Genetic structure of phenotypic robustness in the collaborative cross mouse diallel panel

Developmental stability and canalization describe the ability of developmental systems to minimize phenotypic variation in the face of stochastic micro‐environmental effects, genetic variation and environmental influences. Canalization is the ability to minimize the effects of genetic or environmental effects, whereas developmental stability is the ability to minimize the effects of micro‐environmental effects within individuals. Despite much attention, the mechanisms that underlie these two components of phenotypic robustness remain unknown. We investigated the genetic structure of phenotypic robustness in the collaborative cross (CC) mouse reference population. We analysed the magnitude of fluctuating asymmetry (FA) and among‐individual variation of cranial shape in reciprocal crosses among the eight parental strains, using geometric morphometrics and a diallel analysis based on a Bayesian approach. Significant differences among genotypes were found for both measures, although they were poorly correlated at the level of individuals. An overall positive effect of inbreeding was found for both components of variation. The strain CAST/EiJ exerted a positive additive effect on FA and, to a lesser extent, among‐individual variance. Sex‐ and other strain‐specific effects were not significant. Neither FA nor among‐individual variation was associated with phenotypic extremeness. Our results support the existence of genetic variation for both developmental stability and canalization. This finding is important because robustness is a key feature of developmental systems. Our finding that robustness is not related to phenotypic extremeness is consistent with theoretical work that suggests that its relationship to stabilizing selection is not straightforward.     

EVOLUTION OF VARIATION AND VARIABILITY UNDER FLUCTUATING,STABILIZING, AND DISRUPTIVE SELECTION

How variation and variability (the capacity to vary) may respond to selection remain open questions. Indeed, effects of different selection regimes on variational properties, such as canalization and developmental stability are under debate. We analyzed the patterns of among‐ and within‐individual variation in two wing‐shape characters in populations of Drosophila melanogaster maintained under fluctuating, disruptive, and stabilizing selection for more than 20 generations. Patterns of variation in wing size, which was not a direct target of selection, were also analyzed. Disruptive selection dramatically increased phenotypic variation in the two shape characters, but left phenotypic variation in wing size unaltered. Fluctuating and stabilizing selection consistently decreased phenotypic variation in all traits. In contrast, within‐individual variation, measured by the level of fluctuating asymmetry, increased for all traits under all selection regimes. These results suggest that canalization and developmental stability are evolvable and presumably controlled by different underlying genetic mechanisms, but the evolutionary responses are not consistent with an adaptive response to selection on variation. Selection also affected patterns of directional asymmetry, although inconsistently across traits and treatments.     

Consequences of inter-population crosses on developmental stability and canalization of floral traits in Dalechampia scandens (Euphorbiaceae)

Congruence between changes in phenotypic variance and developmental noise in inter-population hybrids was analysed to test whether environmental canalization and developmental stability were controlled by common genetic mechanisms. Developmental stability assessed by the level of fluctuating asymmetry (FA), and canalization by the within- and among-individual variance, were measured on several floral traits of Dalechampia scandens (Euphorbiaceae). Hybridization affected canalization. Both within- and among-individual phenotypic variance decreased in hybrids from populations of intermediate genetic distance, and strongly increased in hybrids from genetically distant populations. Mean-trait FA differed among cross-types, but hybrids were not consistently more or less asymmetric than parental lines across traits. We found no congruence between changes in FA and changes in phenotypic variance. These results suggest that developmental stability (measured by FA) and canalization are independently controlled. This study also confirms the weak relationship between FA and the breakdown of coadapted gene complexes following inter-population hybridization.     

Canalization and developmental instability of the fetal skull in a mouse model of maternal nutritional stress

Nutritional imbalance is one of the main sources of stress in both extant and extinct human populations. Restricted availability of nutrients is thought to disrupt the buffering mechanisms that contribute to developmental stability and canalization, resulting in increased levels of fluctuating asymmetry (FA) and phenotypic variance among individuals. However, the literature is contradictory in this regard. This study assesses the effect of prenatal nutritional stress on FA and among‐individual variance in cranial shape and size using a mouse model of maternal protein restriction. Two sets of landmark coordinates were digitized in three dimensions from skulls of control and protein restricted specimens at E17.5 and E18.5. We found that, by the end of gestation, maternal protein restriction resulted in a significant reduction of skull size. Fluctuating asymmetry in size and shape exceeded the amount of measurement error in all groups, but no significant differences in the magnitude of FA were found between treatments. Conversely, the pattern of shape asymmetry was affected by the environmental perturbation since the angles between the first eigenvectors extracted from the covariance matrix of shape asymmetric component of protein restricted and control groups were not significantly different from the expected for random vectors. In addition, among‐individual variance in cranial shape was significantly higher in the protein restricted than the control group at E18.5. Overall, the results obtained from a controlled experiment do not support the view of fluctuating asymmetry of cranial structures as a reliable index for inferring nutritional stress in human populations. Am J Phys Anthropol 154:544–553, 2014. © 2014 Wiley Periodicals, Inc.     

Developmental stability and canalization of limb bones of brown haresLepus europaeus with varying levels of heterozygosity

We have studied fluctuating asymmetry (FA), as indicator of developmental stability, and between-individual variation, as surrogate of developmental canalization (DC), in long bones (humerus, ulna, radius, femur, tibia) of 72 wild-living adult-sized brown haresLepus europaeus Pallas, 1778 with variable individual heterozygosity (H).H was calculated from 13 polymorphic allozyme loci. According to the “over-dominance hypothesis”, we expected increased developmental stability and canalization at higherH-levels. But at the individual level we did not find any significant correlation between overall FA (FA_I) andH. Also, standard deviations (SD) of mean length (over both body sides) of bones did not differ between individuals from two intentionally created groups of hares, namely one with high and one with lowH. FA-indices and variances of FA-indices of bone lengths did not differ significantly when compared between two intentionally created groups of hares with high and low SD of bone lengths, respectively. These latter findings suggest that developmental stability and DC are two separate or partly separate mechanisms of developmental homeostasis in the studied appendicular skeleton, and thatH has no traceable effect on develop-mental homeostasis. If there is still such an effect, it should be clearly smaller than a possibly combined effect of (presently uncontrolled) environmental stressors.     

Geographical variation in asymmetry in Gasterosteus aculeatus

We tested whether fluctuating asymmetry (FA) in undisturbed populations is associated with several natural environmental factors and whether FA is negatively correlated with fitness in the wild. We compared the FA of multiple bony structural defences among 87 endemic populations of threespine stickleback ( Gasterosteus aculeatus L.) inhabiting pristine freshwater habitats on the islands of Haida Gwaii, British Columbia, Canada. Multi-trait FA for adults and juvenile fish varied extensively among populations, but only in adults did it correlate with geography and two habitat characteristics (pH and water colour). Mean FA among individual traits varied concordantly among populations but was not correlated within individuals. While asymmetrical fish showed slightly higher levels of parasitism as predicted, selection differentials based on age class comparisons suggested that asymmetrical fish had the same or marginally higher survival than symmetrical fish. Selection differentials of FA varied significantly among traits and may reflect variability in their functional importance and in the strength of selection on their developmental stability. The data imply that FA/fitness associations are heterogeneous and character-specific.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 9–22.     

Effect of mutations on developmental stability and canalization in morphological traits in Drosophila ananassae

The present study was designed to determine the effects of visible mutations of large effect on developmental stability and canalization in different morphological traits, namely, sternopleural bristle number, wing length, wing to thorax ratio, ovariole number, and sex comb tooth number (SCTN) in Drosophila ananassae. We have compared the mean trait size, fluctuating asymmetry (FA) (as an index of developmental stability), and morphological variation (as an index of canalization) of different mutant strains (yellow body color, y; claret eye color, ca; plexus wing, px; spread wing, spr; ebony body and sepia eye color, e se; yellow body and claret eye color, y ca; and cardinal eye color, curled wing, and ebony body color, cd cu e) with wild-type strain. The mean trait size of all morphological traits differs significantly among the wild-type and mutant strains. The wild-type and mutant strains vary significantly for the morphological variation and also for the levels of the FA in different morphological traits. However, we have found no increase in either the variance or in the degree of FA with the increase of the mutations (except in SCTN in y mutant). The plausible reasons for the variation in wild-type and mutant strains with particular reference to developmental stability and canalization have been discussed.     

Asymmetric size and shape variation in the Central European transect across the house mouse hybrid zone

We studied asymmetric variation of the mandible in the Central European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus. Within introgression classes, defined by the share of diagnostic allozymes, we quantified the directional and fluctuating component of asymmetric variation, as well as skewness and kurtosis of individual asymmetry distributions. Furthermore, in the same manner we re‐analysed asymmetric variation of the ventral side of the skull. According to the quadratic polynomial model, the mandible shape‐fluctuating asymmetry, but not size‐fluctuating asymmetry, was significantly decreased in the centre of the hybrid zone (with a minimum predicted for a hybrid index of 0.41). On the contrary, the skull shape‐fluctuating asymmetry non‐monotonically increased towards the musculus side of the hybrid zone (with a peak predicted for a hybrid index of 0.86). Thus, the impact of hybridization on fluctuating asymmetry is trait‐specific in this portion of the house mouse hybrid zone. The only general feature of asymmetric variation we observed was the shift towards the platykurtosis of asymmetry distributions in the centre of the hybrid zone. Taken together, we suggest genetic variability for right–left asymmetries to be generally increased, but the developmental instability of mandible shape to be decreased, by hybridization. We hypothesize the decrease of developmental instability to be caused by overdominant effects on developmental dynamics rather than by increased heterozygosity. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 13–27.     

How accurate is the phenotype? – An analysis of developmental noise in a cotton aphid clone

Background  

The accuracy by which phenotype can be reproduced by genotype potentially is important in determining the stability, environmental sensitivity, and evolvability of morphology and other phenotypic traits. Because two sides of an individual represent independent development of the phenotype under identical genetic and environmental conditions, average body asymmetry (or "fluctuating asymmetry") can estimate the developmental instability of the population. The component of developmental instability not explained by intrapopulational differences in gene or environment (or their interaction) can be further defined as internal developmental noise. Surprisingly, developmental noise remains largely unexplored despite its potential influence on our interpretations of developmental stability, canalization, and evolvability. Proponents of fluctuating asymmetry as a bioindicator of environmental or genetic stress, often make the assumption that developmental noise is minimal and, therefore, that phenotype can respond sensitively to the environment. However, biologists still have not measured whether developmental noise actually comprises a significant fraction of the overall environmental response of fluctuating asymmetry observed within a population.     

PLASTICITY, CANALIZATION, AND DEVELOPMENTAL STABILITY OF THE DROSOPHILA WING: JOINT EFFECTS OF MUTATIONS AND DEVELOPMENTAL TEMPERATURE

The phenotypic effects of genetic and environmental manipulations have been rarely investigated simultaneously. In addition to phenotypic plasticity, their effect on the amount and directions of genetic and phenotypic variation is of particular evolutionary importance because these constitute the material for natural selection. Here, we used heterozygous insertional mutations of 16 genes involved in the formation of the Drosophila wing. The flies were raised at two developmental temperatures (18°C and 28°C). Landmark-based geometric morphometrics was used to analyze the variation of the wing size and shape at different hierarchical levels: among genotypes and temperatures; among individuals within group; and fluctuating asymmetry (FA). Our results show that (1) the phenotypic effects of the mutations depend on temperature; (2) reciprocally, most mutations affect wing plasticity; (3) both temperature and mutations modify the levels of FA and of among individuals variation within lines. Remarkably, the patterns of shape FA seem unaffected by temperature whereas those associated with individual variation are systematically altered. By modifying the direction of available phenotypic variation, temperature might thus directly affect the potential for further evolution. It suggests as well that the developmental processes responsible for developmental stability and environmental canalization might be partially distinct.     

The evolutionary potential of developmental instability

Whether or not developmental instability (DI) has evolutionary potential is subject to much debate. Generally, studies fail to detect significant heritability for fluctuating asymmetry (FA), a trait assumed to reflect DI. In addition, between‐trait correlations in FA are low, suggesting that DI is trait‐ rather than individual‐specific. Among the various attempts to explain these patterns, the overall weak correlation between FA and DI at the individual level has received most attention. Presently, the concept of hypothetical repeatability (R) of individual FA allows us to correct for this weak relationship, transforming patterns of FA into unbiased patterns of DI. By applying R to data presented in the literature, we show that heritability of DI remains lower than predicted but between‐trait correlations in DI substantially increase after transformation. We further provide evidence that DI changes from a trait‐ to an individual‐specific property with higher values of R. As increasing hypothetical repeatability might co‐occur with increased environmental or genetic stress, we discuss the potential implications of our results for the study of evolution of stress resistance. From this we conclude that there is an urgent need for studies that compare the evolutionary potential of developmental instability under a variety of stress conditions.     

A meta-analysis of the heritability of developmental stability

The existence of additive genetic variance in developmental stability has important implications for our understanding of morphological variation. The heritability of individual fluctuating asymmetry and other measures of developmental stability have frequently been estimated from parent-offspring regressions, sib analyses, or from selection experiments. Here we review by meta-analysis published estimates of the heritability of developmental stability, mainly the degree of individual fluctuating asymmetry in morphological characters. The overall mean effect size of heritabilities of individual fluctuating asymmetry was 0.19 from 34 studies of 17 species differing highly significantly from zero (P < 0.0001). The mean heritability for 14 species was 0.27. This indicates that there is a significant additive genetic component to developmental stability. Effect size was larger for selection experiments than for studies based on parent-offspring regression or sib analyses, implying that genetic estimates were unbiased by maternal or common environment effects. Additive genetic coefficients of variation for individual fluctuating asymmetry were considerably higher than those for character size per se. Developmental stability may be significantly heritable either because of strong directional selection, or fluctuating selection regimes which prevent populations from achieving a high degree of developmental stability to current environmental and genetic conditions.     

Fluctuating asymmetry--indicator of what?

In a population the optimal phenotype is promoted by buffering mechanisms that keep inter- and intra-individual variation low. A link exists between canalization, that controls phenotypic variation, and developmental stability, mostly measured as fluctuating asymmetry of bilateral traits (FA). Both types of variation are associated with the functional importance of a trait, and both are increased by stress of various kinds. But there are also several instances of non-congruence. The concept of developmental stability has been found elusive, and low FA is not the unambiguous measure of well being and good genes that has been claimed. It can be concluded that developmental stability is partly governed by specific, as yet unknown, molecular processes.     

Inbreeding,developmental stabilty,and canalization in the sand cricket Gryllus firmus

Inbreeding, the mating of close relatives, is known to have deleterious effects on fitness traits in organisms. Developmental stability (DS) and canalization may represent two processes that allow an organism to maintain a stable development that will produce the fittest phenotype. Inbreeding is thus expected to affect either DS or canalization. We tested if inbreeding affects DS and canalization using an inbreeding experiment on the cricket Gryllus firmus. We compared mean length, fluctuating asymmetry (as an index of DS), and morphological variation (as an index of canalization) of four limb traits between seven highly inbred lines, their F1 crosses, and outbred lines originated from the same stock population and maintained in the same environmental conditions. We show evidence for moderate inbreeding depression on the four measures of leg length. The nonsystematic difference in fluctuating asymmetry indices between breed types indicates that inbreeding or heterozygosity did not affect DS, or that fluctuating asymmetry is not a reliable index of DS. In contrast, inbreeding appears to affect canalization, as shown by the significantly higher variation in inbred lines compared to other lines. Identical low variation values in the crossbred and outbred lines indicate that heterozygosity could affect canalization. High variation in morphological variation and fluctuating asymmetry within crossbred or inbred lines, however, suggest the effect of recessive deleterious alleles on both canalization and DS. Although the strong correlation in morphological variation among traits suggests that identical genetic mechanisms govern canalization for all the limb traits, the absence of significant correlation in fluctuating asymmetry among traits causes us to reject this hypothesis for DS. For most of the traits, morphological variation and fluctuating asymmetry were not significantly correlated, which support the hypothesis that canalization and DS consist in two distinct mechanisms.     

Patterns of leaf asymmetry changes in Plantago major L. (ssp. major) natural populations exposed to different environmental conditions

In the present study, developmental stability of leaf traits was examined in three natural populations of Plantago major L. (ssp. major), representing two polluted environments (Karaburma and Zemun) and an unpolluted area (Crni Lug). Developmental stability was assessed as fluctuating asymmetry (FA). The magnitude of FA is believed to reflect differences in the ability of individuals to buffer their development in natural populations. We hypothesized that there are differences within characters and among characters in response to environmental conditions. Significant patterns of asymmetry correlations and asymmetry changes were detected both within characters and between characters. The manova results revealed a significant effect of individual and a significant individual × environment interaction on actual asymmetry (logL_i ? logR_i) and on the amount of asymmetry |(logL_i ? logR_i)| for leaf width and vein distances within a leaf. Over time, statistically significant and positive correlations of the FA values were detected for each trait separately per sample (population). For both leaf traits, there were differences for (logL_i ? logR_i) and |(logL_i ? logR_i)| asymmetry values among individuals within samples in response to yearly variations. Statistically significant and negative correlations for (logL_i ? logR_i) versus |(logL_i ? logR_i)| asymmetries were detected for both leaf traits. In summary, our results highlight the importance of differences in the ability of individuals to buffer their development under different environmental conditions and point to the concept that developmental stability is character specific.     

Geography of fluctuating asymmetry in the greenfinch, Carduelis chloris

Levels of fluctuating asymmetry (FA) in 12 bilateral skeletal traits were estimated from 12 populations of greenfinches (Carduelis chloris) collected along a north‐south gradient across Europe. Average FA of measured traits was positively correlated with latitude indicating that the younger and genetically less diverse northern European populations are developmentally less stable than the older and genetically more diverse southern populations. Levels of FA differed significantly between different traits being lowest for functionally important traits (limb and wing bones) and highest for functionally less important traits such as foramina (apertures through bones)– a pattern that was highly concordant across different populations. Males tended to exhibit higher levels of FA than females, a finding consistent with the suggestions that males are more prone to developmental perturbations than females. Age differences in levels of FA were relatively clear, but inconsistent across traits with different degree of functionality. Individual heterozygosity – as enumerated from variation in allozyme loci – was unrelated to individual FA. No evidence for existence of individual asymmetry parameter (IAP) was found although traits related to locomotion indicated some degree of integration, which was expressed by correlations in the signed asymmetry. Nevertheless, an individual's overall asymmetry was poorly predicted by asymmetry of individual characters. Evidence for existence of population asymmetry parameter (PAP) was clear since all traits exhibited a similar degree of association with latitude. That the latitudinal cline of increasing FA towards north coincided with decreasing levels of genetic variability across the cline could be indicative of break down of developmental stability in the recently established and genetically impoverished populations. To what extent a reduced heterozygosity, the break up of co‐adapted gene complexes and/or environmental differences contributed to this process cannot be distinguished from our data.     

Associations between leaf developmental stability,variability, canalization,and phenotypic plasticity in Abutilon theophrasti

Developmental stability, canalization, and phenotypic plasticity are the most common sources of phenotypic variation, yet comparative studies investigating the relationships between these sources, specifically in plants, are lacking. To investigate the relationships among developmental stability or instability, developmental variability, canalization, and plasticity in plants, we conducted a field experiment with Abutilon theophrasti, by subjecting plants to three densities under infertile vs. fertile soil conditions. We measured the leaf width (leaf size) and calculated fluctuating asymmetry (FA), coefficient of variation within and among individuals (CV_intra and CV_inter), and plasticity (PI_rel) in leaf size at days 30, 50, and 70 of plant growth, to analyze the correlations among these variables in response to density and soil conditions, at each of or across all growth stages. Results showed increased density led to lower leaf FA, CV_intra, and PI_rel and higher CV_inter in fertile soil. A positive correlation between FA and PI_rel occurred in infertile soil, while correlations between CV_inter and PI_rel and between CV_inter and CV_intra were negative at high density and/or in fertile soil, with nonsignificant correlations among them in other cases. Results suggested the complexity of responses of developmental instability, variability, and canalization in leaf size, as well as their relationships, which depend on the strength of stresses. Intense aboveground competition that accelerates the decrease in leaf size (leading to lower plasticity) will be more likely to reduce developmental instability, variability, and canalization in leaf size. Increased developmental instability and intra‐ and interindividual variability should be advantageous and facilitate adaptive plasticity in less stressful conditions; thus, they are more likely to positively correlate with plasticity, whereas developmental stability and canalization with lower developmental variability should be beneficial for stabilizing plant performance in more stressful conditions, where they tend to have more negative correlations with plasticity.     

Do defoliating insects distinguish between symmetric and asymmetric leaves within a plant?

1. Plants represent a highly heterogeneous resource for herbivores. One dimension of this heterogeneity is reflected by the within‐plant variation in the leaf fluctuating asymmetry (FA), i.e. in the magnitude of the random deviations from the symmetry in leaf shape. 2. This study is the first to test experimentally the hypothesis that variation in the quality of individual leaves for defoliating insects (11 species) within a plant (seven tree and shrub species) is associated with the FA of these leaves. 3. It was demonstrated that specialist defoliators generally distinguish between nearly symmetric (low FA) and highly asymmetric (high FA) leaves, but do not distinguish between discs cut from these leaves. Low‐FA leaves of Salix caprea, Salix myrsinifolia and Populus tremula were of better quality for insects than high‐FA leaves, as demonstrated by both preference tests and performance trials. By contrast, high‐FA leaves of Betula pubescens were of better quality for insects than low‐FA leaves, whereas insects feeding on Alnus incana showed similar responses to high‐ and low‐FA leaves. 4. It is concluded that insect herbivores can distinguish between leaves with high and low FA, and that FA may be associated with the quality of an individual leaf for insects, although the direction and strength of the effect of leaf FA on insect preference and performance vary among study systems. The ecological significance of substantial within‐plant variation in leaf FA remains to be explored.     

Macro-alga population shows low but significant heterogeneity in developmental instability with no detectable association with individual fitness

We studied among-individual variation in developmental instability (DI) and fitness-related parameters in 80 individual plants of Fucus vesiculosus (Phaeophyta). To minimize differential environmental effects, plants were sampled from one environmentally homogenous population. DI was measured as fluctuating asymmetry (FA) of four bilaterally symmetric traits (branch length, receptacle length and width, and bladder width) for an average total of 30 structures per individual. FA levels varied significantly among individual plants, consistent with a coefficient of variation of 0.12 for organism-wide DI or 0.13–0.21 for trait-specific DI. These values are lower than estimates for other organisms, suggesting that the genetic heterogeneity in DI was low. The data provide some evidence for organism-wide DI, but simulations show that organism-wide and trait-specific variation cannot be conclusively separated. Growth rate of branch tips was determined experimentally, demonstrating significant variation among individuals. FA was not significantly correlated with growth rate or with morphological variables associated with fecundity, age, size, and health. At the same time, the signs of all the correlation coefficients were consistent with the expectation of a negative relationship between DI and fitness. The simulations indicated that the correlation between FA and the underlying DI was comparatively strong (high hypothetical repeatability), implying that the lack of significant associations between FA and fitness variables reflected a weak relationship between DI and these fitness parameters. This weak relationship may be related to the low amount of DI variation in the study population.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 277–286.