The Effects of Disruptive and Stabilizing Selection on Body Size in DROSOPHILA MELANOGASTER. II. Analysis of Responses in the Thorax Selection Lines

An analysis was made of changes in mean and variance in some thorax selection lines. The decrease of mean thorax length in the stabilizing selection lines (S) was a consequence of a directional selection component, caused by the skewness of the frequency distributions. The slight or temporary increase of the phenotypic variance and the large increase of the mean value in the disruptive selection lines with random mating (D(R)) could be attributed to differences in reproduction between small and large flies (egg production and mating success). Phenotypic variability was high in two disruptive selection lines with compulsory mating of opposite extremes (D(-)). The mechanism of the change in variability was different in these replicate lines. In D(-)-1 the change was obtained by an increase of the environmental and the nonadditive genetic components of the variance. In D(-)-2 almost exclusively an increase of additive genetic variance occurred.     

Variation of body size within and between wild populations of Drosophila buzzatii

Four populations of the cactophilous species D. buzzatii have been compared with respect to the phenotypic variation of thorax and wing length of wild versus laboratory reared flies. Three of the strains were intercrossed to provide parent, F₁ and F₂ comparisons as a test of co-adaptation. The genetic contribution to phenotypic variation of laboratory reared flies was estimated from the correlation between sibs derived from random pair mating and reared individually in separate cultures. The average natural temperature during development was estimated from the relations between the wing/thorax ratio and temperature in laboratory tests.The variance of thorax and wing length of wild flies was several times greater than that of laboratory reared flies and the increase was attributed primarily to variation in larval food supply although temperature fluctuation is also important. There was no evidence of heterosis or F₂ break-down in the crosses. For two of the populations the heritability of thorax length was high, 60–70%, and substantially lower for the third. The average temperature estimated from the wing/thorax/temperature relationship differed between sites. The reduction of body size below the potential maximum averaged 30% for two and 20% for the other population, with a wide spread about these values. The evidence is discussed in relation to assessing the nature of ecological variation by comparing the variation of morphological traits in wild and laboratory reared flies.     

The evolutionary history of Drosophila buzzatii. XX. Positive phenotypic covariance between field adult fitness components and body size

In the cactophilic species Drosphila buzzatii, it is feasible to infer the action of natural selection by simultaneously sampling different life history stages in the field. During four years of research, samples of mating and non-mating adults and pupae were taken from a natural population. The main adult fitness components, i.e., mating success, longevity, and fecundity, were recorded in relation to body size, as measured by thorax length. The age of flies was estimated by observing the developmental stage of the reproductive system. Our data showed that larger flies can outlive and outmate small flies, and that mating success is related to age. An estimate of the fitness function showed a linear increase of mating success with increasing thorax length. There was no assortative mating for this trait. We advance the hypothesis that mating success is related to the rate of encounter and courtship time through general activity, which in turn may be related to body size. A positive phenotypic correlation between thorax length and ovariole number, which is related to fecundity, was found in females emerged from wild pupae. Neither the phenotypic nor the genetic (additive) correlations between these two traits were statistically different from zero in laboratory reared females. The genetic consequences of the observed phenotypic selection on body size are discussed.     

Genetic Variance for Body Size in a Natural Population of Drosophila Buzzatii

Previous work has shown thorax length to be under directional selection in the Drosophila buzzatii population of Carboneras. In order to predict the genetic consequences of natural selection, genetic variation for this trait was investigated in two ways. First, narrow sense heritability was estimated in the laboratory F2 generation of a sample of wild flies by means of the offspring-parent regression. A relatively high value, 0.59, was obtained. Because the phenotypic variance of wild flies was 7-9 times that of the flies raised in the laboratory, "natural" heritability may be estimated as one-seventh to one-ninth that value. Second, the contribution of the second and fourth chromosomes, which are polymorphic for paracentric inversions, to the genetic variance of thorax length was estimated in the field and in the laboratory. This was done with the assistance of a simple genetic model which shows that the variance among chromosome arrangements and the variance among karyotypes provide minimum estimates of the chromosome's contribution to the additive and genetic variances of the trait, respectively. In males raised under optimal conditions in the laboratory, the variance among second-chromosome karyotypes accounted for 11.43% of the total phenotypic variance and most of this variance was additive; by contrast, the contribution of the fourth chromosome was nonsignificant. The variance among second-chromosome karyotypes accounted for 1.56-1.78% of the total phenotypic variance in wild males and was nonsignificant in wild females. The variance among fourth chromosome karyotypes accounted for 0.14-3.48% of the total phenotypic variance in wild flies. At both chromosomes, the proportion of additive variance was higher in mating flies than in nonmating flies.     

Courtship success and multivariate analysis of sexual selection on morphometric traits inDrosophila buzzatii (Diptera: Drosophilidae)

Using wild-reared flies, we examined sexual selection on five phenotypic traits (thorax length, wing length, wing width, head width, and face width) inDrosophila buzzatii, by scoring copulatory status in nine mass mating cages. Only male face width was identified as a direct target of sexual selection in an analysis of selection gradient, while indirect selection was present on all other studied traits, as expected from their correlations with face width. In contrast to males, there was no indication of selection in females. Nor was there evidence of assortative mating. The suggested direct selection on face width seems to take place during licking behavior of the courtship and might be related to courtship feeding. This study suggests that courtship success gives rise to indirect selection on body size.     

LATITUDINAL VARIATION OF WING:THORAX SIZE RATIO AND WING-ASPECT RATIO IN DROSOPHILA MELANOGASTER

In dipterans, the wing-beat frequency, and, hence, the lift generated, increases linearly with ambient temperature. If flight performance is an important target of natural selection, higher wing:thorax size ratio and wing-aspect ratio should be favored at low temperatures because they increase the lift for a given body weight. We investigated this hypothesis by examining wing: thorax size ratio and wing-aspect ratio in Drosophila melanogaster collected from wild populations along a latitudinal gradient and in their descendants reared under standard laboratory conditions. In a subset of lines, we also studied the phenotypic plasticity of these traits in response to temperature. To examine whether the latitudinal trends in wing:thorax size ratio and wing-aspect ratio could have resulted from a correlated response to latitudinal selection on wing area, we investigated the correlated responses of these characters in lines artificially selected for wing area. In both the geographic and the artificially selected lines, wing:thorax size ratio and wing-aspect ratio decreased in response to increasing temperature during development. Phenotypic plasticity for either trait did not vary among latitudinal lines or selective regimes. Wing:thorax size ratio and wing-aspect ratio increased significantly with latitude in field-collected flies. The cline in wing:thorax size ratio had a genetic component, but the cline in wing-aspect ratio did not. Artificial selection for increased wing area led to a statistically insignificant correlated increase in wing:thorax size ratio and a decrease in wing-aspect ratio. Our observations are consistent with the hypotheses that high wing-thorax size ratio and wing aspect ratio are per se selectively advantageous at low temperatures.     

AN ANALYSIS OF SELECTIONAL RESPONSE IN RELATION TO A POPULATION BOTTLENECK

Selection for increased morphometric shape (ratio of wing length to thorax width) was compared between control (nonbottlenecked) populations and bottlenecked populations founded with two male–female pairs of flies. Contrary to neutral expectation, selectional response was not reduced in bottlenecked populations, and the mean realized heritabilities and additive genetic variances were higher for the bottlenecked lines than for the nonbottlenecked lines. Additive genetic variances based on these realized heritabilities were consistent with independent estimates of genetic variances based on parent–offspring covariances. Joint scaling tests applied to the crosses between selected lines and their controls revealed significant nonadditive components of genetic variance in the ancestor, which were not detected in the crosses involving bottlenecked lines. The nonbottlenecked lines responded principally by changes in one trait or the other (wing length or thorax width) but not in both, and regardless of which trait responded, larger trait size was dominant and epistatic to smaller size. Stabilizing selection for morphometric shape in the ancestor likely molded the genetic architecture to include nonadditive genetic effects.     

Selection on Wing Allometry in Drosophila Melanogaster

Five bivariate distributions of wing dimensions of Drosophila melanogaster were measured, in flies 1) subjected to four defined environmental regimes during development, 2) taken directly from nature in seven U.S. states, 3) selected in ten populations for change in wing form, and 4) sampled from 21 long inbred wild-type lines. Environmental stresses during development altered both wing size and the ratios of wing dimensions, but regardless of treatment all wing dimensions fell near a common allometric baseline in each bivariate distribution. The wings of wild-caught flies from seven widely separated localities, and of their laboratory-reared offspring, also fell along the same baselines. However, when flies were selected divergently for lateral offset from these developmental baselines, response to selection was rapid in every case. The mean divergence in offset between oppositely selected lines was 14.68 SD of the base population offset, after only 15 generations of selection at 20%. Measurements of 21 isofemale lines, founded from wild-caught flies and maintained in small populations for at least 22 years, showed large reductions in phenotypic variance of offsets within lines, but a large increase in the variance among lines. The variance of means of isofemale lines within collection localities was ten times the variance of means among localities of newly established wild lines. These observations show that much additive genetic variance exists for individual dimensions within the wing, such that bivariate developmental patterns can be changed in any direction by selection or by drift. The relative invariance of the allometric baselines of wing morphology in nature is most easily explained as the result of continuous natural selection around a local optimum of functional design.     

An adaptive chromosomal polymorphism affecting size-related traits,and longevity selection in a natural population ofDrosophila buzzatii

Size-related phenotypic variation among second-chromosome karyotypes inDrosophila buzzatii was examined in an Argentinian natural population. For all measured traits (thorax and wing length; wing, head and face width), this inversion polymorphism exhibited a significant and (additive) linear contribution to the phenotypic variance in newly emerged wild flies. The results suggest that only overall body size, and not body shape, is affected. as no karyotypic variation was found for any trait when the effects of differences in within-karyotype size were removed with Burnaby's method. Likewise, in an experiment of longevity selection in the wild, variation in chromosomal frequencies was verified in the direction predicted on the basis of: (i) previous studies on longevity selection for body size in the wild and (ii) the pattern of chromosomal effects we observed on size. The direction of such selection is consistent with a pattern of antagonistic selection detected in previous studies on the inversion polymorphism.     

The genetics of phenotypic plasticity. III. Genetic correlations and fluctuating asymmetries

We examined the relationship of three aspects of development, phenotypic plasticity, genetic correlations among traits, and developmental noise, for thorax length, wing length, and number of sternopleural bristles in Drosophila melanogaster. We used 14 lines which had previously been selected on either thorax length or plasticity of thorax length in response to temperature. A half-sib mating design was used and offspring were raised at 19° C or 25° C. We found that genetic correlations were stable across temperatures despite the large levels of plasticity of these traits. Plasticities were correlated among developmentally related traits, thorax and wing length, but not among unrelated traits, lengths and bristle counts. Amount of developmental noise, measured as fluctuating asymmetry and within-environmental variation, was positively correlated with amount of plasticity only for some traits, thorax length and bristle number, and only at one temperature, 25° C.     

Quantitative variation of four morphological traits in Drosophila melanogaster under larval crowding

Effects of three different larval densities (low, intermediate and high) on phenotypic and genetic variation of four morphological traits (thorax and wing length, sternopleural and abdominal bristle number) were studied in Drosophila melanogaster using the isofemale line technique. Phenotypic variation was found to increase at high larval density in all traits examined. Environmental variance for three traits (exception was sternopleural bristle number) and fluctuating asymmetry for both bilateral traits were also increased under high density conditions. For estimates of genetic variability (among isofemale lines variance, heritability and evolvability), no statistically significant differences among density regimes were detected. However, the trends in changes of these estimates across densities indicated a possibility for enhanced genetic variation under larval crowding for all traits except abdominal bristle number. For the latter trait, genetic variation seemed not to be dependent on density regime. Generally, two metric traits (thorax and wing length) were more affected by larval crowding than two meristic ones (sternopleural and abdominal bristle number). The Results are in complete agreement with those previously obtained for D. melanogaster using extreme temperatures as stress-factors.     

The evolutionary history of Drosophila buzzatii XXVII

The correlation between body size and longevity was tested in an Argentinian natural population of Drosophila buzzatii. Mean thorax length of flies newly emerging from rotting cladodes of Opuntia vulgaris was significantly smaller than that of two samples of flies caught at baits. The present results which might be interpreted as directional selection for longevity favoring larger flies are in agreement with previous results achieved in a Spanish natural population of D. buzzatii. Flies emerging from different substrates showed significant differences in thorax length, suggesting that an important fraction of phenotypic variance can be attributed to environmental variability. However, laboratory and field work in different populations of D. buzzatii showed a significant genetic component for thorax length variation.     

Half-sib analysis of three morphological traits in Drosophila melanogaster under poor nutrition

Variation in thorax length, wing length and sternopleural bristle number was examined in Drosophila melanogaster reared in stressful and nonstressful environments using paternal half-sib design. Low concentration of yeast in the medium was used as a stress factor. Phenotypic variation of thorax length and wing length was higher under poor nutrition than in the control; in bristle number, phenotypic variation was relatively stable regardless of the environment. Heritability of all the traits analyzed was generally lower under nutritional stress. Heritability changes in thorax length and wing length were mainly due to an increase in the environmental variance under stress, whereas in bristle number, stress resulted in a decrease in genetic variation. Genetic variance in thorax length was higher under poor nutrition; in wing length, no difference in genetic variance between environments was found.     

Genetic variability and fluctuating asymmetry of morphological signs of Drosophila melanogaster during development in a pesticide-containing environment

The effects of chlorine-organic insecticide endosulfane (thiodan) on phenotypic and genetic variation in four morphological traits of Drosophila melanogaster (wing length, thorax length, the number of orbital bristles and the number of sternopleural bristles) were examined. In addition, the effect of this pesticide on stability of development measured as fluctuating asymmetry of bilateral traits was estimated. On the medium with endosulfane, phenotypic variation of morphometric traits was significantly higher. No difference in fluctuating asymmetry between the stressed and the control samples was found. The among-line variance of morphometric traits of flies reared on the endosulfane-containing medium was significantly higher as compared to the corresponding variance under control conditions. The efficiency of using fluctuating asymmetry and phenotypic variation of morphometric and meristic traits as indicators of environmental stress in insect populations is discussed.     

Genetic Variability and Fluctuating Asymmetry of Morphological Traits in Drosophila melanogasterReared on a Pesticide-Containing Medium

The effects of chlorine-organic insecticide endosulfane (thiodan) on phenotypic and genetic variation in four morphological traits of Drosophila melanogaster(wing length, thorax length, the number of orbital bristles and the number of sternopleural bristles) were examined. In addition, the effect of this pesticide on stability of development measured as fluctuating asymmetry of bilateral traits was estimated. On the medium with endosulfane, phenotypic variation of morphometric traits was significantly higher. No difference in fluctuating asymmetry between the stressed and the control samples was found. The among-line variance of morphometric traits of flies reared on the endosulfane-containing medium was significantly higher as compared to the corresponding variance under control conditions. The efficiency of using fluctuating asymmetry and phenotypic variation of morphometric and meristic traits as indicators of environmental stress in insect populations is discussed.     

Female Mate Choice, Calling Song and Genetic Variance in the Cricket, Gryllodes sigillatus

Female preferences for song patterns of males of Gryllodes sigillatus and genetic variance of morphological traits correlated with them were analyzed. Females preferred short pulses associated with large males. The males’ thorax width, wing length and femur III length showed stronger relationship with the song pulse duration, whereas the relationship between pulse duration and wing width was not significant. Interestingly, this last trait was the only one that showed significant levels of genetic variance. Perhaps these results could be explained by the evolutionary response to sexual selection. Sexual selection could deplete the genetic variance in the male traits related to male‐mating success.     

Correlated responses to selection on body size in Drosophila melanogaster.

Correlated responses to artificial selection on body size in Drosophila melanogaster were investigated, to determine how the changes in size were produced during development. Selection for increased thorax length was associated with an increase in larval development time, an extended growth period, no change in growth rate, and an increased critical larval weight for pupariation. Selection for reduced thorax length was associated with reduced growth rate, no change in duration of larval development and a reduced critical larval weight for pupariation. In both lines selected for thorax length and lines selected for wing area, total body size changed in the same direction as the artificially selected trait. In large selection lines of both types, the increase in size was achieved almost entirely by an increase in cell number, while in the small lines the decrease in size was achieved predominantly by reduced cell size, and also by a reduction in cell number. The implications of the results for evolutionary-genetic change in body size in nature are discussed.     

Disruptive selection for sternopleural bristle phenotypes in Drosophila ananassae

Disruptive selection is potentially critical in maintaining variation and initiating speciation and plays an important role in the organization of genetic variability in natural populations. It occurs when extreme phenotypes have a fitness advantage over intermediate phenotypes. Disruptive selection for high and low numbers of sternopleural bristles in Drosophila ananassae was applied for 12 generations to test its effect in induction of behavioural isolation. Pattern of mating between flies of high and low lines was tested in an Elens-Wattiaux mating chamber by using a multiple-choice technique after G(5) and G(12). Data was analyzed by calculating χ(2) under the assumption of random mating to test the difference between homoand heterogamic matings, and sexual isolation was tested by calculating the isolation estimate. The results show that there is no evidence for sexual isolation in G(5) and G(12). The realized heritability, standard error of regression coefficient, and t values suggest that disruptive selection for sternopleural bristle phenotypes was effective, but it does not lead to behavioral isolation in D. ananassae. Rather, it induces differences in mating propensity, which is influenced by sternopleural bristle phenotypes.     

The effects of disruptive and stabilizing selection on body size inDrosophila melanogaster. III. Genetic analysis of two lines with different reactions to disruptive selection with mating of opposite extremes

A genetic analysis was made of two lines which when subjected to disruptive selection with compulsary mating of opposite extremes (D^?) showed a different response viz. one, D^?-1, showing predominantly an increase of environmental variance and possibly interaction variance, the other, D^?-2, showing an increase of genetic variance. Crosses between extreme flies within lines revealed that D^?-1 genomes from large flies are dominant to genomes from small individuals. In D^?-2 the genetic variation is predominantly additive variance. Tests for dominant chromosome effect in crosses with an inbred stock with recessive markers showed clear third chromosome differences in D^?-2 and not in D^?-1. Chromosome exchange between extreme flies corroborated the importance of genetic differences in D^?-2. A factor or complex of factors with large effect decreasing body size is located on third chromosomes from small flies in D^?-2. Interaction between chromosomes has a similar magnitude in the two lines. Crowding and temperature experiments did not reveal an increased general sensitivity to environmental factors in D^?-1, which was suggested by the enlarged environmental variance of this line.     

Size Dependent Sexual Selection in Drosophila ananassae

Mate choice based on body size is widespread and can have numerous consequences. We present data, which show the effect of male and female body size on sexual selection in Drosophila ananassae. The relationships between wing size, locomotor activity, mating latency, courtship pattern, fertility and mating success were studied. Mating latency was negatively correlated with wing length and with locomotor activity, while wing length and locomotor activity was positively correlated in males as well as in females. In female- and male-choice, we found that mate choice influenced size-assortative mating by: (1) large and small males preferring to mate with large females, (2) large males successfully competing for large females, leaving small males to mate with small females. Males increased their reproductive success by mating with large and more fecund females. In addition, in pairs of long/short winged flies, long winged flies courted and mated more successfully than short winged flies and they also have longer duration of copulation and more progeny than short winged flies. We found sterile mating in pairs of small winged males and females.