Does fluctuating asymmetry of wing traits capture relative environmental stress in a lepidopteran?

1. Fluctuating asymmetry (FA) is hypothesized to be a useful predictor of population canalization, especially for organisms at risk from environmental change.
2. Identification of traits that meet statistical criteria as FA measures remains a challenge.
3. Here, a laboratory experiment subjected immature butterflies (Vanessa cardui) to diet and temperature conditions of varying stress levels. Variation in dietary macronutrient ratio (protein: carbohydrate) and rearing temperature (optimal: 25°C; elevated: 32°C) was introduced as stressors. Temperature and nutrition are key variables influencing ectotherm growth and fitness and so are likely to be important stressors that influence FA.
4. Individuals subjected to stressful conditions were predicted to show elevated FA of three wing size traits, as well as increased mortality and decreased adult body size.
5. Trait FA did not vary across treatments. Instead, treatment levels impacted viability: The combined incidence of pupal death and expression of significant wing malformations increased in treatment levels designated as stressful. Variation in adult dry mass also reflected predicted stress levels. Results suggest that individuals predicted to display increased FA either died or displayed gross developmental aberrations.
6. This experiment illustrates important constraints on the investigation of FA, including selection of appropriate traits and identification of appropriate levels of stressors to avoid elevated mortality. The latter concern brings into question the utility of FA as an indicator of stress in vulnerable, natural populations, where stress levels cannot be controlled, and mortality and fitness effects are often not quantifiable.

     

Feather development under environmental stress: lead exposure effects on growth patterns in Great Tits Parus major

Capsule Regrowth rate of tail feathers is more strongly affected compared to feather length and symmetry.

Aims To assess the value of avian feathers as bioindicators.

Methods The origin and persistence of fluctuating asymmetry (FA) in homologous pairs of regrowing feathers was studied in captive birds under different levels of environmental stress, respresented by exposure to lead (Pb). Homologous feathers of individually housed birds were plucked synchronously or with a delay of seven days. We measured growth rate, regeneration time, final size and FA of regrown feathers and related them to Pb stress.

Results Asymmetry decreased as feathers reached their final length. This was not due to compensatory growth but rather a consequence of the programmed growth trajectory of single feathers. Tail feathers grown under higher Pb pollution showed increased regeneration times, decreased growth rates and shorter lengths, but no changes in development times nor in FA. For differences between both (i) original and induced feathers and (ii) control and Pb treatment, effect sizes of parameters related to feather development (growth bar width, growth rate, regeneration and development time) were consistently larger than those related to the resulting phenotype (feather length and FA).

Conclusions Growth bar widths in particular provide an applicable, sensitive and reliable indicator of adverse conditions such as Pb pollution and experimental conditions. In general, phenotypic characteristics that retrospectively allow estimation of growth rates may be more suitable for monitoring environmental stress than sizes or asymmetries of full-grown traits.     

Fluctuating asymmetry in antlers of fallow deer (Dama dama): the relative roles of environmental stress and sexual selection

Antler lengths were recorded of a total of 250 male fallow deer (Dama dama (L)). Animals sampled were from marked populations where the majority of individuals were of known age. Asymmetry in antler length was normally distributed with a mean not significantly different from zero, confirming that differences in length between the antler pair constitute a true fluctuating asymmetry (FA). We found no clear relationship between the degree of asymmetry in antler length of an individual male and either population density or actual body mass. We did, however, detect a significant relationship between asymmetry and deviation from maximum cohort bodyweight. If deviation from maximum weight within a cohort may be considered some index of competitive success or increasing environmental stress, this may suggest that asymmetry in antler length relates in some way to developmental stress suffered by the individual concerned. The degree of asymmetry recorded in antler length also showed a significant decline with animal age, with antlers of animals of 2 years or older showing significantly greater symmetry. This is consistent with a hypothesis that despite continued competition for resources, there is a changing balance of selection pressure as animals reach maturity, with increased pressure from sexual selection requiring males to produce significantly more symmetrical ornaments.     

Functional ecological implications of intraspecific differences in wing melanization in Colias butterflies

Variation in the degree of insect wing melanin affects thermoregulation, and is expected to be adapted to local environmental conditions, for example over an elevational gradient. The effects of melanization on flight activity and egg maturation rate were assessed in the closely related butterflies Colias philodice eriphyle and C. eurytheme using experimental manipulation of wing darkness and transplant experiments between high and low elevation sites. Experimental manipulation of wing darkness in C. p. eriphyle demonstrated that light males had reduced flight activity at high elevations, and darkened males had reduced flight activity at low elevations. In contrast, the transplant experiments revealed asymmetrical adaptation for male C. p. eriphyle . At high elevations darker, high-elevation males had higher flight activity than lighter, low-elevation males, but there was no difference between the two groups at low elevation. For females, melanization had no effect on flight activity. However, an increase in female C. eurytheme wing darkness led to a significantly higher egg maturation rate at cold ambient temperatures, which may increase female reproductive output under natural conditions. Therefore, dispersers moving down in elevation may be more successful than those moving up.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 79–87.     

Developmental stability and morphological differences in Plantago major L. leaves under contrasting environmental conditions

Abstract

In the present study, an additional combination of end‐points was applied on the natural populations of the common plantain, previously estimated using morphometric assays. Here, besides measuring developmental instability (DI), by determining the level of fluctuating asymmetry (FA) and the total amount of phenotypic variability (PV), we tried to distinguish the three natural populations under contrasting environmental conditions using the morphological data. Results obtained using both FA indices were the same; higher asymmetry levels in the reference than in the polluted environments were detected for leaf width, vein distances within a leaf and lobe length. The one‐way analysis of variance results revealed that there were significant differences in PV values among populations analysed for each character. When all leaf traits were considered together, the PV median value was significantly higher in Crni Lug leaves compared with leaves from other sites. The multivariate analysis of variance results revealed the significant effect of environment on both FA₄ and PV values. The component scores of first factor (PC1) were significantly different between the Karaburma and Crni Lug populations. Besides, component scores of both PC1 and PC2 were significantly different between the Zemun and Crni Lug samples. The stepwise discriminant functional analysis results allowed us to identify a set of four variables, with a sufficient discriminating ability (75%).     

Developmental stability in yellow dung flies (Scathophaga stercoraria): fluctuating asymmetry,heterozygosity and environmental stress

The genetic basis for developmental stability, the ability of an organism to withstand genetic and environmental disturbance of development, is poorly understood. Fluctuating asymmetry (FA: small random deviations from symmetry in paired, bilateral traits) is the most widely used measure of developmental stability, and evidence suggests FA is weakly and negatively associated with genome‐wide heterozygosity. We investigated the genetic basis of developmental stability in the yellow dung fly. Fly lines were inbred for 16 generations at which time they were homozygous at the phosphoglucomutase (PGM) loci and PGM appears to influence FA in at least one other taxon. After 16 generations of inbreeding, lines homozygous for different PGM alleles were crossed and levels of FA for four metric traits were compared in the inbred and crossed flies. We also compared FA levels in these flies with previously gathered data on wild‐type (second generation outcrossed) flies, and additionally looked at the effects of two environmental stresses (larval food limitation and increased temperature) on FA. There were no significant differences in any measure of FA, nor in mean FA, in any trait when inbred and crossed flies were compared. Comparison of FA in these and wild flies also revealed no significant differences. Food limitation had no influence on FA, whereas heat stress increased FA of naturally, but not sexually, selected traits. Our results do not show a negative relationship between heterozygosity and FA, but support the notion that FA levels are stress, trait and taxon specific.     

Deficiency screening for genomic regions with effects on environmental sensitivity of the sensory bristles of Drosophila melanogaster

Environmental canalization is defined as a reduction in the effect of external environmental perturbations on a phenotype, while phenotypic plasticity is defined as the production of different phenotypes in alternative environments. These terms describe different aspects of the same phenomenon, that is, the sensitivity of the phenotype to the environment. Genetic regulation of the environmental sensitivity has been a central topic in the field of evolutionary biology. In this study, we performed deficiency screening to detect genomic regions with effects on the environmental sensitivity of Drosophila melanogaster sensory bristles. We used a collection of isogenic deficiency strains established by the DrosDel Project for screening. We screened 423 genomic deficiencies that encompassed approximately 63.6% of the entire D. melanogaster genome. We identified 29 genomic deficiencies showing significant effects on environmental sensitivity, suggesting that multiple genomic regions may influence phenotypic variation. We also found significant correlations among the effects of deficiencies on environmental sensitivity for different bristle traits, suggesting that the same genetic mechanism can regulate environmental sensitivity of multiple traits. Current high-resolution mapping will facilitate the examination of individual candidate genes using mutations or RNAi approaches in future studies.     

Genetics of wing size asymmetry in Drosophila buzzatii

Contemporary approaches that use fluctuating asymmetry (FA) as a possible target for natural and sexual selection are based on the premise that FA is a quantifiable expression of developmental instability (DI) that is inherited. Previous work with Drosophila buzzatii found that male mating success was correlated positively to body size (wing length) and negatively to FA, but these relationships seem to be environmentally induced. Heritability of FA was low and not significantly different from zero, but statistical power was also estimated to be very low and, hence, no conclusive evidence could be obtained. A large half‐sib mating design is used here to examine the relationships of different aspects of development for wing size. Consistently with previous findings, I found high heritabilities for wing length (WL) and wing width (WW), and positive correlations between both traits. Heritabilities of FA (FA_WL, FA_WW) were low (0.037) but significantly different from zero, and the genetic correlation between FA_WL and FA_WW was estimated as ?1 because the absolute value for the genetic covariance was similar in magnitude or even larger than the estimated genetic variances of both traits. This suggests that these two traits should be considered to be the same character. The between‐trait phenotypic correlation in FA, which reduces to the repeatability in this situation, was positive and statistically significant thus rendering an estimate of heritability for DI in D. buzzatii of . Nevertheless, the fact that left/right wing sizes were found to be determined by the same set of genes is difficult to reconcile with the presence of special genetic mechanisms that stabilize left/right development in this species. A qualitatively different pattern for asymmetry was observed when the nonlinear composite character wing area (WA ≈ WL × WW) was used, and . Although the results could be made compatible with the existence of a diallelic locus with antagonistic pleiotropic effects on FA_WL and FA_WW that combine multiplicatively to produce overdominance for FA_WA, the available evidence is extremely weak at best. Finally, a test to the null hypothesis of a nongenetic basis of FA, particularly relevant to those situations when directional asymmetry may be heritable, is suggested.     

On the adaptive accuracy of directional asymmetry in insect wing size

Subtle left-right biases are often observed in organisms with an overall bilateral symmetry. The evolutionary significance of these directional asymmetries remains uncertain, however, and scenarios of both developmental constraints and adaptation have been suggested. Reviewing the literature on asymmetry in insect wings, we analyze patterns of directional asymmetry in wing size to evaluate the possible adaptive significance of this character. We found that directional asymmetry in wing size is widespread among insects, with left- and right-biased asymmetries commonly observed. The direction of the asymmetry does not appear to be evolutionarily conserved above the species level. Overall, we argue that the very small magnitude of directional asymmetry, 0.7% of the wing size on average, associated with an extremely imprecise expression, precludes directional asymmetry from playing any major adaptive role.     

Mild stress during development affects the phenotype of great tit Parus major nestlings: a challenge experiment

Conditions experienced during early development may affect both adult phenotype and performance later during life. Phenotypic traits may hence be used to indicate past growing conditions and predict future survival probabilities. Relationships between phenotypic markers and future survival are, however, highly heterogeneous, possibly because poor‐ and high‐quality individuals cannot be morphologically discriminated when developing under good environmental conditions. Sub‐optimal breeding conditions, in contrast, may unmask poor‐quality individuals in a measurable way at the morphological level. We thus predict stronger associations between phenotype and performance under stress. In this field study, we test this hypothesis, experimentally challenging the homeostasis of great tit (Parus major) nestlings by short‐term deprivation of parental care, which had no immediate effect on nestling fitness. The experiment was replicated during two subsequent breeding seasons with contrasting ambient weather conditions. Experimental (short‐term) stress affected tarsus growth but not residual mass at fledging, whereas ambient (continuous) stress affected residual mass but not tarsus growth. Short‐term stress effects on tarsus length and tarsus fluctuating asymmetry were only apparent when ambient conditions were unfavourable. Residual mass and hatching date, but none of the other phenotypic traits, predicted local survival, whereby the strength of the relationship did not vary between both years. Because effects of stress on developmental homeostasis are likely to be trait‐specific and condition‐dependent, studies on the use of phenotypic markers for individual fitness should integrate multiple traits comprising different levels of developmental complexity. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 103–110.     

Fluctuating asymmetry of leaves is a poor indicator of environmental stress and genetic stress by inbreeding in Silene vulgaris

Fluctuating asymmetry (FA) is often used as a measure of developmental instability and has been proposed as an indicator of both environmental and genetic stress. However, the empirical evidence for the effects of stress on FA in plants is inconsistent, and there are few controlled experimental studies. We analyzed different distance- and shape-based measures of the fluctuating asymmetry of the leaves of clonally replicated self- and cross-pollinated lineages of Silene vulgaris (Moench) Garcke (Caryophyllaceae) grown under a control and seven different stress treatments (drought, copper, simulated herbivory, and two levels of nutrient deficiency and of shade). Overall, FA differed among stress treatments, but was not generally higher under stress and even reduced in some treatments. Different measures of FA were only weakly correlated. Inbreeding increased only one measure of leaf FA, and only under high stress intensities. Our findings suggest that in S. vulgaris leaf fluctuating asymmetry does not serve as an indicator of environmental stress or of genetic stress by inbreeding.     

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.     

Developmental stress in wild-living Drosophilids inferred from biometry: metric and meristic traits react differently to heterogeneous environmental conditions

Abstract.  1. During their development in natural conditions, Drosophila larvae and pupae face heterogeneous environmental conditions (HEC). Analysing the morphology of wild-living adults is a means of evaluating the effects of HEC.
2. Two drosophilid species of the Zaprionus genus that recently invaded the Nile delta were investigated, and three metric, size-related traits, and one meristic trait, the sternopleural bristle number, were measured. Data were compared with those of F₁ generation reared under favourable laboratory conditions.
3. Body size was smaller in nature, but also extremely variable among individuals, with an average coefficient of variation (CV) of 9.1 ± 0.3, against a much lesser value of 2.4 ± 0.1 in laboratory flies. Correlations among size traits were also greater in nature (0.96 ± 0.01) than in the laboratory (0.75 ± 0.04).
4. By contrast, sternopleural bristles produced similar results in wild-living and laboratory flies. In nature, mean numbers were only slightly less than in the laboratory and the CVs were almost identical (10.87 ± 0.70 vs 10.80 ± 0.76).
5. Fluctuating asymmetry of sternopleural bristles was also identical in the two kinds of flies, and was not influenced by HEC, indicating a high level of developmental canalisation with respect to HEC.
6. The implications of the results for the problems of field heritability and developmental stability are discussed.     

Interactions of environmental factors influencing pupal coloration in swallowtail butterfly Papilio xuthus

The swallowtail butterfly Papilio xuthus Linné [Lepidoptera: Papilionidae] exhibits pupal protective color polyphenism. Interactions of various environmental factors on pupal coloration were analyzed in non-diapausing individuals. Under sufficient light (200lux), most pupating larvae became green pupae when the surface of the pupation site was smooth, while they became brown when the surface was rough. Tactile signals are the positive environmental factors causing induction of the brown pupal coloration. In dark boxes, the induction of the brown pupal coloration was easily induced even on a smooth surface, suggesting that light suppresses induction of brown coloration. Different colors of pupation sites did not affect pupal coloration under sufficient light. Environmental factors received during a critical period both before girdling and after girdling affected pupal coloration. When tactile signals received from rough surfaces reach threshold levels during pupation, brown pupal coloration is determined. Larvae reared under a daily periodicity of natural light formed a girdle at midnight, subsequently, the prepupae received strong daylight the following day. Under natural light most larvae produced brown pupae on rough surfaces and green pupae on smooth surfaces.     

Nutritional stress reduces flight performance and exploratory behavior in a butterfly

Anthropogenic global change, including agricultural intensification and climate change, poses a substantial challenge to many herbivores due to a reduced availability of feeding resources. The concomitant food stress is expected to detrimentally affect performance, amongst others in dispersal-related traits. Thus, while dispersal is of utmost importance to escape from deteriorating habitat conditions, such conditions may negatively feedback on the ability to do so. Therefore, we here investigate the impact of larval and adult food stress on traits related to dispersal ability, including morphology, physiology, flight performance, and exploratory behavior, in a butterfly. We show that inadequate nutrition during development and in the adult stage dim inishes flight performance, despite som e reallocation of somatic resources. Detrimental effects of food stress on flight performance were mainly caused by reductions in body mass and storage reserves. Similar results were found for exploratory behavior. Furthermore, exploratory behavior was found to be (moderately) repeatable at the individual level, which might indicate the existence of a personality trait. This notion is further supported by the fact that flight performance and exploratory behavior were positively correlated, potentially suggesting the existence of a dispersal syndrome. In summary, our findings may have important implications for dispersal in natural environments, as the conditions requiring dispersal the m ost impair flight ability and thereby likely dispersal rates.     

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.