ONTOGENETIC VARIATION IN PATTERNS OF DEVELOPMENTAL AND FUNCTIONAL INTEGRATION IN SKULLS OF SIGMODON FULVIVENTER

Patterns of variation and covariation within populations can influence how characters respond to natural selection and random genetic drift and so constrain the ability of natural selection to modify the phenotype. We examined several potential developmental and functional explanations of character covariation throughout ontogeny using known-age samples of the cotton rat (Sigmodon fulviventer) to identify the causes of covariation and to assess the variability of patterns of covariation throughout postnatal growth. Competing developmental and functional models were fit to samples of orofacial and neurocranial measures by confirmatory factor analysis and evaluated for their ability to reconstruct observed variance-covariance matrices. Samples of successive ages were simultaneously fit to a common model to test the hypothesis that the patterns of developmental and functional integration were invariant between ages. Orofacial characters derived from the same branchial-arch primordium covary early in ontogeny. Subsequently, there is a repatterning of integration that may reflect a transition from developmental to functional sources of integration. Neurocranial characters exhibit even more variation in patterns of covariation: initially, characters appear to comprise a single integrated unit; before puberty, they appear to respond to localized bone growth; after puberty, they form separate calvarial and basicranial components. This ontogenetic variation in patterns of covariation suggests that developmental constraints are transient and flexible and that the consequences of selection may depend upon the age at which it acts.     

HOW SIMILAR ARE GENETIC CORRELATION STRUCTURES? DATA FROM MICE AND RATS

An integral assumption of many models of morphometric evolution is the equality of the genetic variance-covariance structure across evolutionary time. To examine this assumption, the quantitative-genetic aspects of morphometric form are examined for eight pelvic traits in laboratory rats (Rattus norvegicus) and random-bred ICR mice (Mus musculus). In both species, all traits are significantly heritable, and there are significant phenotypic and genetic correlations among traits, although environmental correlations among the eight traits are low. The size relations among the pelvic variables are isometric. Three matrix-permutation tests are used to examine similarity of phenotypic, genetic, and environmental covariance and correlation matrices within and between species. Independent patterns of morphometric covariation and correlation arise from genetic and environmental effects within each species and from environmental effects between species. The patterns of phenotypic and genetic covariation and correlation are similar within each species, and the phenotypic and genetic correlations are also similar between these species. However, genetic covariance matrices show no significant statistical association between species. It is suggested that the assumption of equality of genetic variance-covariance structures across divergent taxa should be approached with caution.     

Covariation among floral traits in Spergularia marina (Caryophyllaceae): geographic and temporal variation in phenotypic and among-family correlations

Strong covariation among traits suggests the presence of constraints on their independent evolution due to pleiotropy, to linkage, or to selective forces that maintain particular trait combinations. We examined floral trait covariation among individuals, among maternal families within and across populations, and over time, in greenhouse-raised plants of the autogamous Spergularia marina. We had three aims. First, since the phenotype of traits expressed by modular organs often changes as individuals age, estimates of the degree of genetic covariation between such traits may also change over time. To seek evidence for this, we measured weekly (for five weeks) an array of floral traits among plants representing ～ 10 maternal families from each of four populations. The statistical significance of the phenotypic and among-family correlations among traits changed over time. Second, we compared populations with respect to trait covariation to determine whether populations or traits appear to be evolving independently of one another. Differences observed among populations suggest that they have diverged genetically. Third, we sought correlations that might reflect constraints on the independent evolution of floral traits. Investment in another and ovule production per flower vary independently among maternal families; there was no evidence for a “trade-off” between male and female investment. We propose that in autogamous taxa one should not find a negative correlation between pollen and ovule production per flower, as such taxa cannot evolve sexual specialization and should be under strong selection to maintain an efficient pollen:ovule ratio, preventing the evolution of male-biased or female-biased genotypes. We found that other pairs of floral traits, however, expressed highly signficant correlation coefficients, suggesting the presence of some evolutionary constraints, at least within some populations, although their strength depended on exactly when flowers were sampled.     

ONTOGENETIC VARIATION IN PATTERNS OF PHENOTYPIC INTEGRATION IN THE LABORATORY RAT

I used confirmatory factor analysis to evaluate the ability of causal developmental models to predict observed phenotypic integration in limb and skull measures at five stages of postnatal ontogeny in the laboratory rat. To analyze the dynamics of phenotypic integration, I fit successive age-classes simultaneously to a common model. Growth was the principal developmental explanation of observed phenotypic covariation in the limb and skull. No complex morphogenetic model more adequately reconstructed observed covariance structure. Models that could not be interpreted in embryological terms, coupled with a growth component, provide the best models for observed phenotypic integration. During postnatal growth, some aspects of integration vary in both the skull and limb. The covariance between factors and the proportion of variance unique to each character differ between some sequential age-classes. The factor-pattern is invariant in the limb; however, repatterning in the skull occurs in the interval between eye-opening and weaning. The temporal variation in the structure of covariation suggests that functional interactions among characters may create observed patterns of phenotypic integration. The developmental constraints responsible for evolutionary modification of phenotypes might be equally dynamic and responsive to embryonic functional interactions.     

Developmental and Genetic Constraints on Neurocranial Globularity: Insights from Analyses of Deformed Skulls and Quantitative Genetics

Neurocranial globularity is one of the few derived traits defining anatomically modern humans. Variations in this trait derive from multiple and complex interactions between portions of the brain and the size and shape of the cranial base, among other factors. Given their evolutionary and functional importance, neurocranial globularity is expected to present high genetic and developmental constraints on their phenotypic expression. Here we applied two independent approaches to investigate both types of constraints. First, we assessed if patterns of morphological integration are conserved or else disrupted on a series of artificially deformed skulls in comparison to non-deformed (ND) ones. Second, after the estimation of the genetic covariance matrix for human skull shape, we explored how neurocranial globularity would respond to putative selective events disrupting the normal morphological patterns. Simulations on these deviations were explicitly set to replicate the artificial deformation patterns in order to compare developmental and genetic constraints under the same biomechanical conditions. In general terms, our results indicate that putative developmental constraints help to preserve some aspects of normal morphological integration even in the deformed skulls. Moreover, we find that the response to selection in neurocranial globularity is pervasive. In other words, induced changes in the vault generate a global response, indicating that departures from normal patterns of neurocranial globularity are genetically constrained. In summary, our combined results suggest that neurocranial globularity behaves as a highly genetic and developmental constrained trait. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Neus Martínez-Abadías and Rolando González-José contributed equally to this work.     

The ontogenetic complexity of developmental constraints

Developmental constraint is a theoretically important construct bridging ontogenetic and evolutionary studies. We propose a new operationalization of this notion that exploits the unusually rich measurement structure of landmark data. We represent landmark configurations by their partial warps, a basis for morphospace that represents a set of localized features of form. A finding of developmental constraint arises from the interplay between age-varying means and age-specific variances in these subspaces of morphospace. Examination of variances and means in 16 ventral skull landmarks in the cotton rat S. fulviventer at ages 1, 10, 20, and 30 days yielded three types of developmental constraint: canalization (constraint to relatively constant form age by age); chreods (reduction of variance orthogonal to the mean trajectory over ages); and opposition (reduction of age-specific variance along the mean trajectory over ages). While canalization and chreodic constraints have been noted previously, the oppositional type of constraint appears novel. Only one of our characters, relative length and orientation of the incisive foramen, appears to be canalized. Although skull growth becomes increasingly integrated through ontogeny, our characters display a remarkable spatiotemporal complexity in patterns of variance reduction. The specific assortment of constraints observed may be related to the precociality of Sigmodon. We suggest that Waddington's diagrammatic presentation of the “epigenetic landscape” may be misleading in quantitative studies of developmental regulation.     

Reaction norms for developmental time and weight at eclosion in Drosophila mercatorum

We determined reaction norms for developmental time and weight at eclosion for 2 isozygous and 11 genetically mixed strains of Drosophila mercatorum in four culture media differing in yeast concentration. With decreasing yeast concentration, development was delayed, the weight of emerging flies decreased, and the phenotypic variance of both variables increased. Differences among stocks and significant stock × yeast interactions indicated genetic variance for both variables within environment and different phenotypic responses of stocks across environments. The phenotypic correlation between developmental time and weight was negative at low yeast concentrations and disappeared gradually with increasing yeast. The comparison of completely homozygous with genetically heterogenous stocks showed that most of the increase of variability with deteriorating environment was due to the changing expression of genetic variance. The genetic correlation between developmental time and weight turned from negative in poor to positive in rich medium, while the environmental covariance was negative in all media. Plotting the reaction norms in the developmental time-weight plane rather than separately for each trait reveals most of these results at a glance. It also suggests that much of the genetic variance might be additive, because an effect of the half-sib family structure inherent in the design is clearly visible in the plot. We interpret the pattern of changing variances and covariances, pointing out that the special growth physiology of Drosophila and the way environmental factors affect it must be taken into account. We briefly discuss the implications of changing genetic correlations among traits for the evolution of phenotypic plasticity in general.     

NATURAL SELECTION ON QUANTITATIVE TRAITS IN THE BOMBINA HYBRID ZONE

Observations on the means, variances, and covariances of quantitative traits across hybrid zones can give information similar to that from Mendelian markers. In addition, they can identify particular traits through which the cline is maintained. We describe a survey of six traits across the hybrid zone between Bombina bombina and Bombina variegata (Amphibia: Discoglossidae) near Pe??enica in Croatia. We obtained laboratory measuments of the belly pattern, skin thickness, mating call, skeletal form, egg size, and the developmental time of tadpoles. Although offspring from hybrid populations showed no evidence of reduced viability, a third of the F₁ families failed completely, irrespective of the direction of the cross. All traits differed significantly between the taxa. Clines in belly pattern, skin thickness, mating call, and skeletal form were closely concordant with clines in four diagnostic enzyme loci. However, the cline in developmental time was displaced towards bombina, and the cline in egg size was displaced towards variegata. This discordance could be because the traits are not inherited additively or because they are subject to different selection pressures. We favor the latter explanation in the case of developmental time. We show that moderate selection acting directly on a trait suffices to shift its position; rather stronger selection is needed to change its width appreciably. Within hybrid populations, there are significant associations among quantitative traits, and between traits and enzymes. Phenotypic variances also increase in hybrid populations. These observations can be explained by linkage disequilibria among the underlying loci. However, the average magnitude of the covariance between traits is about half that expected from the linkage disequilibria between enzyme loci. The discrepancy is not readily explained by nonadditive gene action. This puzzle is now unresolved and calls for further investigation.     

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.     

Population divergence along lines of genetic variance and covariance in the invasive plant Lythrum salicaria in eastern North America

Evolution during biological invasion may occur over contemporary timescales, but the rate of evolutionary change may be inhibited by a lack of standing genetic variation for ecologically relevant traits and by fitness trade-offs among them. The extent to which these genetic constraints limit the evolution of local adaptation during biological invasion has rarely been examined. To investigate genetic constraints on life-history traits, we measured standing genetic variance and covariance in 20 populations of the invasive plant purple loosestrife (Lythrum salicaria) sampled along a latitudinal climatic gradient in eastern North America and grown under uniform conditions in a glasshouse. Genetic variances within and among populations were significant for all traits; however, strong intercorrelations among measurements of seedling growth rate, time to reproductive maturity and adult size suggested that fitness trade-offs have constrained population divergence. Evidence to support this hypothesis was obtained from the genetic variance-covariance matrix (G) and the matrix of (co)variance among population means (D), which were 79.8% (95% C.I. 77.7-82.9%) similar. These results suggest that population divergence during invasive spread of L. salicaria in eastern North America has been constrained by strong genetic correlations among life-history traits, despite large amounts of standing genetic variation for individual traits.     

QUANTITATIVE GENETICS OF SURVIVAL AND GROWTH IN IMPATIENS CAPENSIS

When variation in life-history characters is caused by many genes of small effect, then quantitative-genetic parameters may quantify constraints on rate and direction of microevolutionary change. I estimated heritabilities and genetic correlations for 16 life-history and morphological characters in two populations of Impatiens capensis, a partially self-pollinating herbaceous annual. The Madison population had little or no additive genetic variance for any of these characters, while the Milwaukee population had significant narrowsense heritabilities and genetic correlations for several traits, including adult size, which is highly correlated with fitness. All genetic correlations among fitness components were positive, hence there is no evidence for antagonistic pleiotropy among these traits. Dissimilarity of heritabilities in the two populations supports theoretical predictions that long-term changes in genetic variance-covariance patterns may occur when population sizes are small and selection is strong, as may occur in many plant species.     

DIVERGENCE AND GENETIC STRUCTURE IN ADJACENT GRASS POPULATIONS. I. QUANTITATIVE GENETICS

Additive genetic variances and covariances were estimated for life history and morphological traits in two adjacent populations of the grass, Holcus lanatus L. Significant phenotypic differentiation was found between the two populations for four of the 15 morphological attributes measured. Significant differences in genetic architecture were found between the two populations for 11 of the 13 traits for which genetic variance components could be calculated. Estimates of genetic correlations also showed considerable divergence between the populations. The genetic divergence was much larger than would have been anticipated from simple measures of phenotypic differentiation. These results show that, even in plant species with relatively large population sizes, differences in genetic variance-covariance patterns can occur between adjacent populations.     

MUTATIONAL CONTRIBUTIONS TO GENETIC VARIANCE-COVARIANCE MATRICES: AN EXPERIMENTAL APPROACH USING INDUCED MUTATIONS IN ARABIDOPSIS THALIANA

Genetic potential for evolutionary change and covariational constraints are typically summarized as the genetic variance-covariance matrix G , and there is currently debate over the extent to which G remains effectively constant during the course of adaptive evolution. However, G provides only a temporally restricted view of constraints that ignores possible biases in how new mutations affect multivariate phenotypes. We used chemical mutagenesis to study the effect of mutations as summarized by the mutational covariance matrix, M , in Arabidopsis thaliana. By introducing mutations into three isogenic strains of A. thaliana, we were able to quantify M directly as the genetic variance-covariance matrix of mutagenized lines. Induced mutations generally did not alter the means of the six morphology and life-history traits we measured, but they did affect the levels of available genetic variation and the covariances among traits. However, these effects were not consistent among the three isogenic lines; that is, there were significant differences among the lines in both the number of mutations produced by ethyl-methane-sulfonate treatment and the M matrices they induced. The evolutionary implications of the dependence of M on the number of mutations, the particular genetic background, and the mutagenic sampling of loci in the genome are discussed in light of commonly applied models of multivariate evolution and the potential for the genetic architecture itself to change in ways that facilitate the coordinated evolution of complex phenotypes.     

Genetic architecture of host use in a widely distributed, polyphagous butterfly (Lepidoptera: Papilionidae): adaptive inferences based on comparison of spatio-temporal populations

Genetic variance-covariance structure of larval performance within and among spatio-temporal populations of the widely distributed, polyphagous tiger swallowtail butterfly, Papilio glaucus , is described. Performance traits were assessed for full-sibling families on three host species: Liriodendron tulipifera, Magnolia virginiana and Prunus serotina . Mean performance varied across hosts, indicating these hosts present unique developmental environments. Although full-sibling families significantly differed in plasticity of across-hosts response in three of the five spatio-temporal populations, additive genetic variation was mostly associated with P. serotina or pupal mass. The relative lack of heritable variation in rate and length of larval development on L. tulipifera and M. virginiana was consistent with an earlier study that established host-associated geographic differentiation of P. glaucus populations. Performance appeared relatively independent across hosts and thus genetic constraints cannot be casually invoked to explain persistence of local adaptation and host specialization in the face of extensive gene flow. I promote the hypothesis that gene flow among geographically distant populations is relatively restricted and that previously established, allozyme-based estimates of panmixia are confounded by effects of Pleistocene glaciations. Significant heterogeneity of variance-covariance structure among spatio-temporal P. glaucus populations supports an interpretation of restricted gene flow and relative evolutionary independence. Despite low precision of estimates of genetic parameters, local variance-covariance structure was remarkably consistent with expectations given the presumed evolutionary history of regional populations.     

Epigenetic interactions and the structure of phenotypic variation in the cranium

Understanding the developmental and genetic basis for evolutionarily significant morphological variation in complex phenotypes such as the mammalian skull is a challenge because of the sheer complexity of the factors involved. We hypothesize that even in this complex system, the expression of phenotypic variation is structured by the interaction of a few key developmental processes. To test this hypothesis, we created a highly variable sample of crania using four mouse mutants and their wild-type controls from similar genetic backgrounds with developmental perturbations to particular cranial regions. Using geometric morphometric methods we compared patterns of size, shape, and integration in the sample within and between the basicranium, neurocranium, and face. The results highlight regular and predictable patterns of covariation among regions of the skull that presumably reflect the epigenetic influences of the genetic perturbations in the sample. Covariation between relative widths of adjoining regions is the most dominant factor, but there are other significant axes of covariation such as the relationship between neurocranial size and basicranial flexion. Although there are other sources of variation related to developmental perturbations not analyzed in this study, the patterns of covariation created by the epigenetic interactions evident in this sample may underlie larger scale evolutionary patterns in mammalian craniofacial form.     

Constancy of the G matrix in ecological time

The constancy of the genetic variance-covariance matrix (G matrix) across environments and populations has been discussed and tested empirically over the years but no consensus has so far been reached. In this paper, I present a model in which morphological traits develop hierarchically, and individuals differ in their resource allocation and acquisition patterns. If the variance in resource acquisition is many times larger than the variance in resource allocation then strong genetic correlations are expected, and with almost isometric relations among traits. As the variation in resource acquisition decreases below a certain threshold, the correlations decrease overall and the relations among traits become a function of the allocation patterns, and in particular reflecting the basal division of allocation. A strong bottleneck can break a pattern of strong genetic correlation, but this effect diminishes rapidly with increasing bottleneck size. This model helps to understand why some populations change their genetic correlations in different environments, whereas others do not, since the key factor is the relation between the variances in resource acquisition and allocation. If a change in environment does not lead to a change in this ratio, no change can be expected, whereas if the ratio is changed substantially then major changes can be expected. This model can also help to understand the constancy of morphological patterns within larger taxa as a function of constancy in resource acquisition patterns over time and environments. When this pattern breaks, for example on islands, larger changes can be expected.     

Morphological variation and covariation in mandibular molars of platyrrhine primates

Molars are highly integrated biological structures that have been used for inferring evolutionary relationships among taxa. However, parallel and convergent morphological traits can be affected by developmental and functional constraints. Here, we analyze molar shapes of platyrrhines in order to explore if platyrrhine molar diversity reflects homogeneous patterns of molar variation and covariation. We digitized 30 landmarks on mandibular first and second molars of 418 extant and 11 fossil platyrrhine specimens to determine the degree of integration of both molars when treated as a single module. We combined morphological and phylogenetic data to investigate the phylogenetic signal and to visualize the history of molar shape changes. All platyrrhine taxa show a common shape pattern suggesting that a relatively low degree of phenotypic variation is caused by convergent evolution, although molar shape carries significant phylogenetic signal. Atelidae and Pitheciidae show high levels of integration with low variation between the two molars, whereas the Cebinae/Saimiriinae, and especially Callitrichinae, show greater variation between molars and trend toward a modular organization. We hypothesize that biomechanical constraints of the masticatory apparatus, and the dietary profile of each taxon are the main factors that determine high covariation in molars. In contrast, low molar shape covariation may result from the fact that each molar exhibits a distinct ecological signal, as molars can be exposed to distinct occlusal loadings during food processing, suggesting that different selective pressures on molars can reduce overall molar integration.     

Developmental associations between traits: covariance and beyond

Statistical associations between phenotypic traits often result from shared developmental processes and include both covariation between the trait values and more complex associations between higher moments of the joint distribution of traits. In this article, an analytical technique for calculating the covariance between traits is presented on the basis of (1). the distribution of underlying genetic and environmental variation that jointly influences the traits and (2). the mechanics of how these underlying factors influence the development of each trait. It is shown that epistasis can produce patterns of covariation between traits that are not seen in additive models. Applying this approach to a trait in parents and the same trait in their offspring allows us to study the consequences of epistasis for the evolution of additive genetic variance and heritability. This analysis is then extended to the study of more complicated associations between traits. It is shown that even traits that are not correlated may exhibit developmental associations that influence their joint evolution.     

HOMOGENEITY OF THE GENETIC VARIANCE-COVARIANCE MATRIX FOR ANTIPREDATOR TRAITS IN TWO NATURAL POPULATIONS OF THE GARTER SNAKE THAMNOPHIS ORDINOIDES

Quantitative genetic models of evolution rely on the genetic variance-covariance matrix to predict the phenotypic response to selection. Both prospective and retrospective studies of phenotypic evolution across generations rely on assumptions about the constancy of patterns of genetic covariance through time. In the absence of robust theoretical predictions about the stability of genetic covariances, this assumption must be tested with empirical comparisons of genetic parameters among populations and species. Genetic variance-covariance matrices were estimated for a suite of antipredator traits in two populations of the northwestern garter snake, Thamnophis ordinoides. The characters studied include color pattern and antipredator behaviors that interact to facilitate escape from predators. Significant heritabilities for all traits were detected in both populations. Genetic correlations and covariances were found among behaviors in both populations and between color pattern and behavior in one of the populations. Phenotypic means differed among populations, but pairwise comparisons revealed no heterogeneity of genetic parameters between the populations. The structure of the genetic variance-covariance matrix has apparently not changed significantly during the divergence of these two populations.     

Serial homology and the evolution of mammalian limb covariation structure

The tetrapod forelimb and hindlimb are serially homologous structures that share a broad range of developmental pathways responsible for their patterning and outgrowth. Covariation between limbs, which can introduce constraints on the production of variation, is related to the duplication of these developmental factors. Despite this constraint, there is remarkable diversity in limb morphology, with a variety of functional relationships between and within forelimb and hindlimb elements. Here we assess a hierarchical model of limb covariation structure based on shared developmental factors. We also test whether selection for morphologically divergent forelimbs or hindlimbs is associated with reduced covariation between limbs. Our sample includes primates, murines, a carnivoran, and a chiropteran that exhibit varying degrees of forelimb and hindlimb specialization, limb size divergence, and/or phylogenetic relatedness. We analyze the pattern and significance of between-limb morphological covariation with linear distance data collected using standard morphometric techniques and analyzed by matrix correlations, eigenanalysis, and partial correlations. Results support a common limb covariation structure across these taxa and reduced covariation between limbs in nonquadruped species. This result indicates that diversity in limb morphology has evolved without signficant modifications to a common covariation structure but that the higher degree of functional limb divergence in bats and, to some extent, gibbons is associated with weaker integration between limbs. This result supports the hypothesis that limb divergence, particularly selection for increased functional specialization, involves the reduction of developmental factors common to both limbs, thereby reducing covariation.