The allometry of fluctuating asymmetry in southern African plants: flowers and leaves

Several studies of fluctuating asymmetry (FA) in animals show that secondary sexual characters used in signalling have a negative relationship between size and asymmetry. Larger sexual traits are presumably more costly to produce, which should lead to greater developmental stress and corresponding increases in asymmetry. In the absence of among individual variation in the ability to handle these costs, the relationship between size and asymmetry should thus be positive. A negative relationship therefore suggests that expression of these traits is condition-dependent. In plants, flowers act as signals for pollinators and may show similar trends to animal signals. Leaves which are uninvolved in signalling should not. Moller & Eriksson (1994) found that 89% of species ( n = 16 of 18) with insect-pollinated flowers showed a negative relationship between petal size and asymmetry, while 79% of species ( n = 15 of 19) showed a positive relationship between leaf size and asymmetry. I carried out a similar study of 18 plant species. The average relationship between petal size and asymmetry did not differ significantly from zero in those species showing measurable FA in flowers ( n = 12). The relationship was significantly negative in one species, and significandy positive in another. On average, leaves in species with FA did not show a significant positive relationship between size and asymmetry ( n = 7). There was no significant difference in the slopes of the relationship between size and asymmetry for leaves and flowers. Levels of floral asymmetry for species with FA were significandy repeatable on individual plants in 33% ( n = 4 of 12) of species, but leaf asymmetry was not significantly repeatable in any species. It is argued that condition-dependence of traits need not result in a negative relationship between size and asymmetry.     

先天性心脏病患者皮纹波动不对称性的研究

本文采用随机整体抽样的方法分析了先天性心脏病患者129例(男性59例, 女性70例) 和正常对照人群133例(男性69例, 女性64例) 13项皮纹波动不对称性(Fluctuating asymmetry, FA)的分布特征。结果表明: (1)先天性心脏病患者组与正常对照组在13项皮纹波动不对称性指标中均未出现显著性别差异; (2)先天性心脏病患者组与正常对照组在FAⅥ(P<0.05)和FAⅦ(P<0.01)两项有显著性差异, 表现为患者组明显增高, 提示先天性心脏病患者在胚胎发育早期易受到环境因素影响, 具有较高的发育不稳定性。     

精神分裂症患者皮纹a-b嵴线数波动性不对称的研究

本文分析了精神分裂症患者134例(男性70例,女性64例)和正常对照人群331例(男性170例,女性161例)皮纹a-b嵴线数波动性不对称的分布特征。结果表明:⑴精神分裂症患者双手皮纹a-b嵴线数明显低于正常对照组(P<0.01),表现出明显增高。     

Developmental stability in flowers of Clarkia tembloriensis (Onagraceae)

Developmental instability of floral traits is examined in four populations of Clarkia tembloriensis (Onagraceae) with different natural outcrossing rates. Developmental instability is estimated using fluctuating asymmetry (FA) and within plant variance. The results are coupled with those from a previous study of leaf traits. In the first experiment, flowers were collected from the same growth chamber-grown plants that had been previously used to estimate leaf developmental stability in two C. tembloriensis populations. These populations differed in FA for only one floral trait, long filament length. After adjusting for organ size differences, we found floral FA values were about half those of leaves. These are the first quantitative data indicating that flowers are more developmentally stable than leaves. In a second experiment, greenhouse grown plants from two other C. tembloriensis populations (one highly outcrossing and one predominantly self-pollinating) did not differ significantly in floral FA or in within-plant variance of floral traits, though earlier studies of the same populations revealed significant differences in FA of leaf traits. In both experiments, FA values of different floral traits were uncorrelated. We attribute the lack of significant differences in floral stability between populations to the greater canalization of floral organs and to the magnification of measurement error that occurs when calculating FA. We also found that the shorter styles of selfers are the greatest difference in flower form between predominantly self-pollinating and predominantly outcrossing populations of C. tembloriensis.     

Developmental stability,sexual selection and speciation

Fluctuating asymmetry occurs when an individual is unable to undergo identical development of an otherwise bilaterally symmetric trait on both sides of its body. Since both sides of a bilaterally symmetric trait are the result of the actions of a single genome, fluctuating asymmetry represents an epigenetic measure of the sensitivity of development to stress. Different morphological traits may show a direct relationship between their functional importance and their degree of developmental canalization. This may explain why some characters show high degrees of fluctuating asymmetry, and why these characters more often become exaggerated secondary sexual ornaments. The degree of fluctuating asymmetry is generally larger in small marginal populations living in novel environments, and this will particularly lead to relatively large degrees of asymmetry in the least developmentally canalized traits. More stringent selection against heterozygotes in marginal populations may further break down developmental stability and linkage groups which would lead to increased genetic variance. Females may prefer to mate with males having large, but relatively symmetric morphological characters, because it is more difficult to make large traits (a good genes argument), a large trait is more easily perceived (a sensory bias preference), and because symmetry signals ability to cope with stress (a good genes argument). The low degree of developmental stability and the large amount of genetic variance in secondary sexual characters in small, marginal populations could set the scene for rapid development of divergence and speciation in marginal populations.     

Morphological variability and developmental instability in subpopulations of the Eurasian badger (Meles meles) in Denmark

Aim Local populations from different geographical regions may differ in the selection regimes to which they are exposed. Differences in environmental factors and population density may affect the relative importance of different selective forces (e.g. natural vs. sexual selection). We suggest a direction of investigation concerned with the developmental instability of morphological traits. The goal is to disclose putative small‐scale geographical differences in the evolutionary forces, which may be hard to detect. Location Craniometrical investigations were carried out on ninety‐eight skulls and teeth of the Eurasian badger (Meles meles) collected during the period 1995–97 from three different populations in Denmark. One of these thrives at low population density, whereas the two others are characterized by high local density. Methods The skulls were investigated for developmental instability (DI) using fluctuating asymmetry (FA) as its estimator. FA was measured on canines, molars, premolar teeth and other skull and mandible traits. For the statistical analyses, we applied nonparametric permutation tests. Results Evidence was found suggesting differentiation among populations in mean degree of FA, and the FA values measured on canines were higher in the high‐density populations. FA of the canines was significantly higher in males than females, in contrast to FA of the other traits. Evidence of a negative relationship between canine size and their FA was found, whereas no significant correlations were found between the molar and premolar teeth measures and their FA. Main conclusions Our results suggest that canines could be under directional selection stemming from intrasexual competition, which may be stronger in high‐density zones. The other teeth investigated seem to be under a stabilizing regime hence their FA is mainly affected by environmental stresses. The negative relationship between canine size and FA found in males suggests the capacity of badgers to respond in an evolutionary way to environmental changes, despite the low genetic variability previously found at the molecular level.     

Parasites differentially increase the degree of fluctuating asymmetry in secondary sexual characters

The degree of fluctuating asymmetry has been demonstrated to reflect the ability of individuals to cope with different kinds of environmental stress (Parsons 1990). Parasites and diseases are one kind of environmental stress which most individuals encounter during their lifetime. Parasites have also been suggested to play an important role in sexual selection and the development of ornaments, since the full expression of ornaments may reflect the ability of hosts to cope with the debilitating effects of parasites. Here I report for the first time that a parasite, the haematophagous tropical fowl mite Ornithonyssus bursa (Macronyssidae, Gamasida), directly affects the degree of fluctuating asymmetry in a secondary sexual character of its host, the elongated tail of the swallow Hirundo rustica (Aves: Hirundinidae). I experimentally manipulated the mite load of swallow nests during one season by either increasing or reducing the number of mites, or keeping nests as controls. The degree of fluctuating asymmetry was measured in the subsequent year after the swallows had grown new tail ornaments under the altered parasite regime. The degree of fluctuating asymmetry was larger at increasing levels of parasites for male tail length, but not for the length of the shortest tail feather or wing length or for tail and wing length in females. These results suggest that the degree of fluctuating asymmetry in tail ornaments, but not in other feather traits, reliably reveals the level of parasite infestation. This has important implications for the ability of conspecifics to use the size and the expression of ornaments in assessment of phenotypic quality and thus in sexual selection.     

The association of three indicators of developmental instability with mating success in humans

Developmental instability (DI) has been proposed to relate negatively to aspects of evolutionary fitness, like mating success. One suggested indicator is fluctuating asymmetry (FA), random deviations from perfect symmetry in bilateral bodily traits. A meta-analytically robust negative association between FA and number of lifetime sexual partners has been previously shown in men and women. We examined the relationship between bodily FA across twelve traits and indicators of quantitative mating success in 284 individuals (141 males, age 19–30 years). Two further indicators of DI, minor physical anomalies (MPAs) and asymmetry in palmar atd angles, were also assessed. For men, no significant associations were detected, whereas for women, unexpected positive relationships of FA with the number of lifetime sexual partners and one-night stands emerged. Thus, in a large sample and using a more highly aggregated FA index, our study fails to replicate previous findings, though equivalence testing also did not support deviation from previous meta-analytic estimates, especially for men. No associations were found for MPAs and FA in atd angles in either sex.     

Fitness advantages of the biased use of paired laterally symmetrical penises in an insect

The evolution of laterality, that is the biased use of laterally paired, morphologically symmetrical organs, has attracted the interest of researchers from a variety of disciplines. It is, however, difficult to quantify the fitness benefits of laterality because many organs, such as human hands, possess multimodal functions. Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population‐level laterality. Our population studies revealed that in 10 populations, located at 2.23–43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left‐handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left‐handed males in copulation (the probability of establishing genital coupling during the 1‐hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right‐handed advantage may be due to the coiled morphology of the sperm storage organ with a right‐facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.     

Flower orientation enhances pollen transfer in bilaterally symmetrical flowers

Zygomorphic flowers are usually more complex than actinomorphic flowers and are more likely to be visited by specialized pollinators. Complex zygomorphic flowers tend to be oriented horizontally. It is hypothesized that a horizontal flower orientation ensures effective pollen transfer by facilitating pollinator recognition (the recognition-facilitation hypothesis) and/or pollinator landing (the landing-control hypothesis). To examine these two hypotheses, we altered the angle of Commelina communis flowers and examined the efficiency of pollen transfer, as well as the behavior of their visitors. We exposed unmanipulated (horizontal-), upward-, and downward-facing flowers to syrphid flies (mostly Episyrphus balteatus), which are natural visitors to C. communis. The frequency of pollinator approaches and landings, as well as the amount of pollen deposited by E. balteatus, decreased for the downward-facing flowers, supporting both hypotheses. The upward-facing flowers received the same numbers of approaches and landings as the unmanipulated flowers, but experienced more illegitimate landings. In addition, the visitors failed to touch the stigmas or anthers on the upward-facing flowers, leading to reduced pollen export and receipt, and supporting the landing-control hypothesis. Collectively, our data suggested that the horizontal orientation of zygomorphic flowers enhances pollen transfer by both facilitating pollinator recognition and controlling pollinator landing position. These findings suggest that zygomorphic flowers which deviate from a horizontal orientation may have lower fitness because of decreased pollen transfer.     

Patterns of fluctuating asymmetry in flowers: Implications for sexual selection in plants

Characters in animals used in signalling and subjected to strong directional selection often demonstrate (i) an elevated level of fluctuating asymmetry (small random deviations from bilateral symmetry) and (ii) a negative relationship between the degree of individual fluctuating asymmetry and the size of a given character. We tested these two predictions in plants since flowers are subjected to strong directional selection and are involved in signalling to pollinators, whereas leaves are supposed not to be directly involved in signalling. The overall level of fluctuating asymmetry in a number of plant species with bilaterally or radially symmetric flowers was not generally higher in floral traits than in leaves. The level of fluctuating asymmetry in plants was sometimes significantly consistent within individuals. The absolute degree of individual fluctuating asymmetry in floral traits was generally negatively related to the size of the trait, while there was a positive relationship for leaves. The degree of individual fluctuating asymmetry in floral traits was marginally negatively related to the degree of individual fluctuating asymmetry in leaf traits. These patterns of fluctuating asymmetry in plants suggest that (i) the degree of asymmetry in flowers signals different aspects of quality than does the degree of asymmetry in leaves, and that (ii) fluctuating asymmetry in flowers often reflects the phenotypic quality of individual plants.     

Tolerance assessment of Cistus ladanifer to serpentine soils by developmental stability analysis

Developmental instability is the result of random environmental perturbations during development. Its absence (developmental stability) depends on an organism's ability to buffer environmental disturbances. Both genotype and environment influence the phenotypic expression of developmental instability and it is susceptible to selection pressure. We studied developmental instability (as indicated by increased within-individual asymmetry of repeated traits) in vegetative and reproductive structures of three populations of Cistus ladanifer L. living in different soil substrates (serpentine, siliceous and contact zone) to detect tolerance to serpentine soils. Serpentine soils, characterized by high concentrations of heavy metals (Ni, Cr, and Co), low levels of Ca/Mg ratio and high water deficit, can adversely affect plant performance. In this study we demonstrated that asymmetry and within-plant variance were higher in the contact zone population than either the silica or serpentine populations, proving the adaptation of C. ladanifer to serpentine soils. Within-population estimates of developmental instability were concordant for both vegetative and reproductive traits. There was little or no within-individual correlation among estimates of developmental instability based on different structures, i.e., plants that had highly asymmetric leaves always had high developmental instability in translational symmetry. Radial asymmetry of petals was negatively correlated with petal size, especially in silica soil plants, providing evidence of selection for symmetric and large petals. While leaf size was positively correlated with absolute fluctuating asymmetry, suggesting selection for small or intermediate size leaves. Serpentine soils presented the largest foliar and floral traits, as well as shoot elongation, while silica soil plants had the smallest scores. On the contrary, aboveground plant biomass was larger in silica soil plants, while the contact zone plants had the lowest biomass.     

Human fluctuating asymmetry in relation to health and quality: a meta-analysis

Developmental instability (DI) reflects the inability of a developing organism to buffer its development against random perturbations, due either to frequent, large perturbations or to a poor buffering system. The primary measure used to assess DI experienced by an individual organism is fluctuating asymmetry (FA), asymmetry of bilateral features that are, on average in a population, symmetrical. A large literature on FA in humans in relation to measures of health and quality (close to 100 studies and nearly 300 individual effect size estimates) has accumulated. This paper presents the first quantitative meta-analysis of this literature. The mean effect size (scaled as Pearson r) was about 0.2. Effect sizes covaried negatively with sample size, consistent with effects of publication bias, the tendency for significant effects to be published. Conservative correction for this bias reduced the mean effect to about 0.1. Associations with FA underestimate effects of underlying DI due to imprecise measurement of the latter. A model-based best estimate of the mean effect of DI on outcomes is about 0.3, a theoretically meaningful, relatively large effect, albeit of moderate absolute size. The data are consistent, however, with a range of true effect sizes between 0.08 and 0.67, partly due to large study effects. Study-specific effect sizes in DI ranged between −0.2 and 1.0. A humbling and perhaps sobering conclusion is that, in spite of a large body of literature involving nearly 50 000 participants, we can only confidently state that there is on average a robust positive average effect size. An accurate estimate of that effect size was not possible, and between-study variation remained largely unexplained. We detected no robust variation across six broad categories of outcomes (health and disease, fetal outcomes, psychological maladaptation, reproduction, attractiveness and hormonal effects), though examination of narrower domains reveal some corrected effects close to 0.2 and others near zero. The meta-analysis suggests fruitful directions for future research and theory.     

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.     

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.     

Change in male coloration associated with artificial selection on foraging colour preference

Sensory drive proposes that natural selection on nonmating behaviours (e.g. foraging preferences) alters sensory system properties and results in a correlated effect on mating preferences and subsequently sexual traits. In colour‐based systems, we can test this by selecting on nonmating colour preferences and testing for responses in colour‐based female preferences and male sexual coloration. In guppies (Poecilia reticulata), individual functional links of sensory drive have been demonstrated providing an opportunity to test the process over more than one link. We measured male coloration and female preferences in populations previously artificially selected for colour‐based foraging behaviour towards two colours, red and blue. We found associated changes in male coloration in the expected direction as well as weak changes in female preferences. Our results can be explained by a correlated response in female preferences due to artificial selection on foraging preferences that are mediated by a shared sensory system or by other mechanisms such as colour avoidance, pleiotropy or social experiences. This is the first experimental evidence that selection on a nonmating behaviour can affect male coloration and, more weakly, female preferences.     

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.