FLUCTUATING ASYMMETRY IN CENTRAL AND MARGINAL POPULATIONS OF LYCHNIS VISCARIA IN RELATION TO GENETIC AND ENVIRONMENTAL FACTORS

Directional asymmetry and direction-giving factors: Lessons from flowers with complex symmetry

Directional asymmetry is a systematic difference between the left and right sides for structures with bilateral symmetry or a systematic differentiation among repeated parts for complex symmetry. This study explores factors that produce directional asymmetry in the flower of Iris pumila, a structure with complex symmetry that makes it possible to investigate multiple such factors simultaneously. The shapes and sizes of three types of floral organs, the falls, standards, and style branches, were quantified using the methods of geometric morphometrics. For each flower, this study recorded the compass orientations of floral organs as well as their anatomical orientations relative to the two spathes subtending each flower. To characterize directional asymmetry at the whole-flower level, differences in the average sizes and shapes according to compass orientation and relative orientation were computed, and the left-right asymmetry was also evaluated for each individual organ. No size or shape differences within flowers were found in relation to anatomical position; this may relate to the terminal position of flowers in Iris pumila, suggesting that there may be no adaxial-abaxial polarity, which is very prominent in many other taxa. There was clear directional asymmetry of shape in relation to compass orientation, presumably driven by a consistent environmental gradient such as solar irradiance. There was also clear directional asymmetry between left and right halves of every floral organ, most likely related to the arrangement of organs in the bud. These findings indicate that different factors are acting to produce directional asymmetry at different levels. In conventional analyses not recording flower orientations, these effects would be impossible to disentangle from each other and would probably be included as part of fluctuating asymmetry.

Long-term stress level in a small mammal species undergoing range expansion

Rapid climate change is currently altering species distribution ranges. Evaluating the long-term stress level in wild species undergoing range expansion may help better understanding how species cope with the changing environment. Here, we focused on the white-footed mouse (Peromyscus leucopus), a widespread small mammal species in North-America whose distribution range is rapidly shifting northward. We evaluated long-term stress level in several populations of P. leucopus in Quebec (Canada), from the northern edge of the species distribution to more core populations in Southern Quebec. We first tested the hypothesis that populations at the range margin are under higher stress than more established populations in the southern region of our study area. We then compared four measures of long-term stress level to evaluate the congruence between these commonly used methods. We did not detect any significant geographical trend in stress level across our study populations of P. leucopus. Most notably, we found no clear congruence between the four measures of stress level we used, and conclude that these four commonly used methods are not equivalent, thereby not comparable across studies.

Inbreeding and competition, but not abiotic stresses, increase fluctuating asymmetry of Mimulus guttatus flowers

Genetic and environmental disturbances are expected to increase developmental instability, which may result in higher fluctuating asymmetry (FA), i.e. small random deviations from symmetry. Plant leaves often do not show this pattern, possibly due to high phenotypic plasticity of leaf shape and low adaptive significance of leaf symmetry. In contrast, symmetry in many animal traits but also in flower shape is considered to be under selection, and FA in such traits may better reflect developmental instability. Using geometric morphometrics, I analysed the symmetry of flowers of inbred and outbred Mimulus guttatus (Phrymaceae) plants grown under five stress treatments with and without grass competition. Flower FA was not increased by abiotic stress, but by inbreeding and competition. As inbreeding and competition affected different principal components of flower FA, different mechanisms may be involved in their effects on FA. FA decreased with individual biomass particularly in selfed offspring, which suggests that inbreeding increased FA particularly when growth was limited by environmental or genetic constraints. Increased flower FA of inbred offspring may explain increased flower handling time and reduced pollinator preference for inbred plants in other M. guttatus studies, and could thus have important consequences for plant demography and plant–pollinator interactions.

Phenotypic plasticity in response to environmental heterogeneity contributes to fluctuating asymmetry in plants: first empirical evidence

Fluctuating asymmetry (FA) is widely used to quantify developmental instability (DI) in ecological and evolutionary studies. It has long been recognized that FA may not exclusively originate from DI for sessile organisms such as plants, because phenotypic plasticity in response to heterogeneities in the environment might also produce FA. This study provides the first empirical evidence for this hypothesis. We reasoned that solar irradiance, which is greater on the southern side than on the northern side of plants growing in the temperate zone of the Northern Hemisphere, would cause systematic morphological differences and asymmetry associated with the orientation of plant parts. We used geometric morphometrics to characterize the size and shape of flower parts in Iris pumila grown in a common garden. The size of floral organs was not significantly affected by orientation. Shape and particularly its asymmetric component differed significantly according to orientation for three different floral parts. Orientation accounted for 10.4% of the total shape asymmetry within flowers in the falls, for 11.4% in the standards and for 2.2% in the style branches. This indicates that phenotypic plasticity in response to a directed environmental factor, most likely solar irradiance, contributes to FA of flowers under natural conditions. That FA partly results from phenotypic plasticity and not just from DI needs to be considered by studies of FA in plants and other sessile organisms.

Testing the relationship between human occupancy in the landscape and tadpole developmental stress

Amphibian population declines are widespread; the main causal factors are human related and include habitat fragmentation due to agriculture, mining, fires, and urban development. Brazil is the richest country in species of amphibians, and the Brazilian regions with the greatest amphibian diversity are experiencing relatively high rates of habitat destruction, but there are presently relatively few reports of amphibian declines. It is thus important to develop research methods that will detect deterioration in population health before severe declines occur. We tested the use of measurements of fluctuating asymmetry (FA) taken on amphibian larvae to detect anthropogenic stress. We hypothesized that greater human occupancy in the landscape might result in more stressful conditions for amphibians. We conducted this study at the Espinhaço mountain range in southeastern Brazil, using as a model an endemic species (Bokermannohyla saxicola, Hylidae). We chose two tadpole denticle rows and eye-nostril distance as traits for FA measurement. We measured percent cover of human-altered habitats in the landscape around tadpole sampling points and measured FA levels in sampled tadpoles. We found FA levels to differ among localities but found no relationship between human modification of the landscape and tadpole FA levels. Levels of FA in the traits we examined may not be strongly affected by environmental conditions, or may be affected by local variables that were not captured by our landscape-scale measures. Alternatively, populations may be genetically differentiated, affecting how FA levels respond to stress and obscuring the effects of anthropogenic disturbance.

High levels of fluctuating asymmetry in isolated stickleback populations

Background

Fluctuating asymmetry (FA), defined as small random deviations from the ideal bilateral symmetry, has been hypothesized to increase in response to both genetic and environmental stress experienced by a population. We compared levels of FA in 12 bilateral meristic traits (viz. lateral-line system neuromasts and lateral plates), and heterozygosity in 23 microsatellite loci, among four marine (high piscine predation risk) and four pond (zero piscine predation risk) populations of nine-spined sticklebacks (Pungitius pungitius).

Results

Pond sticklebacks had on average three times higher levels of FA than marine fish and this difference was highly significant. Heterozygosity in microsatellite markers was on average two times lower in pond (H_E ≈ 0.3) than in marine (H_E ≈ 0.6) populations, and levels of FA and heterozygosity were negatively correlated across populations. However, after controlling for habitat effect on heterozygosity, levels of FA and heterozygosity were uncorrelated.

Conclusions

The fact that levels of FA in traits likely to be important in the context of predator evasion were elevated in ponds compared to marine populations suggests that relaxed selection for homeostasis in ponds lacking predatory fish may be responsible for the observed habitat difference in levels of FA. This inference also aligns with the observation that the levels of genetic variability across the populations did not explain population differences in levels of FA after correcting for habitat effect. Hence, while differences in strength of selection, rather than in the degree of genetic stress could be argued to explain habitat differences in levels of FA, the hypothesis that increased FA in ponds is caused by genetic stress cannot be rejected.

Beyond bilateral symmetry: geometric morphometric methods for any type of symmetry

BACKGROUND

Studies of symmetric structures have made important contributions to evolutionary biology, for example, by using fluctuating asymmetry as a measure of developmental instability or for investigating the mechanisms of morphological integration. Most analyses of symmetry and asymmetry have focused on organisms or parts with bilateral symmetry. This is not the only type of symmetry in biological shapes, however, because a multitude of other types of symmetry exists in plants and animals. For instance, some organisms have two axes of reflection symmetry (biradial symmetry; e.g. many algae, corals and flowers) or rotational symmetry (e.g. sea urchins and many flowers). So far, there is no general method for the shape analysis of these types of symmetry.

RESULTS

We generalize the morphometric methods currently used for the shape analysis of bilaterally symmetric objects so that they can be used for analyzing any type of symmetry. Our framework uses a mathematical definition of symmetry based on the theory of symmetry groups. This approach can be used to divide shape variation into a component of symmetric variation among individuals and one or more components of asymmetry. We illustrate this approach with data from a colonial coral that has ambiguous symmetry and thus can be analyzed in multiple ways. Our results demonstrate that asymmetric variation predominates in this dataset and that its amount depends on the type of symmetry considered in the analysis.

CONCLUSIONS

The framework for analyzing symmetry and asymmetry is suitable for studying structures with any type of symmetry in two or three dimensions. Studies of complex symmetries are promising for many contexts in evolutionary biology, such as fluctuating asymmetry, because these structures can potentially provide more information than structures with bilateral symmetry.

Developmental stability covaries with genome-wide and single-locus heterozygosity in house sparrows

Fluctuating asymmetry (FA), a measure of developmental instability, has been hypothesized to increase with genetic stress. Despite numerous studies providing empirical evidence for associations between FA and genome-wide properties such as multi-locus heterozygosity, support for single-locus effects remains scant. Here we test if, and to what extent, FA co-varies with single- and multilocus markers of genetic diversity in house sparrow (Passer domesticus) populations along an urban gradient. In line with theoretical expectations, FA was inversely correlated with genetic diversity estimated at genome level. However, this relationship was largely driven by variation at a single key locus. Contrary to our expectations, relationships between FA and genetic diversity were not stronger in individuals from urban populations that experience higher nutritional stress. We conclude that loss of genetic diversity adversely affects developmental stability in P. domesticus, and more generally, that the molecular basis of developmental stability may involve complex interactions between local and genome-wide effects. Further study on the relative effects of single-locus and genome-wide effects on the developmental stability of populations with different genetic properties is therefore needed.

Limited phenotypic plasticity in range-edge populations: a comparison of co-occurring populations of two Agrimonia species with different geographical distributions

Increased importance of genetic drift and selection for stress resistance have been predicted to lead to a reduction in the degree of phenotypic plasticity in populations at margins of a species' geographical range, relative to those in the centre. We examined the effect of population positioning within the species range on degree of active morphological plasticity to vegetation shade. Importantly, we discriminated between active, size-independent morphological adjustments in response to shade and passive changes in morphology caused by the dependence of morphological traits on plant size, as only the former are considered to be adaptive. Two closely related and ecologically similar Agrimonia species were examined in the same geographical location, where one species reaches the edge of its distribution (Agrimonia pilosa) and the other does not (A. eupatoria). Plasticity to light availability is likely to be advantageous for both species as they occupy habitats with variable light conditions. However, we hypothesised that high levels of environmental stress should lead to reduced active plasticity in marginal compared with more central populations. Agrimonia eupatoria exhibited active adjustments in leaf morphology in response to tree shade, and in elongation of stems and inflorescences in response to herbaceous shade. In contrast, A. pilosa exhibited very limited active plasticity. High active plasticity allowed A. eupatoria to retain constant shoot growth in a wide range of light conditions, while the lack of active plasticity in A. pilosa resulted in a strong dependence of shoot growth on light availability. We propose that high levels of environmental stress in marginal areas of a species' range may lead to a significant reduction in the degree of active plasticity. Our results clearly indicate that discrimination between active and passive plasticity is crucial for reaching valid conclusions about differences in adaptive plasticity between marginal and non-marginal populations.

Relationship between heterozygosity and asymmetry: a test across the distribution range

The genetic basis of developmental stability, as measured by bilateral asymmetry, has been debated for over 50 years among developmental and evolutionary biologists. One of the central theories dealing with this relationship suggests that higher levels of genetic diversity, as reflected in heterozygosity, result in increased stability during development and thus in lower asymmetry. In this study, we aimed to test the relationship between asymmetry and heterozygosity at two levels: (1) the population level, where mean heterozygosity within a population is predicted to be negatively correlated with mean population asymmetry and (2) the individual level, where the proportion of heterozygous loci of an individual and its bilateral asymmetry estimates are predicted to be negatively correlated. While previous studies often focused on local populations, work across species ranges can answer the following questions. Are levels of heterozygosity correlated with levels of developmental instability, as estimated by bilateral asymmetry? Are patterns consistent across the distribution range, from the periphery towards the core? Does the relationship between genetic stress and bilateral asymmetry depend on the degree of environmental stress? We tested heterozygosity levels in 26 loci and asymmetry in third toe length in 11 populations of the chukar partridge (Alectoris chukar) across a sharp climatic gradient in Israel from the arid periphery, through the Mediterranean-desert ecotone towards the Mediterranean areas located further away from the range boundaries. Genetic diversity, as estimated using both observed and expected heterozygosity, was not associated with asymmetry at either the population or at the individual level. Whereas heterozygosity showed a hump-shaped pattern, peaking at the ecotone, asymmetry monotonically increased towards the range periphery. We argue that whereas asymmetry may serve as a useful tool for estimating changes in environmental stress, it may not be widely applicable for estimating genetic stress.

Food stress, nestling growth, and fluctuating asymmetry

It has been suggested that fluctuating asymmetry (FA) is an indicator of perturbed development. Our study tests the validity of a fundamental assumption of asymmetry theory: that environmental stress imposed during development increases FA. We deprived tree swallow (Tachycineta bicolor) nestlings of food for 3 h every morning from days 3 to 15 of brood age. We obtained 2 repeated measurements of the right and left ninth primaries, outer rectrices, and tarsi, and one measure of body mass on days 8, 12, and 16. Food-deprived nestlings had significantly smaller character sizes (all characters for all days), significantly lower wing and tail feather growth rates, and significantly higher tarsal and mass growth rates than control nestlings. However, food deprivation did not cause experimental nestlings to exhibit significantly greater FA than control nestlings. Nestling mass and FA were not associated within treatments or across broods, and brood size had no significant effect upon the character size, growth rate, or FA level in nestlings. Measurement errors were small and size scaling, antisymmetry, and directional asymmetry did not confound FA estimates. Our results suggest that the ninth primaries and outer rectrices of tree swallows are unreliable indicators of FA-stress relations because of strong stabilizing selection for symmetry in this aerially insectivorous species.

Morphological Developmental Stability in Plants: Patterns and Causes

Minor bilateral or radial asymmetry of leaves or flowers, the frequency of phenodeviants, intraindividual variation in repeated characters, and fractal dimensions of morphology are considered to represent measures of developmental instability since deviations from regularity of the phenotype constitute a measure of the inability to maintain developmental precision during ontogeny. First, we review patterns of fluctuating asymmetry in plants and show that levels of asymmetry are considerably greater than in animals. While petal asymmetry tends to decrease with petal size within species, leaf asymmetry tends to increase with leaf size. Intraspecific correlations of petal asymmetry and leaf asymmetry are weakly positive. Second, a meta-analysis of the effects of environmental factors hypothesized to increase asymmetry in leaves and flowers, such as radiation, ultraviolet light, excess artificial fertilizer, pollutants, extreme saline conditions, herbivory, and competition, showed intermediate (i.e., explaining 10% of the variance) to large (i.e., explaining 25% of the variance) effects. Third, a meta-analysis of the effects of genetic factors hypothesized to contribute to increased asymmetry in plants, such as homozygosity, hybridization, mutation, and quantitative genetic differences among individuals, showed variable but usually significant effects, although the number of studies generally was small. Controlled experimental studies of environmental and genetic effects on developmental instability of plants may increase our understanding of the mechanisms causing developmental instability.

A meta-analysis of fluctuating asymmetry in relation to heterozygosity

Fluctuating asymmetry, the random departure from perfect bilateral symmetry, is a common measure of developmental instability that has been hypothesized to be inversely correlated with heterozygosity. Although this claim has been widely repeated, several studies have reported no such association. Therefore, we test the generality of this association, using meta-analysis, by converting test statistics for the relationship between heterozygosity (H) and fluctuating asymmetry (FA) into a common effect size, the Pearson's product-moment correlation coefficient. We have analysed a database containing 41 studies with a total of 118 individual samples. Overall we found an unweighted mean negative effect size; r=-0.09 (i. e. a negative correlation between H and FA). Significant heterogeneity in effect size was mainly caused by a difference between ectothermic and endothermic animals, and to a lesser extent by the use of different study designs (i.e. within-population vs. among-populations). Mean effect size for endothermic animals was positive and significantly different from the mean effect size for ectothermic animals. Only for within-population studies of ectothermic animals did we find a significantly negative effect size (r=-0.23 +/- 0.09). The distribution of effect sizes in relation to sample size provided little evidence for patterns typical of those produced by publication bias. Our analysis suggests, at best, only a weak association between H and FA, and heterozygosity seems to explain only a very small amount of the variation in developmental instability among individuals and populations (r2=0.01 for the total material).