DIRECT AND CORRELATED RESPONSES TO SELECTION AMONG LIFE-HISTORY TRAITS IN MILKWEED BUGS (ONCOPELTUS FASCIATUS)

Offspring-parent regressions provided initial estimates of heritabilities and genetic correlations among wing length, body length, pronotum width, head-capsule width, development time, age at first reproduction, and fecundity in an Iowa population of the large milkweed bug, Oncopeltus fasciatus. Replicated, bidirectional selection for wing length was imposed for nine generations. The direct response to selection revealed the existence of substantial additive genetic variance for wing length in this population. Traits were assayed for correlated responses to selection after seven generations. Body length, pronotum width, head capsule width, and fecundity showed consistent, positive correlated responses. Development time showed a negative correlated response. Age at first reproduction showed no consistent correlated response to selection on wing length. These pleiotropic effects among wing length and fecundity, development time, and body size characters provide the potential for these traits to evolve together in O. fasciatus, independently of age at first reproduction.     

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.     

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.     

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.     

REML estimates of genetic parameters of sexual dimorphism for wing and thorax length in<Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

Restricted maximum likelihood was used to estimate genetic parameters of male and female wing and thorax length in isofemale lines ofDrosophila melanogaster, and results compared to estimates obtained earlier with the classical analysis of variance approach. As parents within an isofemale line were unknown, a total of 500 parental pedigrees were simulated and mean estimates computed. Full and half sibs were distinguished, in contrast to usual isofemale studies in which animals were all treated as half sibs and hence heritability was overestimated. Heritability was thus estimated at 0.33, 0.38, 0.30 and 0.33 for male and female wing length and male and female thorax length, respectively, whereas corresponding estimates obtained using analysis of variance were 0.46, 0.54, 0.35 and 0.38. Genetic correlations between male and female traits were 0.85 and 0.62 for wing and thorax length, respectively. Sexual dimorphism and the ratio of female to male traits were moderately heritable (0.30 and 0.23 for wing length, 0.38 and 0.23 for thorax length). Both were moderately and positively correlated with female traits, and weakly and negatively correlated with male traits. Such heritabilities confirmed that sexual dimorphism might be a fast evolving trait inDrosophila. An erratum to this article is available at .     

Longevity differences among lines artificially selected for developmental time and wing length in Drosophila buzzatii

We assessed the indirect response of longevity in lines selected for wing length (WL) and developmental time (DT). Longevity in selection lines was compared to laboratory control lines and the offspring of recently collected females. Wild flies (W lines), flies from lines selected for fast development (F lines), and for fast development and large wing length (L lines) outlived control laboratory lines (C lines) and lines selected for fast development and short wing (S lines). The decline in longevity in S lines is in line with the idea that body size and longevity are correlated and may be the result of the fixation of alleles at loci affecting pleiotropically the two traits under selection and longevity. In addition, inbreeding and artificial selection affected the correlation between wing length and longevity that occurs in natural populations of Drosophila buzzatii, suggesting that correlations between traits are not a perdurable feature in a population.     

GENETIC CORRELATIONS AMONG MORPHOMETRIC TRAITS AND RATES OF GROWTH AND DIFFERENTIATION IN THE GREEN TREE FROG,HYLA CINEREA

It is often proposed that the morphometric shape of animals often evolves as a correlated response to selection on life-history traits such as whole-body growth and differentiation rates. However, there exists little empirical information on whether selection on rates of growth or differentiation in animals could generate correlated response in morphometric shape beyond that owing to the correlation between these rates and body size. In this study genetic correlations were estimated among growth rate, differentiation rate, and body-size-adjusted head width in the green tree frog, Hyla cinerea. Head width was adjusted for size by using the residuals from log-log regressions of head width on snout-vent length. Size-adjusted head width at metamorphosis was positively genetically correlated with larval period length. Thus, size-independent shape might evolve as a correlated response to selection on a larval life-history trait. Larval growth rate was not significantly genetically correlated with size-adjusted head width. An additional morphometric trait, size-adjusted tibiofibula length, had a nonnormal distribution of breeding values, and so was not included in the analysis of genetic correlations (offspring from one sire had unusually short legs). This result is interesting because, although using genetic covariance matrices to predict long-term multivariate response to selection depends on the assumption that all loci follow a multivariate Gaussian distribution of allelic effects, few data are available on the distribution of breeding values for traits in wild populations. Size at metamorphosis was positively genetically correlated with larval period and larval growth rate. Quickly growing larvae that delay metamorphosis therefore emerge at a large size. The genetic correlation between larval growth rate and juvenile (postmetamorphic) growth rate was near zero. Growth rate may therefore be an example of a fitness-related trait that is free to evolve in one stage of a complex life cycle without pleiotropic constraints on the same trait expressed in the other stage.     

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.     

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.     

Realized evolvability: quantifying phenotypic evolution in a Drosophila clade

Understanding the evolutionary potential of morphological evolution is still a major problem in evolutionary biology. In this study, we tried to quantify the amount of variation of different traits among species of a Drosophila clade reared under standard conditions. Nineteen different traits have been measured on nine species of the same clade, the Neotropical saltans group of Drosophila. Measured traits can be distributed into five categories: size‐traits (wing and thorax), shape indices (ratios among the size traits), number of sternopleural bristles on the thorax, number of abdominal bristles on successive sternites, and dorsal pigmentation of abdomen. All species are of medium size with a generally dark pigmentation. A remarkable feature is the presence of numerous bristles on the 6th sternite of the males, while this segment is bare in other Drosophila species. A multivariate analysis revealed that it was possible to discriminate all the investigated species by using the complete set of measured traits. For each trait, phenotypic variability was investigated at the within‐ and between‐species levels. As a rule, the interspecific coefficient of variation (CV) was much greater than the within species CV, and it is proposed to call it realized evolvability. All possible correlations were calculated between traits across species, providing many unexpected results. Size traits were highly correlated among them, but not correlated with shape indices. Abdominal traits (bristles and pigmentation) were correlated, but often in opposite directions, with thoracic shape indices. Tergite pigmentation was negatively correlated with bristle number on sternite. For the moment, most of the correlations cannot be explained by developmental processes or parallel selective pressures. Nonetheless, mapping the evolution of the two characters on a molecular phylogeny of the studied species revealed two opposite phylogenetic trends for abdominal pigmentation and setation, respectively. Our data suggest a need for similar studies in other well‐defined Drosophila clades.     

Phenotypic plasticity in Drosophila cactophilic species: the effect of competition,density, and breeding sites

Changes in the environmental conditions experienced by naturally occurring populations are frequently accompanied by changes in adaptive traits allowing the organism to cope with environmental unpredictability. Phenotypic plasticity is a major aspect of adaptation and it has been involved in population dynamics of interacting species. In this study, phenotypic plasticity (i.e., environmental sensitivity) of morphological adaptive traits were analyzed in the cactophilic species Drosophila buzzatii and Drosophila koepferae (Diptera: Drosophilidae) considering the effect of crowding conditions (low and high density), type of competition (intraspecific and interspecific competition) and cacti hosts (Opuntia and Columnar cacti). All traits (wing length, wing width, thorax length, wing loading and wing aspect) showed significant variation for each environmental factor considered in both Drosophila species. The phenotypic plasticity pattern observed for each trait was different within and between these cactophilic Drosophila species depending on the environmental factor analyzed suggesting that body size‐related traits respond almost independently to environmental heterogeneity. The effects of ecological factors analyzed in this study are discussed in order to elucidate the causal factors investigated (type of competition, crowding conditions and alternative host) affecting the election of the breeding site and/or the range of distribution of these cactophilic species.     

Size-related sexual selection and yeast diet inDrosophila buzzatii (Diptera: Drosophilidae)

Size-related sexual selection (SRSS) was examined on four traits (thorax and wing length and head and face width) inDrosophila buzzatii, by scoring male copulatory status in two mass-mating experiments. Using axenic females, experiment 1 was carried out with axenic males, and experiment 2 with yeast-supplemented males. While there was no indication of SRSS in experiment 1, such selection was substantial in yeast-supplemented males, which transmitted yeasts to mating females. Multivariate analyses of selection indicated that face width is the measured trait on which directional SRSS essentially acted in yeast-supplemented males, resulting in indirect selection on body size. Because this selection was affected by yeast diet in males, its possible interaction with the yeast transmission from males to females during the courtship is discussed.     

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.     

Drosophila wing modularity revisited through a quantitative genetic approach

To predict the response of complex morphological structures to selection it is necessary to know how the covariation among its different parts is organized. Two key features of covariation are modularity and integration. The Drosophila wing is currently considered a fully integrated structure. Here, we study the patterns of integration of the Drosophila wing and test the hypothesis of the wing being divided into two modules along the proximo‐distal axis, as suggested by developmental, biomechanical, and evolutionary evidence. To achieve these goals we perform a multilevel analysis of covariation combining the techniques of geometric morphometrics and quantitative genetics. Our results indicate that the Drosophila wing is indeed organized into two main modules, the wing base and the wing blade. The patterns of integration and modularity were highly concordant at the phenotypic, genetic, environmental, and developmental levels. Besides, we found that modularity at the developmental level was considerably higher than modularity at other levels, suggesting that in the Drosophila wing direct developmental interactions are major contributors to total phenotypic shape variation. We propose that the precise time at which covariance‐generating developmental processes occur and/or the magnitude of variation that they produce favor proximo‐distal, rather than anterior‐posterior, modularity in the Drosophila wing.     

RESPONSES TO SELECTION AMONG LIFE-HISTORY TRAITS IN A NONMIGRATORY POPULATION OF MILKWEED BUGS (ONCOPELTUS FASCIATUS)

Genetic parameters were assessed in the nonmigratory Puerto Rico population of the milkweed bug, Oncopeltus fasciatus, and compared with parameters estimated in a migratory population from Iowa (Palmer and Dingle, 1986). Offspring-parent regression analysis provided initial estimates of heritabilities and phenotypic and genetic correlations among wing length, head-capsule width, female age at first reproduction, fecundity for the first and second five days of reproduction by females, and clutch size for the first and second five days of reproduction by females. Replicated bidirectional selection for wing length was then imposed, with a direct response to selection revealing substantial additive genetic variance for this trait, as was also the case with the Iowa population. Assays for correlated response to selection yielded two further similarities to Iowa: a positive response in head-capsule width and no consistent response in age at first reproduction. In contrast to the results with Iowa bugs, neither regression analysis nor selection revealed statistically significant genetic correlations between fecundity measures and those of other traits. In both populations the potential exists for body-size characters to evolve together independently of age at first reproduction; but in the nonmigratory Puerto Rico bugs, fecundity does not contribute to a life-history syndrome involving genetic correlations among these traits.     

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.     

Direct and correlated responses to artificial selection on developmental time and wing length in Drosophila buzzatii

Abstract.— Developmental time and body size are two positively correlated traits closely related to fitness in many organisms including Drosophila . Previous work suggested that these two traits are involved in a trade-off that may result from a negative genetic correlation between their effects on pre-adult and adult fitness. Here, we examine the evolution of developmental time and body size (indexed by wing length) under artificial selection applied to one or both traits in replicated D. buzzatii populations. Directional changes in both developmental time and wing length indicate the presence of substantial additive genetic variance for both traits. The strongest response to selection for fast development was found in lines selected simultaneously to reduce both developmental time and wing length, probably as an expected consequence of a synergistic effect of indirect selection. When selection was applied in the direction opposite to the putative genetic correlation, that is, large wing length but fast development, no responses were observed for developmental time. Lines selected to reduce both wing length and developmental time diverged slightly faster from the control than lines selected to increase wing length and reduce developmental time. However, wing length did not diverge from the control in lines selected only for fast development. These results suggest a complex genetic basis of the correlation between developmental time and wing length, but are generally consistent with the hypothesis that both traits are related in a trade-off. However, we found that this trade-off may disappear under uncrowded conditions, with fast-developing lines exhibiting a higher pre-adult viability than other lines when tested at high larval density.     

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.     

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.     

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.