EVOLVABILITY OF INDIVIDUAL TRAITS IN A MULTIVARIATE CONTEXT: PARTITIONING THE ADDITIVE GENETIC VARIANCE INTO COMMON AND SPECIFIC COMPONENTS

Genetic covariation among multiple traits will bias the direction of evolution. Although a trait's phenotypic context is crucial for understanding evolutionary constraints, the evolutionary potential of one (focal) trait, rather than the whole phenotype, is often of interest. The extent to which a focal trait can evolve independently depends on how much of the genetic variance in that trait is unique. Here, we present a hypothesis‐testing framework for estimating the genetic variance in a focal trait that is independent of variance in other traits. We illustrate our analytical approach using two Drosophila bunnanda trait sets: a contact pheromone system comprised of cuticular hydrocarbons (CHCs), and wing shape, characterized by relative warps of vein position coordinates. Only 9% of the additive genetic variation in CHCs was trait specific, suggesting individual traits are unlikely to evolve independently. In contrast, most (72%) of the additive genetic variance in wing shape was trait specific, suggesting relative warp representations of wing shape could evolve independently. The identification of genetic variance in focal traits that is independent of other traits provides a way of studying the evolvability of individual traits within the broader context of the multivariate phenotype.     

The phenotypic and genetic covariance structure of drosphilid wings

Evolutionary constraint results from the interaction between the distribution of available genetic variation and the position of selective optima. The availability of genetic variance in multitrait systems, as described by the additive genetic variance-covariance matrix (G), has been the subject of recent attempts to assess the prevalence of genetic constraints. However, evolutionary constraints have not yet been considered from the perspective of the phenotypes available to multivariate selection, and whether genetic variance is present in all phenotypes potentially under selection. Determining the rank of the phenotypic variance-covariance matrix (P) to characterize the phenotypes available to selection, and contrasting it with the rank of G, may provide a general approach to determining the prevalence of genetic constraints. In a study of a laboratory population of Drosophila bunnanda from northern Australia we applied factor-analytic modeling to repeated measures of individual wing phenotypes to determine the dimensionality of the phenotypic space described by P. The phenotypic space spanned by the 10 wing traits had 10 statistically supported dimensions. In contrast, factor-analytic modeling of G estimated for the same 10 traits from a paternal half-sibling breeding design suggested G had fewer dimensions than traits. Statistical support was found for only five and two genetic dimensions, describing a total of 99% and 72% of genetic variance in wing morphology in females and males, respectively. The observed mismatch in dimensionality between P and G suggests that although selection might act to shift the intragenerational population mean toward any trait combination, evolution may be restricted to fewer dimensions.     

THE QUANTITATIVE GENETICS AND COEVOLUTION OF MALE AND FEMALE REPRODUCTIVE TRAITS

Studies of experimental sexual selection have tested the effect of variation in the intensity of sexual selection on male investment in reproduction, particularly sperm. However, in several species, including Drosophila pseudoobscura, no sperm response to experimental evolution has occurred. Here, we take a quantitative genetics approach to examine whether genetic constraints explain the limited evolutionary response. We quantified direct and indirect genetic variation, and genetic correlations within and between the sexes, in experimental populations of D. pseudoobscura. We found that sperm number may be limited by low heritability and evolvability whereas sperm quality (length) has moderate V_A and CV_A but does not evolve. Likewise, the female reproductive tract, suggested to drive the evolution of sperm, did not respond to experimental sexual selection even though there was sufficient genetic variation. The lack of genetic correlations between the sexes supports the opportunity for sexual conflict over investment in sperm by males and their storage by females. Our results suggest no absolute constraint arising from a lack of direct or indirect genetic variation or patterns of genetic covariation. These patterns show why responses to experimental evolution are hard to predict, and why research on genetic variation underlying interacting reproductive traits is needed.     

Measuring Evolutionary Constraints Through the Dimensionality of the Phenotype: Adjusted Bootstrap Method to Estimate Rank of Phenotypic Covariance Matrices

The potential and direction of phenotypic evolution is constrained by the distribution of genetic variation for the traits as described by the phenotypic (P) and genetic covariance matrices (G). The rank of the covariance matrix reflects the number of independent variational dimensions of the phenotype. Covariance matrices with less than full rank indicate lack of variation in some directions of the phenotype space and thus are an indication of absolute evolutionary constraints. Because selection acts upon phenotypic variation, the rank of P represents the upper limit of the dimensionality in G, relevant for selection response. The limitations of current methods to estimate matrix rank motivated us to analyze and adjust a bootstrap method and evaluate its performance by simulation. The results show that the modified bootstrap method (ABRE) gives reliable and rather conservative rank estimates when the sample size is sufficient for the number of variables studied (the sample size is at least five-fold the number of variables). Applying the method to various datasets suggests high phenotypic dimensionality in all cases. The analysis thus provides no evidence for absolute evolutionary constraints. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Thermal evolution of pre-adult life history traits, geometric size and shape, and developmental stability in Drosophila subobscura

Replicated lines of Drosophila subobscura originating from a large outbred stock collected at the estimated Chilean epicentre (Puerto Montt) of the original New World invasion were allowed to evolve under controlled conditions of larval crowding for 3.5 years at three temperature levels (13, 18 and 22 degrees C). Several pre-adult life history traits (development time, survival and competitive ability), adult life history related traits (wing size, wing shape and wing-aspect ratio), and wing size and shape asymmetries were measured at the three temperatures. Cold-adapted (13 degrees C) populations evolved longer development times and showed lower survival at the highest developmental temperature. No divergence for wing size was detected following adaptation to temperature extremes (13 and 22 degrees C), in agreement with earlier observations, but wing shape changes were obvious as a result of both thermal adaptation and development at different temperatures. However, the evolutionary trends observed for the wing-aspect ratio were inconsistent with an adaptive hypothesis. There was some indication that wing shape asymmetry has evolutionarily increased in warm-adapted populations, which suggests that there is additive genetic variation for fluctuating asymmetry and that it can evolve under rapid environmental changes caused by thermal stress. Overall, our results cast strong doubts on the hypothesis that body size itself is the target of selection, and suggest that pre-adult life history traits are more closely related to thermal adaptation.     

Cross‐sex genetic covariances limit the evolvability of wing‐shape within and among species of Drosophila

The independent evolution of males and females is potentially constrained by both sexes inheriting the same alleles from their parents. This genetic constraint can limit the evolvability of complex traits; however, there are few studies of multivariate evolution that incorporate cross‐sex genetic covariances in their predictions. Drosophila wing‐shape has emerged as a model high‐dimensional phenotype; wing‐shape is highly evolvable in contemporary populations, and yet perplexingly stable across phylogenetic timescales. Here, we show that cross‐sex covariances in Drosophila melanogaster, given by the B ‐matrix, may considerably bias wing‐shape evolution. Using random skewers, we show that B would constrain the response to antagonistic selection by 90%, on average, but would double the response to concordant selection. Both cross‐sex within‐trait and cross‐sex cross‐trait covariances determined the predicted response to antagonistic selection, but only cross‐sex within‐trait covariances facilitated the predicted response to concordant selection. Similar patterns were observed in the direction of extant sexual dimorphism in D. melanogaster, and in directions of most and least dimorphic variation across the Drosophila phylogeny. Our results highlight the importance of considering between‐sex genetic covariances when making predictions about evolution on both macro‐ and microevolutionary timescales, and may provide one more explanatory piece in the puzzle of stasis.     

Natural genetic variation in fluctuating asymmetry of wing shape in Drosophila melanogaster

Fluctuating asymmetry (FA), defined as random deviation from perfect symmetry, has been used to assay the inability of individuals to buffer their developmental processes from environmental perturbations (i.e., developmental instability). In this study, we aimed to characterize the natural genetic variation in FA of wing shape in Drosophila melanogaster, collected from across the Japanese archipelago. We quantified wing shapes at whole wing and partial wing component levels and evaluated their mean and FA. We also estimated the heritability of the mean and FA of these traits. We found significant natural genetic variation in all the mean wing traits and in FA of one of the partial wing components. Heritability estimates for mean wing shapes were significant in two and four out of five wing traits in males and females, respectively. On the contrary, heritability estimates for FA were low and not significant. This is a novel study of natural genetic variation in FA of wing shape. Our findings suggest that partial wing components behave as distinct units of selection for FA, and local adaptation of the mechanisms to stabilize developmental processes occur in nature.     

Sexual variation in heritability and genetic correlations of morphological traits in house sparrow (Passer domesticus)

Estimates of genetic components are important for our understanding of how individual characteristics are transferred between generations. We show that the level of heritability varies between 0.12 and 0.68 in six morphological traits in house sparrows (Passer domesticus L.) in northern Norway. Positive and negative genetic correlations were present among traits, suggesting evolutionary constraints on the evolution of some of these characters. A sexual difference in the amount of heritable genetic variation was found in tarsus length, wing length, bill depth and body condition index, with generally higher heritability in females. In addition, the structure of the genetic variance-covariance matrix for the traits differed between the sexes. Genetic correlations between males and females for the morphological traits were however large and not significantly different from one, indicating that sex-specific responses to selection will be influenced by intersexual differences in selection differentials. Despite this, some traits had heritability above 0.1 in females, even after conditioning on the additive genetic covariance between sexes and the additive genetic variances in males. Moreover, a meta-analysis indicated that higher heritability in females than in males may be common in birds. Thus, this indicates sexual differences in the genetic architecture of birds. Consequently, as in house sparrows, the evolutionary responses to selection will often be larger in females than males. Hence, our results suggest that sex-specific additive genetic variances and covariances, although ignored in most studies, should be included when making predictions of evolutionary changes from standard quantitative genetic models.     

Direct and correlated responses to artificial selection on male mating frequency in the stalk-eyed fly Cyrtodiopsis dalmanni

Traditionally it was thought that fitness-related traits such as male mating frequency, with a history of strong directional selection, should have little additive genetic variance and thus respond asymmetrically to bidirectional artificial selection. However, recent findings and theory suggest that a balance between selection for increased male mating frequency and opposing selection pressures on physiologically linked traits will cause male mating frequency to have high additive genetic variation and hence respond symmetrically to selection. We tested these hypotheses in the stalk-eyed fly, Cyrtodiopsis dalmanni, in which males hold harems comprising many females and so have the opportunity to mate at extremely high frequencies. We subjected male stalk-eyed flies to artificial selection for increased ('high') and decreased ('low') mating frequency in the presence of ecologically realistic, high numbers of females. High line males mated significantly more often than control or low line males. The direct response to selection was approximately symmetric in the high and low lines, revealing high additive genetic variation for, and no significant genetic constraints on, increased male mating frequency in C. dalmanni. In order to investigate trade-offs that might constrain male mating frequency under natural conditions we examined correlated responses to artificial selection. We measured accessory gland length, testis length and eyespan after 7 and 14 generations of selection. High line males had significantly larger accessory glands than low line males. No consistent correlated responses to selection were found in testis length or eyespan. Our results suggest that costs associated with the production and maintenance of large accessory glands, although yet to be identified, are likely to be a major constraint on mating frequency in natural populations of C. dalmanni.     

Genetic architecture of wing morphology in Drosophila simulans and an analysis of temperature effects on genetic parameter estimates

The Drosophila wing has been used as a model to investigate the mechanisms responsible for size and shape changes in nature, since such changes might underlie morphological evolution. To improve the understanding of wing morphological variation and the interpretation of genetic parameters estimates, we have established 59 lines from a Drosophila simulans laboratory population through single pair random matings. The offspring of each line were reared at three different temperatures, and the wing morphology of 12 individuals was analyzed by adjusting an ellipse to the wings' contour. Temperature, sex and line significantly affected wing trait variation, which was mainly characterized by longer wings having the second, fourth and fifth longitudinal veins closer together at the wing tip. As for the genetic parameter estimates, while the cross-environment heritability of some traits, such as wing size (SI), decreased with an increasing difference between the temperatures at which parents and offspring were reared, wing shape (SH) heritability did not seem to change. Since we found indications that neither an increase in the phenotypic variation nor the occurrence of genotype-environment interactions could fully explain the low heritabilities of SI estimated by cross-environment regressions, we discuss the importance of other effects for explaining this discrepancy between the SI and SH heritability estimates. In addition, although the genetic matrix was not entirely represented in the phenotypic matrix, several correspondences were identified, suggesting that the observed patterns of wing morphology variation are genetically controlled.     

Similarity in G matrix structure among natural populations of Arabidopsis lyrata

Understanding the stability of the G matrix in natural populations is fundamental for predicting evolutionary trajectories; yet, the extent of its spatial variation and how this impacts responses to selection remain open questions. With a nested paternal half‐sib crossing design and plants grown in a field experiment, we examined differences in the genetic architecture of flowering time, floral display, and plant size among four Scandinavian populations of Arabidopsis lyrata. Using a multivariate Bayesian framework, we compared the size, shape, and orientation of G matrices and assessed their potential to facilitate or constrain trait evolution. Flowering time, floral display and rosette size varied among populations and significant additive genetic variation within populations indicated potential to evolve in response to selection. Yet, some characters, including flowering start and number of flowers, may not evolve independently because of genetic correlations. Using a multivariate framework, we found few differences in the genetic architecture of traits among populations. G matrices varied mostly in size rather than shape or orientation. Differences in multivariate responses to selection predicted from differences in G were small, suggesting overall matrix similarity and shared constraints to trait evolution among populations.     

Multiple capacitors for natural genetic variation in Drosophila melanogaster

Cryptic genetic variation (CGV) or a standing genetic variation that is not ordinarily expressed as a phenotype is released when the robustness of organisms is impaired under environmental or genetic perturbations. Evolutionary capacitors modulate the amount of genetic variation exposed to natural selection and hidden cryptically; they have a fundamental effect on the evolvability of traits on evolutionary timescales. In this study, I have demonstrated the effects of multiple genomic regions of Drosophila melanogaster on CGV in wing shape. I examined the effects of 61 genomic deficiencies on quantitative and qualitative natural genetic variation in the wing shape of D. melanogaster. I have identified 10 genomic deficiencies that do not encompass a known candidate evolutionary capacitor, Hsp90, exposing natural CGV differently depending on the location of the deficiencies in the genome. Furthermore, five genomic deficiencies uncovered qualitative CGV in wing morphology. These findings suggest that CGV in wing shape of wild‐type D. melanogaster is regulated by multiple capacitors with divergent functions. Future analysis of genes encompassed by these genomic regions would help elucidate novel capacitor genes and better understand the general features of capacitors regarding natural genetic variation.     

A test for Y‐linked additive and epistatic effects on surviving bacterial infections in Drosophila melanogaster

Y‐ and W‐chromosomes offer a theoretically powerful way for sexual dimorphism to evolve. Consistent with this possibility, Drosophila melanogaster Y‐chromosomes can influence gene regulation throughout the genome; particularly immune‐related genes. In order for Y‐linked regulatory variation (YRV) to contribute to adaptive evolution it must be comprised of additive genetic variance, such that variable Ys induce consistent phenotypic effects within the local gene pool. We assessed the potential for Y‐chromosomes to adaptively shape gram‐negative and gram‐positive bacterial defence by introgressing Ys across multiple genetic haplotypes from the same population. We found no Y‐linked additive effects on immune phenotypes, suggesting a restricted role for the Y to facilitate dimorphic evolution. We did find, however, a large magnitude Y by background interaction that induced rank order reversals of Y‐effects across the backgrounds (i.e. sign epistasis). Thus, Y‐chromosome effects appeared consistent within backgrounds, but highly variable among backgrounds. This large sign epistatic effect could constrain monomorphic selection in both sexes, considering that autosomal alleles under selection must spend half of their time in a male background where relative fitness values are altered. If the pattern described here is consistent for other traits or within other XY (or ZW) systems, then YRV may represent a universal constraint to autosomal trait evolution.     

NO EFFECT OF ENVIRONMENTAL HETEROGENEITY ON THE MAINTENANCE OF GENETIC VARIATION IN WING SHAPE IN DROSOPHILA MELANOGASTER

Theory suggests that heterogeneous environments should maintain more genetic variation within populations than homogeneous environments, yet experimental evidence for this effect in quantitative traits has been inconsistent. To examine the effect of heterogeneity on quantitative genetic variation, we maintained replicate populations of Drosophila melanogaster under treatments with constant temperatures, temporally variable temperature, or spatially variable temperature with either panmictic or limited migration. Despite observing differences in fitness and divergence in several wing traits between the environments, we did not find any differences in the additive genetic variance for any wing traits among any of the treatments. Although we found an effect of gene flow constraining adaptive divergence between cages in the limited migration treatment, it did not tend to increase within‐population genetic variance relative to any of the other treatments. The lack of any clear and repeatable patterns of response to heterogeneous versus homogeneous environments across several empirical studies suggests that a single general mechanism for the maintenance of standing genetic variation is unlikely; rather, the relative importance of putative mechanisms likely varies considerably from one trait and ecological context to another.     

Genetic and maternal influences on life history plasticity in milkweed bugs (Oncopeltus): response to temperature

This study was designed to examine life history flexibility arising from phenotypic plasticity in response to temperature and from maternal effects in response to reproductive diapause in a temperate zone population of the milkweek bug (Oncopeltus fasciatus). We employed a split-family, first-cousin, full-sib design with siblings reared at different temperatures in order to quantify phenotypic plasticity, maternal effects, and variation for each. The following traits were analyzed: development time, age at first reproduction, longevity, early-life fecundity, and wing length. We found both life history plasticity and maternal effects on life history traits which tend to enhance the colonizing ability of offspring born to mothers that have undergone reproductive diapause. We were unable to demonstrate additive genetic variation for plasticity for any of the traits, while for development time and wing length we found variation due to non-additive genetic or common-environmental sources. We were also unable to demonstrate additive genetic variation for maternal effects, although variation may exist at low levels that are difficult to detect using cousin-families. The apparent lack of variation in this population would constrain evolution of life history flexibility even though considerable flexibility exists in the phenotype.     

JOINT ALLELIC EFFECTS ON FITNESS AND METRIC TRAITS

Theoretical explanations of empirically observed standing genetic variation, mutation, and selection suggest that many alleles must jointly affect fitness and metric traits. However, there are few direct demonstrations of the nature and extent of these pleiotropic associations. We implemented a mutation accumulation (MA) divergence experimental design in Drosophila serrata to segregate genetic variants for fitness and metric traits. By exploiting naturally occurring MA line extinctions as a measure of line‐level total fitness, manipulating sexual selection, and measuring productivity we were able to demonstrate genetic covariance between fitness and standard metric traits, wing size, and shape. Larger size was associated with lower total fitness and male sexual fitness, but higher productivity. Multivariate wing shape traits, capturing major axes of wing shape variation among MA lines, evolved only in the absence of sexual selection, and to the greatest extent in lines that went extinct, indicating that mutations contributing wing shape variation also typically had deleterious effects on both total fitness and male sexual fitness. This pleiotropic covariance of metric traits with fitness will drive their evolution, and generate the appearance of selection on the metric traits even in the absence of a direct contribution to fitness.     

THE GENETIC ARCHITECTURE OF WING SIZE DIVERGENCE AT VARYING SPATIAL SCALES ALONG A BODY SIZE CLINE IN DROSOPHILA MELANOGASTER

Latitudinal clines in quantitative traits are common, but surprisingly little is known about the genetic bases of these divergences and how they vary within and between clines. Here, we use line‐cross analysis to investigate the genetic architecture of wing size divergences at varying spatial scales along a body size cline in Drosophila melanogaster. Our results revealed that divergences in wing size along the cline were due to strong additive effects. Significant nonadditive genetic effects, including epistasis and maternal effects, were also detected, but they were relatively minor in comparison to the additive effects and none were common to all crosses. There was no evidence of increased epistasis in crosses between more geographically distant populations and, unlike in previous studies, we found no significant dominance effects on wing size in any cross. Our results suggest there is little variation in the genetic control of wing size along the length of the Australian cline. They also highlight marked inconsistencies in the magnitude of dominance effects across studies, which may reflect different opportunities for mutation accumulation while lines are in laboratory culture.     

Adaptation and constraint in the evolution of Drosophila melanogaster wing shape

SUMMARY We have taken advantage of parallel instances of natural selection on body size in Drosophila melanogaster to investigate constraints and adaptation affecting wing shape. Using recently developed techniques for statistical shape analysis, we have examined variation in wing shape in similar body size clines on three continents. Gender-related shape differences were constant among all populations, suggesting that gender differences represent a developmental constraint on wing shape. In contrast, the underlying shape varied significantly between continents and shape change within each cline (i.e., between small and large body size populations) also varied between continents. Therefore, variation at these two levels presumably results from either drift or natural selection. Functional considerations suggest that shape variation between the continents is unlikely to be adaptive. However, cline-related shape change, which we show has a significant allometric component, may be adaptive. The overall range of wing shape variation, across a large range of wing size, is extremely small, and the possibility that wing shape is subject to stabilizing selection (or canalization) is discussed.     

Swift laboratory thermal evolution of wing shape (but not size) in Drosophila subobscura and its relationship with chromosomal inversion polymorphism

Latitudinal clinal variation in wing size and shape has evolved in North American populations of Drosophila subobscura within about 20 years since colonization. While the size cline is consistent to that found in original European populations (and globally in other Drosophila species), different parts of the wing have evolved on the two continents. This clearly suggests that 'chance and necessity' are simultaneously playing their roles in the process of adaptation. We report here rapid and consistent thermal evolution of wing shape (but not size) that apparently is at odds with that suggestion. Three replicated populations of D. subobscura derived from an outbred stock at Puerto Montt (Chile) were kept at each of three temperatures (13, 18 and 22 degrees C) for 1 year and have diverged for 27 generations at most. We used the methods of geometric morphometrics to study wing shape variation in both females and males from the thermal stocks, and rates of genetic divergence for wing shape were found to be as fast or even faster than those previously estimated for wing size on a continental scale. These shape changes did not follow a neat linear trend with temperature, and are associated with localized shifts of particular landmarks with some differences between sexes. Wing shape variables were found to differ in response to male genetic constitution for polymorphic chromosomal inversions, which strongly suggests that changes in gene arrangement frequencies as a response to temperature underlie the correlated changes in wing shape because of gene-inversion linkage disequilibria. In fact, we also suggest that the shape cline in North America likely predated the size cline and is consistent with the quite different evolutionary rates between inversion and size clines. These findings cast strong doubts on the supposed 'unpredictability' of the geographical cline for wing traits in D. subobscura North American colonizing populations.     

Between‐sex genetic covariance constrains the evolution of sexual dimorphism in Drosophila melanogaster

Males and females share much of their genome, and as a result, intralocus sexual conflict is generated when selection on a shared trait differs between the sexes. This conflict can be partially or entirely resolved via the evolution of sex‐specific genetic variation that allows each sex to approach, or possibly achieve, its optimum phenotype, thereby generating sexual dimorphism. However, shared genetic variation between the sexes can impose constraints on the independent expression of a shared trait in males and females, hindering the evolution of sexual dimorphism. Here, we examine genetic constraints on the evolution of sexual dimorphism in Drosophila melanogaster cuticular hydrocarbon (CHC) expression. We use the extended G matrix, which includes the between‐sex genetic covariances that constitute the B matrix, to compare genetic constraints on two sets of CHC traits that differ in the extent of their sexual dimorphism. We find significant genetic constraints on the evolution of further dimorphism in the least dimorphic traits, but no such constraints for the most dimorphic traits. We also show that the genetic constraints on the least dimorphic CHCs are asymmetrical between the sexes. Our results suggest that there is evidence both for resolved and ongoing sexual conflict in D. melanogaster CHC profiles.