Evidence for organism-wide asymmetry in five bird species of a fragmented afrotropical forest

Empirical evidence for between-trait correlation in fluctuating asymmetry (FA) at the individual level is generally lacking or contradictory. Yet the assumption of organism-wide asymmetry, estimated by the asymmetry of any given trait, is inherent to most studies that use FA as a measure of developmental stability (DS). A commonly assumed reason for this weak between-trait correlation is the low repeatability of individual, single-trait asymmetry. In this paper we describe high repeatability and significant between-trait correlation in population- and individual-level FA in five afrotropical bird species inhabiting a fragmented cloud forest. Absence of anti-symmetry and of between-trait correlation in signed FA levels permits us to translate the observed patterns into the presumed underlying DS, using the concept of hypothetical repeatability. This correction, which has not been applied before in this context, proved adequate as it yielded correlations comparable to those found at the population level.     

How repeatable is the estimation of developmental stability by fluctuating asymmetry?

The estimation of individual fluctuating asymmetry (FA) is subject to large sampling variabilities. Heritability estimates, as well as correlations between developmental stability and any other individual character and/or between-trait correlations, are consequently biased downward if FA is used as an estimate of an individual''s ability to buffer its development against developmental noise. The estimation of the hypothetical repeatability, defined as the ratio of the between-individual component of variation in the unsigned FA divided by the total variance, allows correction for these biases such that patterns observed for FA can be translated to make inferences about the presumed underlying developmental stability. In this paper I show that previous estimates of this repeatability are incorrect. I provide a new method and show by means of simulations that the hypothetical repeatability is in most cases even lower than previously thought. This has important consequences for the analysis of FA with respect to statistical power and the interpretation of patterns in FA.     

Variation in bilateral asymmetry of the Lusitanian toadfish along the Portuguese coast

Three types of bilateral asymmetry [antisymmetry (AS), directional asymmetry (DA) and fluctuating asymmetry (FA)] have long been recognized in the literature. It is often assumed that DA and AS have a larger genetic basis than FA, which therefore provides a measure of developmental stability. To examine changes in bilateral asymmetry of the Lusitanian toadfish, Halobatrachus didactylus (Bloch & Schneider, 1801), collected in 10 localities along the Portuguese coast, the distribution of left–right values of three external and two internal morphometric variables and one meristic variable were evaluated for normality and deviations of the mean to 0; kurtosis and skewness were also determined. AS was found for the majority of variables in all locations, although DA was also very frequent. FA was observed in only four locations for three variables. A general pattern of negative and low Spearman correlations between asymmetry levels and concentrations of heavy metals and pollutants and mean heterozygosity was found. However, exceptions to this pattern indicate that different asymmetry types can occur across the distribution range in a single trait and that the three types of asymmetry are dynamically interrelated, the shifts from FA to the other two types possibly being due to environmental stress.     

Heritability of directional and fluctuating asymmetry for mandibular characters in random-bred mice

In bilateral characters, two kinds of asymmetries are common: fluctuating asymmetry (FA), or nondirectional variation between left and right sides, and directional asymmetry (DA), in which one side is consistently larger than the other. FA has been extensively used as a measure of developmental stability because of its presumed environmental basis whereas DA has not typically been recommended because it has been presumed to have at least some genetic basis. To test these two hypotheses, heritabilities were calculated via parent–offspring regression for both DA and FA in 10 triply measured mandible characters in random-bred mice. Midparent estimates of heritabilities of DA in the 10 characters were quite low (mean = 0.06), but significant for one character as well as the sum of the DA values over all characters (0.21). Midparent estimates of heritability of FA in the 10 characters also were low (mean = 0.03), but not significant for any individual character or the sum of the FA values over all characters. Heritabilities of developmental stability calculated from heritabilities and repeatabilities of FA in the mandible characters were higher in magnitude (mean of midparent estimates = 0.45), but all still were not statistically significant. It was concluded that both hypotheses were supported, but that genetic variation in DA was so small that the potential for DA as an indicator of developmental stability should be explored.     

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.     

High variation in developmental instability under non-normal developmental error: a Bayesian perspective

The developmental mechanisms behind developmental instability (DI) are only poorly understood. Nevertheless, fluctuating asymmetry (FA) is often used a surrogate for DI. Based on statistical arguments it is often assumed that individual levels of FA are only weakly associated with the underlying DI. Patterns in FA therefore need to be interpreted with caution, and should ideally be transformed into patterns in DI. In order to be able to achieve that, assumptions about the distribution of developmental errors must be made. Current models assume that errors during development are additive and independent such that they yield a normal distribution. The observation that the distribution of FA is often leptokurtic has been interpreted as evidence for between-individual variation in DI. This approach has led to unrealistically high estimates of between-individual variation in DI, and potentially incorrect interpretations of patterns in FA, especially at the individual level. Recently, it has been suggested that the high estimates of variation in DI may be biased upward because either developmental errors are log-normal or gamma distributed and/or low measurement resolution of FA. A proper estimation of the amount (and shape) of heterogeneity in DI is crucial for the interpretation of patterns in FA and their transformation into patterns in DI. Yet, incorrect model assumptions may render misleading inferences. We therefore develop a statistical model to evaluate the sensitivity of results under the normal error model against the two alternative distributions as well as to investigate the importance of low measurement resolution. An analysis of simulated and empirical data sets indicated that bias due to misspecification of the developmental error distribution can be substantial, yet, did not appear to reduce estimates of variation in DI in empirical data sets to a large extent. Effects of low measurement resolution were neglectable. The importance of these results are discussed in the context of the interpretation of patterns in FA.     

The normal distribution as appropriate model of developmental instability in Opuntia cacti flowers

Developmental instability (DI) as measured by fluctuating asymmetry (FA) has been proposed to reflect fitness and stress. Furthermore, the associated developmental buffering may reduce morphological variation, conceal the expression of genetic variation and as such play an important role in evolutionary biology. However, observed associations between FA and various forms of stress and quality appear very heterogeneous. Presently it is difficult to interpret the biological relevance of this heterogeneity because little is known about the link between FA and the underlying process of DI, casting doubt whether DI can be viewed as an individual property and how closely FA reflects the underlying process of DI. Therefore, studies that explicitly test the validity of assumptions of the proposed theoretical models and estimate between‐individual variations in DI are needed. We present data on Opuntia cacti floral traits confirming that the normal distribution can be viewed as an appropriate approximation of the distribution of DI and that the concept of hypothetical repeatability can provide useful insights into the interpretation of patterns in FA as a measure of DI. Furthermore, we detected significant between‐individual variation in DI. Measuring petals from several flowers within individual plants allowed making inference of individual DI.     

Unbiased estimation of individual asymmetry

The importance of measurement error (ME) for the estimation of population level fluctuating asymmetry (FA) has long been recognized. At the individual level, however, this aspect has been studied in less detail. Recently, it has been shown that the random slopes of a mixed regression model can estimate individual asymmetry levels that are unbiased with respect to ME. Yet, recent studies have shown that such estimates may fail to reflect heterogeneity in these effects. In this note I show that this is not the case for the estimation of individual asymmetry. The random slopes adequately reflect between‐individual heterogeneity in the underlying developmental instability. Increased levels of ME resulted in, on average, lower estimates of individual asymmetry relative to the traditional unsigned asymmetry. This well‐known shrinkage effect in Bayesian analysis adequately corrected for ME and heterogeneity in ME resulting in unbiased estimates of individual asymmetry that were more closely correlated with the true underlying asymmetry.     

The repeatability of fluctuating asymmetry: a revision and extension

Fluctuating asymmetry (FA) is an imperfect measure of the developmental stability of an individual. Estimates of the heritability of FA will underestimate the heritability of developmental stability; the extent of this bias can be estimated and corrected by estimating the repeatability of developmental stability. This note corrects an error in a previous derivation of this repeatability using the absolute value of the difference between sides as the measure of FA, and derives the repeatability of developmental stability for another common measure of FA, the variance among sides within an individual.     

Correlated development, organism‐wide asymmetry and patterns of asymmetry in two moth species

Understanding the mechanisms that determine the development of a bilaterally symmetrical trait is crucial to the interpretation of patterns of fluctuating asymmetry (FA). Experimental and theoretical studies have indicated that feedback mechanisms both within and between developing traits, may participate in the developmental control of asymmetry. This study provides evidence that naturally occurring patterns of FA are affected by interactions between different traits. We found positive between‐trait correlations in signed FA values for tibia lengths on different legs, but not between wing and tibia FA in two moth species. Further research should investigate if trait functionality is related to this presumed correlated development. An extension of the Rashevsky–Turing model of morphogenesis further showed that correlations between the signed FA values can be generated by feedback mechanisms that regulate growth patterns between traits. We argue that such feedback mechanisms can be expected to be widespread and show that between‐trait correlations in the unsigned FA then become confounded with correlations in the signed FA. In addition, correlated development appeared to invalidate the use of the hypothetical repeatability to translate correlations between the unsigned FA values into correlations in the presumed underlying developmental instability. In conclusion, the presence of an organism‐wide asymmetry, which are most frequently found in morphologically integrated traits, may be even less common than previously thought. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quantitative-genetic analysis of wing form and bilateral asymmetry in isochromosomal lines ofDrosophila subobscura using Procrustes methods

Fluctuating asymmetry (FA) is often used as a measure of underlying developmental instability (DI), motivated by the idea that morphological variance is maladaptive. Whether or not DI has evolutionary potential is a highly disputed topic, marred by methodological problems and fuzzy prejudices. We report here some results from an ongoing study of the effects of karyotype, homozygosity and temperature on wing form and bilateral asymmetry using isochromosomal lines ofDrosophila subobscura. Our approach uses the recently developed methodologies in geometric morphometrics to analyse shape configurations of landmarks within the standard statistical framework employed in studies of bilateral asymmetries, and we have extended these methods to partition the individual variation and the variation in asymmetries into genetic and environmental causal components. The analyses revealed temperaturedependent expression of genetic variation for wing size and wing shape, directional asymmetry (DA) of wing size, increased asymmetries at suboptimal temperature, and a transition from FA to DA in males as a result of increase in the rearing temperature. No genetic variation was generally detected for FA in our samples, but these are preliminary results because no crosses between lines were carried out and, therefore, the contribution of dominance was not taken into account. In addition, only a subset of the standing genetic variation was represented in the experiments.     

Assemblage and population-level consequences of forest fragmentation on bilateral asymmetry in tropical montane birds

Habitat fragmentation has the potential to influence the development and thus the phenotype of organisms. The asymmetry of bilateral traits may be indicative of the extent to which developmental stability is compromised by the stressful conditions underlying fragmentation. Using an assemblage- and population-level approach, we explored asymmetry differences in tarsus and outermost tail feathers of birds inhabiting fragmented landscapes in the tropical Andes of Colombia. More than 2500 individuals of 185 species were mist-netted at nine forest sites representing continuous forest (> 1000 ha), medium- (70–110 ha), and small-sized (8–20 ha) fragments. Feathers showed true fluctuating asymmetry (FA), whereas tarsus presented a mixture of FA and directional asymmetry. Overall, asymmetry was lowest in continuous forest, and highest in small and medium fragments. These patterns remained unchanged when directionality and differences in species composition, abundance, and foraging tactics were considered. The population-level analyses showed a general trend of increased asymmetry variation in fragments, yet the responses were not always in the same direction. Increased asymmetry may represent an outcome of processes that contribute to the persistence of species in changing environments, and to the generation of phenotypic innovation, which suggests individual adjustments of development to deal with stress. This calls into question the deliberated application of FA as a biomonitoring tool for conservation.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 119–133.     

Fluctuating asymmetry and developmental instability in evolutionary biology: past, present and future

The role of developmental instability (DI), as measured by fluctuating asymmetry (FA), in evolutionary biology has been the focus of a wealth of research for more than half a century. In spite of this long period and many published papers, our current state of knowledge reviewed here only allows us to conclude that patterns are heterogeneous and that very little is known about the underlying causes of this heterogeneity. In addition, the statistical properties of FA as a measure of DI are only poorly grasped because of a general lack of understanding of the underlying mechanisms that drive DI. If we want to avoid that this area of research becomes abandoned, more efforts should be made to understand the observed heterogeneity, and attempts should be made to develop a unifying statistical protocol. More specifically, and perhaps most importantly, it is argued here that more attention should be paid to the usefulness of FA as a measure of DI since many factors might blur this relationship. Furthermore, the genetic architecture, associations with fitness and the importance of compensatory growth should be investigated under a variety of stress situations. In addition, more focus should be directed to the underlying mechanisms of DI as well as how these processes map to the observable phenotype. These insights could yield more efficient statistical models and a unified approach to the analysis of patterns in FA and DI. The study of both DI and canalization is indispensable to obtain better insights in their possible common origin, especially because both have been suggested to play a role in both micro- and macro-evolutionary processes.     

Individual differences in developmental precision and fluctuating asymmetry: a model and its implications

In many studies, fluctuating asymmetry (FA) has been used as a measure of individual differences in developmental imprecision. A model of how variation in developmental imprecision is associated with variation in asymmetry is described and applied to important issues about FA. If individual differences in developmental imprecision exist, asymmetry due to developmental error should be leptokurtically distributed. Moreover, the greater the magnitude of individual differences, the greater the leptokurtosis. Asymmetry purportedly due to developmental error in a variety of species is indeed leptokurtically distributed. The level of leptokurtosis suggests that the CV in individual differences in underlying developmental imprecision is generally 20–25, consistent with it being a fitness trait. In addition, data suggest that: (1) the individual differences that underlie the developmental imprecision of different traits are largely shared across traits and not trait-specific; (2) the heritability of these individual differences may average between 35 and 55%, despite small heritabilities of individual trait FAs; and (3) correlations between FA and fitness traits or components suggest high correlations between underlying variation in developmental precision and fitness in many species. Theoretical implications are discussed.     

Lack of response to selection for lower fluctuating asymmetry of mutant eyespots in the butterfly Bicyclus anynana

Fluctuating asymmetry (FA) is claimed both to provide a means of evaluating developmental stability, and to reflect an individual's quality or the stress experienced during development. FA refers to the nondirectional variation between left and right sides, whereas directional asymmetry (DA) refers to a significant directional variation between the sides. We studied four eyespots on the dorsal forewing of the tropical butterfly, Bicyclus anynana. Two of the eyespots were specified by a mutant allele, Spotty, that was fixed in the stock. These eyespots showed higher FA than the two flanking, wild-type eyespots, although they are all formed by the same developmental pathway. We applied artificial selection for lower FA of the novel eyespots in an attempt to increase their developmental stability. There was significant variation present in individual FA in our study. However, this did not change as a result of the artificial selection. Most of the variation in FA can be accounted for by individual differences in developmental stability rather than by the applied selection or by environmental variation. Thus, it was not possible to produce any increased developmental stability of the novel eyespots by selecting for low FA. The estimates of realized heritability for both FA and DA of each eyespot were not significantly different from zero. The results suggest that FA provides little, if any, potential for exploring the mechanistic basis of developmental stability.     

On hidden heterogeneity in directional asymmetry – can systematic bias be avoided?

Directional asymmetry (DA) biases the analysis of fluctuating asymmetry (FA) mainly because among-individual differences in the predisposition for DA are difficult to detect. However, we argue that systematic bias mainly results from predictable associations between signed right-left asymmetry and other factors, i.e. from systematic variation in DA. We here demonstrate methods to test and correct for this, by analysing bilateral asymmetry in size and shape of an irregular sea urchin. Notably, in this model system, DA depended significantly on body length and geographic origin, although mean signed asymmetry (mean DA) was not significant in the sample as a whole. In contrast to the systematic variation in DA, undetectable, random variability in the underlying DA mainly leads to reduced statistical power. Using computer simulations, we show that this loss of power is probably slight in most circumstances. We recommend future studies on FA to routinely test and correct for not only as yet for mean DA, but also for systematic variation in DA.     

Morphological asymmetry of insular freshwater populations of threespine stickleback

Fluctuating asymmetry (FA), random deviations from perfect symmetry in a bilateral organism, has been widely used as a proxy for developmental instability in stressed populations. In order to test the utility of FA of resident freshwater threespine stickleback (Gasterosteus aculeatus) as a biomonitoring tool for contaminated sites, we compared levels of asymmetry of seven morphological traits of threespine stickleback collected from lakes at three islands with a history of military contamination and three islands with no military history, in the Aleutian Archipelago, Alaska. Traits examined include eye diameter, operculum width, pectoral fin ray number, pectoral fin width, lateral plate number, lateral plate length, and pelvic spine length. All morphometric traits demonstrated some degree of FA or directional asymmetry (DA), but the military history of a lake was not a predictor of the degree or type of asymmetry. Overall, the patterns of asymmetry were similar for all traits, irrespective of military contamination at a lake. Our results demonstrate that the suite of threespine stickleback traits measured are not suitable for examination of FA as a proxy for aquatic pollution in this region. DA also does not appear to be suitable as an indicator of aquatic pollution, but may instead be driven by local ecological factors such as predation.     

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.     

Fluctuating asymmetry and age in children: evolutionary implications for the control of developmental stability

Fluctuating asymmetry (FA) is small deviations from perfect symmetry which reflect one component of fitness, i.e. developmental stability. There is accumulating evidence that low FA is important in inter- and intra-sexual selection in humans. However, there is little information on the pattern of FA in children. Data from cross-sectional studies of 680 participants from 2–18 years suggests that (1) both absolute and relative FA reduces with age until 10 years (2) there is an increase in FA in adolescents (11–15 years) with a peak at 13 years for males and 14 years for females (3) after 15 years there is reduction in FA which is maintained until 18 years. The importance of growth rate, metabolic maintenance and sex steroids on developmental stability is discussed.     

Response of fluctuating and directional asymmetry to selection on wing shape in Drosophila melanogaster

We tested whether directional selection on an index-based wing character in Drosophila melanogaster affected developmental stability and patterns of directional asymmetry. We selected for both an increase (up selection) and a decrease (down selection) of the index value on the left wing and compared patterns of fluctuating and directional asymmetry in the selection index and other wing traits across selection lines. Changes in fluctuating asymmetry across selection lines were predominantly small, but we observed a tendency for fluctuating asymmetry to decrease in the up-selected lines in both replicates. Because changes in fluctuating asymmetry depended on the direction of selection, and were not related to changes in trait size, these results fail to support existing hypotheses linking directional selection and developmental stability. Selection also produced a pattern of directional asymmetry that was similar in all selected lines whatever the direction of selection. This result may be interpreted as a release of genetic variance in directional asymmetry under selection.