in the region where migration must be substantial suggests strong selection pressure acting in natural populations of D. melanogaster.

Quantitative genetic analysis of the body size and shape of Drosophila buzzatii

Body size in Drosophila is known to be closely related to a number of traits with important life history consequences, such as fecundity, dispersal ability and mating success. We examine the quantitative genetic basis of body size in three populations of the cactophilic species Drosophila buzzatii, which inhabit climatically different areas of Australia. Flies were reared individually to eliminate any common environmental component in a full-sib design with families split between two temperatures (18° and 25 °C). The means of several size measures differ significantly among populations while the genetic correlations among these traits generally do not differ, either among populations from different natural environments or between the different laboratory temperatures. This stability of correlation structure is necessary if laboratory estimates of genetic correlations are to have any connection with the expression of genetic variation in the field. The amount of variance due to genotype-by-environment interactions (family x temperature of development) varied among populations, apparently in parallel with the magnitudes of seasonal and diurnal variation in temperature experienced by the different populations. A coastal population, inhabiting a relatively thermally benign environment, showed no interaction, while two inland populations, inhabiting thermally more extreme areas, showed interaction. This interaction term is a measure of the amount of genetic variation in the degree of phenotypic plasticity of body size in response to temperature of development. Thus the inland flies vary in their ability to attain a given body size at a particular temperature while the coastal flies do not. This phenotypic plasticity is shown to be due primarily to differences among genotypes in the amount of response to the change in temperature. A possible selective basis for the maintenance of genetic variation for the levels of phenotypic plasticity is proposed.

Heterosis in crosses between lines of Drosophila melanogaster selected for adaptation to different environments

Experiments were designed to examine whether heterosis would occur in crosses of Drosophila melanogaster populations adapted to 18 °C or 28 °C environments. Crosses were examined in parental environments, an intermediate environment (23 °C) and a mixed environment (alternating 18°/28°C). Parental populations did not show divergence for larval viability, cold shock or high temperature mortalities when tested in a common environment. However, the 28 °C population was less fecund than the 18 °C population, but had higher larval competitive ability and higher adult longevity. Heterosis for viability, cold shock mortality and high temperature mortality occurred in crosses between a population adapted to 18 °C and another adapted to 28 °C, but not in crosses between two populations adapted to the same temperature. The results suggest that, in the absence of drift, heterosis is expected in crosses between lines or populations with different histories of selection but not between lines with the same selection histories.

Pupation-temperature range in 12 Drosophila species from different ecological backgrounds

A comparison of pupation-temperature range was made in the laboratory on a temperature gradient (3-38 degrees C) using 12 species of Drosophila representing four species groups and four different ecological backgrounds (temperate-montane forest: virilis group; desert: repleta group; cosmopolitan: melanogaster group; tropical forest: willistoni group). Within groups, differences are found which usually reflect species' distributions. Comparisons of species' mating-, oviposition- and pupation-temperature ranges reveal that pupation most often occurs at temperatures beyond those for mating and oviposition. Each species reflects a different combination of temperature effects. Individual species have different temperature-limits for mating, oviposition and pupation. Temperatures permissive for one response are not predictive of limits on other responses. Among species, temperature can affect a particular response differently. Within groups, species differences can be at high and/or low temperatures for any response, and temperature effects among closely related species can manifest themselves in one, or any combination of responses. One cannot predict which responses will be most and least limited, or at which end of the temperature scale a response will be most limited. Among groups, common, but not absolute temperature ranges generally correspond to the geographic distributions and ecological backgrounds of the species triads. The evaluation of temperature effects on species, based on a single activity, may not be adequate for predicting adaptive strategies.

Enzymatic and quantitative variation in European and African populations of Drosophila simulans

Allozyme polymorphism at 15 loci of D. simulans was studied in 7 natural populations from Europe, North and tropical Africa. Morphological traits were studied in nine European and eleven Afrotropical strains. Within a population, the biochemical polymorphisms of Drosophila simulans and its sibling Drosophila melanogaster are not very different, although D. simulans has a lower heterozygosity. Between-populations genetic differentiation is however much lower in D. simulans than in D. melanogaster. Several loci of D. simulans do exhibit latitudinal trends but these are relatively weak. For morphological traits, both species show an increase of size with latitude, but geographic variation is again less pronounced in D. simulans. Both species are native to tropical Africa and have colonised the rest of the world. During this process, D. simulans has undergone much less geographic differentiation than has D. melanogaster, so that the ecological success of the two species is not correlated with similarities in their genetic properties.

Strategic differences in thermal adaptation between two Drosophila species, D. virilis and D. immigrans

Preadult viability and developmental time at four different temperatures, heat and cold resistances of adult flies, effects of acclimatization on heat resistance, and preferred temperature of adult flies were compared between two species of Drosophila, D. virilis and D. immigrans. Four Japanese local populations were surveyed for each species. As compared with immigrans, virilis was higher in its ability to tolerate both heat and cold stresses and was viable over a broader temperature range. On the other hand, immigrans revealed a superior ability to acclimatize and a rigid preference for gradually changing thermal environment. Differences between geographical populations are remarkable for heat tolerance in virilis and cold tolerance in immigrans. In conclusion, thermal adaptation of virilis seems to be based on the high tolerance to extreme temperatures and that of immigrans mainly on the behavioural preference for viable temperatures.

Alcohol tolerance: An ecological parameter in the relative success of Drosophila melanogaster and Drosophila simulans

Laboratory experiments have shown D. melanogaster adults to be more tolerant to alcohol in the environment than D. simulans, with the females being more tolerant than the males of their species. Larval development on alcohol supplemented media also demonstrated an increased tolerance by D. melanogaster although the effect was not as clear cut as for the adult survival. Oviposition choice experiments demonstrated a marked rejection of alcohol impregnated laying sites by D. simulans when compared to standard medium sites. D. melanogaster showed a slight preference for alcohol supplemented sites.Collections in the maturation cellar of a vineyard produced, with the exception of a single D. simulans fly, entirely D. melanogaster adults while larvae and pupae from the cellar were also all D. melanogaster. Away from the alcohol resource, outside the cellar, both species were collected with D. simulans being the more common. However, the outside distribution of the two species was affected by alcohol fumes during vintage, as was the distribution of the sexes of D. melanogaster, with the more tolerant species or sex being closer to the source. The field results were thus in agreement with the laboratory predictions that D. melanogaster is better able to utilize an alcohol resource than D. simulans.