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.     

Temperature and genotypic effects on life history and fluctuating asymmetry in a field strain of Culex pipiens

Fluctuating asymmetry (FA) has been proposed as a tool to measure levels of stress experienced by populations of organisms during development. To be of value as a bio-marker to highlight conditions at particular sites, it is important that variation in FA is due to environmental (eg pollution) variation and not genetic variation among populations and families, in other words heritability for FA should be very close to zero. A full-sib design was set up in which families of Culex pipiens mosquitoes collected from the field were reared at three different developmental temperatures. The effects of temperature and family on developmental rate, egg to adult survival and four wing morphological measures were assessed. There was both a temperature and a family effect on development rate and survival. Temperature affected all four wing traits, but an influence of family was only evident in two of the wing traits. Two separate measures of FA for each of the wing traits were obtained. The mean estimates of FA were mainly around 1% of the value of the character measured. There was evidence of an increase in FA with increase in temperature stress. Heritability was estimated for the wing traits and wing trait FA's using restricted estimation maximum likelihood. The estimates of heritability for the wing traits were small and, individually, did not differ significantly from zero. There was also no evidence of heritable genetic variation for any of the wing trait FA's. The results are discussed in relation to other studies where FA heritabilities have been estimated and in relation to the use of FA as an indicator of environmental stress.     

Zinc and cadmium accumulation in cabbage aphid (Brevicoryne brassicae) host plants and developmental instability

Developmental instability in morphological characters can occur during individual development due to various environmental stresses. Fluctuating asymmetry (FA) is often used as a measurement of developmental instability, but within-environment variation (CV_e) is also considered an indicator of developmental instability. Cabbage aphid ( Brevicoryne brassicae ) populations were reared on zinc- (Zn) or cadmium- (Cd) contaminated cabbage and radish plants. Developmental instability indicators were measured and their relations with fitness were explored. Results revealed that cabbage aphids exposed to Cd and Zn displayed considerable developmental instability, particularly fluctuating asymmetry. Differences in developmental instability between the two metals were also detected, as well as differences between the two developmental instability measurements. For almost all measured traits, FA was greater on Cd- and Zn-contaminated compared to non-contaminated host plants. In contrast, CV_e of some traits was greater on non-contaminated host plants, yet for other traits CV_e was greater on contaminated host plants. There were also non-significant inverse relationships between FA and fitness of cabbage aphid populations. Due to weak correlations between FA and different patterns of two developmental instability measurements, this study does not support the hypothesis that developmental instability is a useful bioindicator of environmental quality.     

Is phenotypic canalization involved in the decline of the endemic Aquilegia thalictrifolia? Rethinking relationships between fluctuating asymmetry and species conservation status

Fluctuating asymmetry (FA), the deviation from the normal symmetrical condition of a morphological trait having specific morphological symmetry, increases in response to environmental and genetic stress, is related to phenotypic plasticity and is considered a tool for monitoring a species conservation status. However, FA–stress relations are dependent on measured traits or species‐specific characteristics such as mating system and habitat. This study investigates the relationships between FA, genetic diversity, population size, density and individual fitness traits (plant height, fruit and seed set), in the endemic Aquilegia thalictrifolia, a mixed breeder that is declining, but maintaining high levels of heterozygosity. Leaf and flower FA and other traits were investigated in 10 populations of A. thalictrifolia, the whole species range. As a result, we found similar patterns of FA in leaves and flowers between populations, indicating a homogenous level of stress between populations that differed for other traits. FA and the other traits were not related, including heterozygosity. Heterozygosity was not related to individual fitness traits with the exception of plant height. In accordance with other studies, we found that the role of FA as a tool for assessing the conservation status of a species or population should be reconsidered. However, we conclude that a low level of FA should not automatically be considered an indicator of good conservation status or low level of stress, because in species that evolved in highly stable environments it may indicate a scarce ability to plastically respond to environmental changes, as a consequence of environmental and genetic canalization. Further investigation of this point is needed.     

The use of fluctuating asymmetry as a measure of environmentally induced developmental instability: A meta-analysis

Random and subtle deviations from bilateral symmetry (fluctuating asymmetry) have long been of interest to biologists who wish to study the susceptibility of organisms to changes in environmental quality. However, the reliability of FA as a biomarker has come under question due to inconsistent results in the literature. We conducted a meta-analysis of published literature to test the hypothesis that FA is a reliable biomarker of environmental stress in insects and identify possible sources of variation amongst studies. We expected studies to detect larger, positive magnitudes of effect on FA in lab populations due to the lack of confounding effects from other environmental factors compared to wild populations. Additionally, we predicted that studies that used geometric morphometric approaches to FA in shape and size would be more sensitive to changes in environmental quality compared to linear and meristic measures and thus show larger effects on FA. We also expected anthropogenic stressors to generate significantly larger effects on FA compared to naturally occurring stressors due to the organisms’ inability to buffer developmental pathways against a novel stressor. Finally, we predicted comparatively larger magnitudes of effect in studies that verified the environmental factor acting on the organism was a stressor by detecting negative effects on fitness-related traits. Overall, we found that FA is a sensitive biomarker of environmental stress. Environmental stressors explained 36% of the variation of effect on FA across studies. Studies that demonstrated a negative effect of the stressor on fitness-related traits showed significantly larger, positive magnitudes of effect on FA compared to studies that did not detect an effect from the environmental stressor. Additionally, studies conducted under laboratory conditions detected significantly larger, effects on FA compared to field-based studies. The kind of trait measured and the novelty of the stressor did not significantly account for differences amongst studies. Thus, the use of FA as a biomarker of environmental stress is a legitimate tool particularly when studies verify the biological relevance of stressors for the study organism.     

Fluctuating asymmetry in certain morphological traits in laboratory populations of Drosophila ananassae.

Fluctuating asymmetry (FA, subtle random deviations from perfect bilateral symmetry) is often used as a measure of developmental instability (DI), which results from perturbations in developmental pathways caused by genetic or environmental stressors. During the present study, we estimated FA in 5 morphological traits, viz. wing length (WL), wing to thorax ratio (W:T), sternopleural bristle number (SBN), sex-comb tooth number (SCTN), and ovariole number (ON) in 18 laboratory populations of Drosophila ananassae. FA levels of measured traits differed significantly among populations except for SBN (in males and females) and W:T ratio (in females). Positional fluctuating asymmetry (PFA), a sensitive measure of DI, also varied significantly among the populations for SBN in females and SCTN in males. Interestingly, both males and females were similar for nonsexual traits. However, when FA across all traits (sexual and nonsexual) was combined into a single composite index (CFA), significant differences were found for both populations and sexes. Males showed higher CFA values than females, suggesting that males are more prone to developmental perturbations. The magnitude of FA differed significantly among traits, being lowest for nonsexual traits (SBN, WL, W:T ratio) and highest for sexual traits (SCTN and ON). The trait size of sexual traits (SCTN and ON) was positively correlated with their asymmetry. The possible reasons for variation in FA both among traits and among populations, and the usefulness of FA as an indicator of developmental stress and phenotypic quality in D. ananassae are discussed.     

Environmental stress as an indicator of anthropogenic impact across the African Albertine Rift: a case study using museum specimens

We examined fluctuating asymmetry (FA) and body condition (BC) as two measures of environmental stress using museum specimens of Lophuromys aquilus, a rodent species complex wide-spread across the African Albertine Rift. We related FA and BC to a spatially-derived index of anthropogenic impact using a principal components analysis (PCA). We found no relationship between the four PCA scores and mean FA or BC, but did find that FA variance was higher in areas with lower anthropogenic impact. There was also a negative, albeit non-significant, trend for PC3, suggesting that populations with higher than average BC were in areas with higher anthropogenic impact. Overall, our case study does not support FA and BC as effective predictors of environmental stress with low to moderate habitat disturbance. In fact, L. aquilus, as a habitat generalist, may be positively affected by some aspects of anthropogenic change. Studies relating environmental stress to anthropogenic impact should examine sites with a wide range of habitat qualities and human impact and utilize multiple measures of environmental stress to characterize the health of one or more populations.     

Relationship between fluctuating asymmetry and fitness within and between stressed and unstressed populations of the wolf spider Pirata piraticus

Although developmental instability, measured as fluctuating asymmetry (FA), is expected to be positively related to stress and negatively to fitness, empirical evidence is often lacking or contradictory when patterns are compared at the population level. We demonstrate that two important properties of stressed populations may mask such relationships: (i) a stronger relationship between FA and fitness, resulting in stronger selection against low quality (i.e. developmental unstable) individuals and (ii) the evolution of adaptive responses to environmental stress. In an earlier study, we found female wolf spiders Pirata piraticus from metal exposed populations to be characterized by both reduced clutch masses and increased egg sizes, the latter indicating an adaptive response to stress. By studying the relationship between these two fitness related traits and levels of FA at individual level, we here show a significant negative correlation between FA and clutch mass in metal stressed populations but not in unstressed reference populations. As a result, levels of population FA may be biased downward under stressful conditions because of the selective removal of developmentally unstable (low quality) individuals. We further show that females that produced larger eggs in stressed populations exhibited lower individual FA levels. Such interaction between individual FA and fitness with stress may confound the effect of metal stress on FA, resulting in an absence of relationships between FA, fitness and stress at the population level.     

Fluctuating asymmetry and genetic variability in the roe deer (Capreolus capreolus): a test of the developmental stability hypothesis in mammals using neutral molecular markers

Fluctuating asymmetry (FA), used as an indicator of developmental stability, has long been hypothesized to be negatively correlated with genetic variability as a consequence of more variable organisms being better suited to buffer developmental pathways against environmental stress. However, it is still a matter of debate if this is due to metabolic properties of enzymes encoded by certain key loci or rather to overall genomic heterozygosity. Previous analyses suggest that there might be a general difference between homeo- and poikilotherms in that only the latter tend to exhibit the negative correlation predicted by theory. In the present study, we addressed these questions by analysing roe deer (Capreolus capreolus) from five German populations with regard to FA in metric and non-metric skull and mandible traits as well as variability at eight microsatellite loci. Genetic variability was quantified by heterozygosity and mean d2 parameters, and although the latter did not show any relationship with FA, we found for the first time a statistically significant negative correlation of microsatellite heterozygosity and non-metric FA among populations. Because microsatellites are non-coding markers, this may be interpreted as evidence for the role of overall genomic heterozygosity in determining developmental stability. To test if the threshold character of non-metric traits is responsible for the metric vs non-metric difference we also carried out calculations where we treated our metric traits as threshold values. This, however, did not yield significant correlations between FA and genetic variability either.     

Fluctuating asymmetry in the otoliths of larval fish as an indicator of condition: conceptual and methodological aspects

Developmental instability, measured as fluctuating asymmetry (FA) in bilateral traits, has been used widely as an indicator of genetic or environmental stress in a variety of plant and animal taxa. FA arises as small deviations from perfect bilateral symmetry which reflect 'mistakes' in developmental processes resulting from the inability of the genotype to buffer itself effectively against environmental perturbations. Recently, it has been proposed that FA in the otoliths can be used as an indicator of condition in larval fish. This paper reviews the conceptual and methodological aspects of FA relevant to its potential use as a measure of well-being. Its simplicity makes FA an attractive tool to measure developmental precision and condition. However, there are several pitfalls, such as measuring error or potentially size dependence. Subtle asymmetries, i.e. between sides variation of a trait at the individual level, may not always be indicative of condition and should be interpreted with caution. The past-growth record of otoliths may provide a powerful means of studying the development of asymmetries at the level of the individual.     

Environmental stress increases skeletal fluctuating asymmetry in the moor frog Rana arvalis

Whether fluctuating asymmetry (FA) provides a useful metric indicator of the degree of environmental stress experienced by populations is still a contentious issue. We investigated whether the degree of FA in skeletal elements is useful in elucidating the degree of environmental stress experienced by frog populations, and further, tested the proposition that a trait’s sensitivity to stress—as reflected in the degree of FA—is related to the degree of directional selection experienced by the given trait. We compared the degree of FA in four bilateral skeletal elements of male and female moor frogs (Rana arvalis) originating from low (acidified) and neutral pH populations. While the degree of uncorrected FA was unrelated to the degree of acidity, the growth rate and age of the individuals, the size-corrected FA was significantly higher in low than in neutral pH populations and decreased with individual ages and growth rates. In addition, both measures of FA were significantly higher in males and in particular in traits presumably under high sexual selection as indicated by the degree of sexual size dimorphism. All in all, the results indicate that individuals from acidified localities are smaller, younger and exhibit a significantly higher degree of FA than individuals from neutral pH populations. These results constitute the first assessment of FA in amphibians and suggest that the degree of FA in skeletal traits can be a useful indicator of the degree of environmental stress experienced by amphibian populations.     

Potential local adaptation in populations of invasive reed canary grass (Phalaris arundinacea) across an urbanization gradient

Urban stressors represent strong selective gradients that can elicit evolutionary change, especially in non‐native species that may harbor substantial within‐population variability. To test whether urban stressors drive phenotypic differentiation and influence local adaptation, we compared stress responses of populations of a ubiquitous invader, reed canary grass (Phalaris arundinacea). Specifically, we quantified responses to salt, copper, and zinc additions by reed canary grass collected from four populations spanning an urbanization gradient (natural, rural, moderate urban, and intense urban). We measured ten phenotypic traits and trait plasticities, because reed canary grass is known to be highly plastic and because plasticity may enhance invasion success. We tested the following hypotheses: (a) Source populations vary systematically in their stress response, with the intense urban population least sensitive and the natural population most sensitive, and (b) plastic responses are adaptive under stressful conditions. We found clear trait variation among populations, with the greatest divergence in traits and trait plasticities between the natural and intense urban populations. The intense urban population showed stress tolerator characteristics for resource acquisition traits including leaf dry matter content and specific root length. Trait plasticity varied among populations for over half the traits measured, highlighting that plasticity differences were as common as trait differences. Plasticity in root mass ratio and specific root length were adaptive in some contexts, suggesting that natural selection by anthropogenic stressors may have contributed to root trait differences. Reed canary grass populations in highly urbanized wetlands may therefore be evolving enhanced tolerance to urban stressors, suggesting a mechanism by which invasive species may proliferate across urban wetland systems generally.     

Stability in development and variability in morphological signs in a natural population of Drosophila melanogaster: seasonal dynamics in 1999

Seasonal dynamics of developmental stability and variability of morphological traits was examined in a natural population of Drosophila melanogaster in order to compare these two parameters as indicators of temperature stress. Morphometric (thorax length and wing length) and meristic (number of sternopelural and orbital bristles) were studied. Variability was measured as the coefficient of variation. Stability of development was estimated as fluctuating asymmetry (FA) of bilateral traits. Thorax length and wing length did not exhibit consistent seasonal trends whereas wing loading significantly decreased. Significant seasonal changes in FA were not detected in any trait examined. Two traits showed reduced variation in autumn. The use of FA as an indicator of ecological stresses in insects is discussed on the basis of these results and the literature evidence.     

Does stress mess with rodents’ heads? Influence of habitat amount and genetic factors in mandible fluctuating asymmetry in South American water rats (Nectomys squamipes,Sigmodontinae) from Brazilian Atlantic rainforest remnants

Loss of developmental stability can lead to deviations from bilateral symmetry (i.e. Fluctuating Asymmetry ‐ FA), and is thought to be caused by environmental and genetic factors associated with habitat loss and stress. Therefore, levels of FA might be a valuable tool to monitor wild populations if FA serves as an indicator of exposure to stress due to impacts of habitat loss and fragmentation. In studies examining FA and habitat fragmentation, FA levels are often explained by loss of genetic variation, though few studies have addressed FA’s use as indicator of environmental impact. Here, we investigated whether habitat loss, genetic variation, and/or inbreeding affect the developmental instability in Brazilian Atlantic forest populations of a Neotropical water rat (Nectomys squamipes). We sampled individuals from eight sites within Atlantic forest remnants with different amounts of available forest habitat and assessed FA levels with geometric morphometric techniques using adult mandibles. We used observed heterozygosity (H_o) and inbreeding coefficient (F_is), from seven microsatellite markers, as a proxy of genetic variation at individual and population levels. Populations were not significantly different for shape or size FA levels. Furthermore, interindividual variation in both shape and size FA levels and interpopulational differences in size FA levels were best explained by chance. However, habitat amount was negatively associated with both interpopulational variance and average shape FA levels. This association was stronger in populations living in areas with <28% of forest cover, which presented higher variance and higher average FA, suggesting that Nectomys squamipes might have a tolerance threshold to small availability of habitat. Our work is one of the first to use FA to address environmental stress caused by habitat loss in small mammal populations from a Neotropical biome. We suggest that shape FA might serve as a conservation tool to monitor human impact on natural animal populations.     

不同Bt棉种植年代棉铃虫种群的波动性不对称

波动性不对称 (Fluctuating asymm etry,FA)是指相对于生物两侧对称发育的一种细微的随机偏离 ,通常用来指示生物在外界环境压力下发育的稳定性程度。 2 0 0 2年在我国北方棉区 1 3个具有 1～ 5 a转 Bt基因棉种植史的市、县采集了棉铃虫上灯蛾 ,测定了各点成虫后足腿节的波动性不对称 (Fluctuating asymmetry,FA)。结果表明 ,1 2个采集地的棉铃虫后足腿节均表现出 FA;相关性分析发现 ,各采集点的 FA值与 Bt棉的种植年代间存在着显著的负相关关系 (r=- 0 .77) ,显示出棉铃虫自然种群对 Bt棉的适应性随着 Bt棉种植时间延长而渐增的趋势。讨论了棉铃虫对 Bt棉的适应以及 FA作为棉铃虫田间种群对 Bt棉早期抗性监测指标的可能性     

Evolution of morphological integration: developmental accommodation of stress-induced variation

Extreme environmental change during growth often results in an increase in developmental abnormalities in the morphology of an organism. The evolutionary significance of such stress-induced variation depends on the recurrence of a stressor and on the degree to which developmental errors can be accommodated by an organism's ontogeny without significant loss of function. We subjected populations of four species of soricid shrews to an extreme environment during growth and measured changes in the patterns of integration and accommodation of stress-induced developmental errors in a complex of mandibular traits. Adults that grew under an extreme environment had lower integration of morphological variation among mandibular traits and highly elevated fluctuating asymmetry in these traits, compared to individuals that grew under the control conditions. However, traits differed strongly in the magnitude of response to a stressor--traits within attachments of the same muscle (functionally integrated traits) had lower response and changed their integration less than other traits. Cohesiveness in functionally integrated complexes of traits under stress was maintained by close covariation of their developmental variation. Such developmental accommodation of stress-induced variation might enable the individual's functioning and persistence under extreme environmental conditions and thus provides a link between individual adaptation to stress and the evolution of stress resistance.     

Leaf developmental stability of Platanus acerifolia under urban environmental stress and its implication as an environmental indicator

The developmental stability indices,leaf width based fluctuating asymmetry (FA1),and lateral vein length based directional asymmetry (RDA1) of Platanus acerifolia were studied.All the leaves were sampled from 14 sites that were categorized based on different urban environmental stress levels (UESL) in Shanghai metropolitan,China.Besides,foliar stomatal density and stomatal length were also studied as the subsidiary indices to test the availability of developmental stability indices as the indicator under a stressful environment.Results showed seasonal variation of FA1 and RDA1 existed among the 14 sites,but the data showed significant negative correlation between FA1 and UESL (FA1=0.029-0.000 9UESL+0.000 3UESL2,r=0.766 5,P=0.001 4).However,a similar trend was not found between RDA1 and UESL.Furthermore,the significant correlation among FA1 and leaf stomatal length and stomatal density implied they could be used as indicators of urban stress levels on a small scale.It seemed that RDA1 was possibly a normal parameter during leaf development but it was unavailable for use as an indicator of urban stresses.     

Hybridization, developmental stability, and functionality of morphological traits in the ground beetle Carabus solieri (Coleoptera, Carabidae)

The assessment of developmental stability in hybrids can provide valuable information in the study of species formation because it allows an evaluation of the degree of incompatibility of genetic systems that control developmental processes. The present study assessed the impact of two hybridization events, assumed to have occurred at different times, on developmental instability in the ground beetle Carabus solieri . Developmental instability was estimated in 678 individuals from 27 populations from the fluctuating asymmetry (FA) levels of four morphological traits: the tibia length of middle and hind legs, which are functional structures, and the length and the proximal width of the hind wings, which are vestigial and thus nonfunctional structures. Significant variations of FA levels between populations were shown only for the wing width. For this trait, FA levels in hybrids were higher than in their parental entities for both hybridization events, indicating a significant divergence of the gene systems controlling development between the parental entities in the two hybridization cases. As expected, wing traits exhibited FA levels at least three times higher than leg trait. Finally, the potential interest of vestigial traits in the particular context of hybridization is discussed.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 151–158.     

Does fluctuating asymmetry constitute a sensitive biomarker of nutritional stress in house sparrows (Passer domesticus)?

Small random deviations from left–right symmetry in bilateral traits, termed fluctuating asymmetry (FA), are theoretically predicted to increase with environmental stress and believed to constitute a potential biomarker in conservation. However, reported relationships between FA and stress are generally weak and variable among organisms, traits and stresses. Here we test if, and to what extent, FA increases with nutritional stress, estimated from independent feather growth measurements, in free-ranging house sparrows (Passer domesticus). Ptilochronological feather marks showed significant heterogeneity among study plots, indicating that house sparrow populations were exposed to variable levels of nutritional stress during development. However, individuals from more stressed populations did not show increased levels of fluctuating asymmetry in tarsus or rectrix length, nor was there evidence for significant between-trait concordance in FA at the individual or the population level. Lack of support for FA in tarsus and rectrix length as estimator of nutritional stress in house sparrows may indicate that developmental instability is insensitive to nutritional stress in this species, poorly reflected in patterns of fluctuating asymmetry due to ecological or statistical reasons, or highly context-specific. Such uncertainty continues to hamper the use of FA as a biomarker tool in conservation planning.     

Fluctuating Asymmetry as a Bioindicator of Stress: Comparing Efficacy of Analyses Involving Multiple Traits

Researchers have suggested fluctuating asymmetry (FA) as an indicator of environmental stress and have usually tested this assertion by examining relations between FA of single traits and stress. Fluctuating asymmetry stress relations are real but are typically weak and difficult to detect. Researchers would like to maximize the probability of detecting FA-stress relations when they exist. We assert that analyses based on the FA of multiple traits may provide better methods for detecting stress. In this article, we used computer simulations to compare the ability of six analyses to detect differences in FA between stressed and unstressed populations. We show that the optimal analysis depends upon the underlying form of the FA distributions. We also show that two of the analyses had inflated Type I errors in some situations. Finally, we quantify the advantage of our preferred analysis over those of single-trait FA in detecting stress.