Habitat dependent associations between parasitism and fluctuating asymmetry among endemic stickleback populations

Inconsistencies in the relationship between fluctuating asymmetry (FA) and fitness may be due to selection acting on the degree of trait asymmetry that differs among populations or among traits. We assessed relationships between parasite susceptibility and fluctuating asymmetry in the number of bony lateral plates among 83 populations of freshwater Gasterosteus aculeatus (three spined stickleback) and among lateral plate positions that vary in the selection they experience for symmetry. The correlation between FA and parasite infection was highly variable among samples. Excess of infected asymmetric G. aculeatus increased significantly as the robustness of structural predator defences decreased. This effect was found for one parasite species only (Eustrongylides sp.) and was slightly stronger in females. In addition, there was a trend for there to be an excess of infected females asymmetric in those lateral plates positions that did not experience selection for their symmetry, although the trend only approached significance. These results suggest that selection for trait symmetry can obscure relationships between fitness and individual-wide developmental stability, providing one possible explanation for some of the heterogeneity in FA/fitness relationships seen in the literature. These results are also consistent with previous reports showing that ecological segregation between symmetric and asymmetric G. aculeatus and between sexes can alter the FA/fitness relationship.     

Morphological asymmetry of insular freshwater populations of threespine stickleback

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

Condition-dependent asymmetry is fluctuating asymmetry

Asymmetry has been demonstrated to play a role in signalling systems such as sexual selection and pollination, with receivers showing a preference for symmetrical signals. Large signals often have the smallest degree of asymmetry, a finding that is consistent with signal asymmetry being condition-dependent. The kind of asymmetry displayed by signals was supposed or shown to be fluctuating asymmetry, and signals revealing individual differences in the ability to stabilize developmental processes, despite a hostile developmental environment, was supposed to be the basis for the preference for symmetric signals. Recently, it has been suggested that condition-dependent signals display antisymmetry rather than fluctuating asymmetry, based on analyses of the relationship between asymmetry and mean length of the left and the right character in a few published graphs of absolute asymmetry of signals. Here I demonstrate on the basis of a much larger number of data sets, including those previously published, that the previous results are biased because of the methods used for the analyses, and that characters with condition-dependent asymmetry show fluctuating asymmetry rather than antisymmetry. In particular, frequency distributions of signed left-minus-right character values display leptokurtosis, as predicted if asymmetry distributions reflected individual differences in developmental precision, rather than platykurtosis. Platykurtosis is predicted if the traits are antisymmetric. The preponderance of leptokurtic distributions is consistent with recent modelling showing that inherent differences in the ability of individuals to control developmental processes invariably leads to leptokurtic distributions of signed left-minus-right character values.     

Strong nonlinear selection against fluctuating asymmetry in wild populations of a marine fish

Theoretical links between fluctuating asymmetry (FA) and fitness have led many to use FA as a proxy for average fitness. However, studies examining whether asymmetry actually correlates with individual fitness in wild populations are relatively rare and often use simple measures of association (e.g., correlation coefficients). Consequently, the pattern of selection on asymmetry in the wild is seldom clear. We examined selection on FA of pectoral fin morphology in two wild populations of a marine fish (the kelp perch; Brachyistius frenatus). As expected, variance in signed FA in each initial sample was significantly greater than that found in the surviving population, indicating selection against FA. Our estimate of the fitness surface confirmed perfect symmetry as the phenotypic optimum and indicated strong, nonlinear selection against asymmetry. No difference in the form of selection was detected between populations. However, the level of FA in the initial samples varied among populations, leading to an overall difference in the level of selective mortality. Our results suggest that selection on asymmetry in wild populations may be strongly nonlinear, and indicate that the demographic costs of asymmetry may play a substantial role in the dynamics of populations.     

Ecological opportunity and levels of morphological variance within freshwater stickleback populations

The 'ecological opportunity' hypothesis predicts that when interspecific competition or predation is reduced, populations will exhibit increases in phenotypic variance as a result of colonization and adaptation to vacant or underutilized ecological niches (i.e. character release). We assessed this hypothesis by examining morphological diversity within stickleback populations in 40 undisturbed lakes from six islands off the mid-coast of British Columbia, Canada. Because larger lakes with well-developed littoral and limnetic zones will have greater trophic niche diversity than smaller lakes with only littoral zones, we predicted a positive association between lake size and variation in trophic morphology. Conversely, reduced vertebrate predation in small bog lakes allows increased variance in defensive structures without costs to fitness. Consistent with both predictions, we observed that phenotypic variance in two traits that are involved in feeding (gape width and pectoral fin length) increased with lake size while variability in defence structures (lateral plate number and dorsal spine length) was inversely related to lake size. Moreover, increased variance in defence morphology was accentuated in populations with severe armour reduction (spine loss, decreased plate overlap), another strong indicator of reduced vertebrate predation. In the majority of cases, these patterns were repeatable among islands, independent from the geographical distance between lakes, and arose from a combination of high variance within each of the sexes and increases in sexual dimorphism. These findings suggest that character release can be trait-specific and reflect the combined effects of competition, predation and habitat heterogeneity.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 297–308.     

The ontogeny of fluctuating asymmetry

We tested seven hypotheses regarding the mechanisms by which fluctuating asymmetry (FA) originates. We did this by analyzing data on four bilateral characters measured repeatedly during the development of individual domestic fowl. Immediately posthatching, there was substantial directional asymmetry, which rapidly decreased. We detected FA at significant levels in all characters in the majority of our measurements over the remainder of development. We also examined the effects of known environmental stressors (food and density stress) on levels of FA. At the levels we examined, changes in these stressors did not alter the degree of asymmetry we found in fowl. Time series of asymmetry for individuals did not exhibit regular oscillations, as much of the relevant literature predicts. Asymmetry levels reflected the combined effects of developmental noise, which was random in degree and direction, and feedback processes, which decreased asymmetry by altering growth rates on both sides of the body. Our findings best fit the predictions of the residual asymmetry and compensatory growth hypotheses, which suggest that levels of asymmetry reflect only recent growth history.     

The ecology of asymmetry in stickleback defense structures

Slight departures from bilateral symmetry are usually associated with reduced fitness. Here we show that an insular freshwater population of Gasterosteus aculeatus exhibits spatial and temporal segregation according to the incidence and direction of asymmetry in the number of bony lateral plates, which are important predator-defense structures. We analyzed 11,263 fish collected from 19 full lake transects over three years, and found that signed asymmetries for plate number were slightly left-side biased. Asymmetries occurred in 55% of the stickleback with more asymmetrics found during cold weather, and more left-biased individuals found during windy conditions, possibly due to environmentally driven activity levels that differ among asymmetric forms. Absolute plate asymmetries were randomly distributed in the lake, but there was a strong shift in signed plate asymmetry from a mean of zero in littoral zones to left-side bias in limnetic zones, probably due to microhabitat choices among asymmetric forms. Video data of avian piscivores on the lake show laterality in prey handling, providing a potential mechanism for asymmetric prey morphology. Our results imply a complex relationship between slight asymmetries and niche space that is relevant to the broader literature on the evolutionary implications of developmental instability and intrapopulation variability.     

Directional asymmetry of pelvic vestiges in threespine stickleback

Extensive reduction of the size and complexity of the pelvic skeleton (i.e., pelvic reduction) has evolved repeatedly in Gasterosteus aculeatus. Asymmetrical pelvic vestiges tend to be larger on the left side (i.e., left biased) in populations studied previously. Loss of Pitx1 expression is associated with pelvic reduction in G. aculeatus, and pelvic reduction maps to the Pitx1 locus. Pitx1 knockouts in mice have reduced hind limbs, but the left limb is larger. Thus left-biased directional asymmetry of stickleback pelvic vestiges may indicate the involvement of Pitx1 in pelvic reduction. We examined 6,356 specimens from 27 Cook Inlet populations of G. aculeatus with extensive pelvic reduction. Samples from 20 populations exhibit the left bias in asymmetrical pelvic vestiges expected if Pitx1 is involved, and three have a slight, non-significant left bias. However, samples from three populations have a significant right bias, and one large sample from another population has equal frequencies of specimens with larger vestiges on the left or right side. A sample of fossil threespine stickleback also has significantly left-biased pelvic vestiges. These results suggest that silencing of Pitx1 or the developmental pathway in which it functions in the pelvis is the usual cause of pelvic reduction in most Cook Inlet populations of G. aculeatu, and that it caused pelvic reduction at least 10 million years ago in a stickleback population. A different developmental genetic mechanism is implicated for three populations with right-biased pelvic vestiges and for the population without directional asymmetry.     

Variation in fluctuating asymmetry levels across a Robertsonian polymorphic zone of the house mouse

Fluctuating asymmetry (FA) levels were assessed within the Barcelona Robertsonian polymorphic zone of Mus musculus domesticus as a measure of developmental instability (DI). This zone is characterized by populations with a reduced diploid number (2n = 27–39) surrounded by others with standard karyotype (2n = 40). Mice were distributed into four classes according to mean diploid number at each collecting site, and differences in their FA levels were studied. Three different FA indices were calculated on interlandmark distances from two major morphogenetic units of the mandible: the teeth and muscular areas. Three kinds of analysis were performed: trait‐by‐trait, considering the two regions separately and pooling all traits. In univariate analyses few differences were detected between classes. Nevertheless, when characters were pooled, differences in FA values were found between one Robertsonian group and standard mice, and between several Robertsonian classes. Teeth area traits showed higher FA levels in Rb I mice, i.e. those geographically close to the standard populations. However, FA levels decreased in more introgressed Robertsonian groups and showed significant differences with Rb I. Muscular area traits showed lower DI levels in the Robertsonian mice, especially those with an intermediate mean diploid number (Rb II). When all traits were pooled, FA levels increased in Rb I, whilst the other Robertsonian groups showed similar, or even lower, FA values to those of the standard mice. The higher FA levels found in Rb I suggest an increase in the disruption of genetic coadaptation. Furthermore, in Rb I the presence of metacentrics is more recent than in more introgressed populations; we therefore suggest that Rb I have had less time to restore genetic coadaptation. In addition, the teeth region of the mandible seems to be more sensitive to genomic stress than the muscular region. In the light of these results we suggest that hybrids should be separated into groups and that mandible morphogenetic units be differentiated when using FA in hybrid zones to analyse DI.     

Facial masculinity and fluctuating asymmetry

Recently, women have been found to prefer the scent of symmetrical men and relatively masculine male faces more during the fertile (late follicular and ovulatory) phases of their menstrual cycles than during their infertile (e.g., luteal) phases. These findings make most theoretical sense if men's symmetry is associated with the masculinity of their faces and, therefore, men's symmetry and facial masculinity tap a shared underlying quality. This study examined associations between masculine facial features and nonfacial body symmetry as well as facial symmetry in samples of 141 men and 154 women. As predicted, a component of facial features that discriminates the sexes and reflects masculinization of the face significantly covaried with symmetry in men. No significant correlation was observed for women. These findings suggest that men's facial masculinity partly advertises underlying developmental stability.     

Confirmation bias in studies of fluctuating asymmetry

Fluctuating asymmetry (FA) represents small, non-directional deviations from perfect symmetry in morphological characters. FA is generally accepted to increase in response to stress; therefore, FA is frequently used in ecological studies as an index of stress experienced by an organism, in particular due to environmental pollution. We experimentally tested the hypothesis that the outcomes of studies based on FA measurements may have been influenced by confirmation bias, i.e. the tendency of humans to seek out evidence in a manner that confirms their hypotheses and beliefs. We collected 100 leaves of downy birch (Betula pubescens) from a single tree, grouped them haphazardly into ten samples, scanned every sample, and then asked each of 31 scientists (experienced in studying FA) to measure FA from the scanned images of all 100 leaves. Three groups of participants were provided with false information about the origin of each sample and about the hypothesis to be tested, and one group (control) was provided with true information. The participants who believed that the leaves originated from a heavily polluted site reported significantly higher values of FA when compared to the participants who believed that the leaves were collected from an unpolluted site. When the participants were told that half the samples originated from a polluted site and half from a clean site and were asked to attribute each sample to either of these sites based on leaf FA, the differences in FA between samples classified as ‘polluted’ and ‘unpolluted’ were much higher than the differences obtained from measurements of the same sets of samples made by the control group. We conclude that when scientists expected to find high FA in some samples, the results of their measurements confirmed their expectations. This effect, classified as confirmation bias, may considerably influence the outcomes of the research on FA. This confirmation bias can be avoided by using a blind method, where the person conducting measurements is not aware of the origin of samples being measured. We argue that the use of blind methods is critically important for any study addressing environmental or genetic impacts on FA.     

The quantitative genetics of fluctuating asymmetry

Fluctuating asymmetry (subtle departures from identical expression of a trait across an axis of symmetry) in many taxa is under stabilizing selection for reduced asymmetry. However, lack of reliable estimates of genetic parameters for asymmetry variation hampers our ability to predict the evolutionary outcome of this selection. Here we report on a study, based on analysis of variation within and between isofemale lines and of generation means (line-cross analysis), designed to dissect in detail the quantitative genetics of positional fluctuating asymmetry (PFA) in bristle number in natural populations of Drosophila falleni. PFA is defined as the difference between the two sides of the body in the placement or position of components of a meristic trait. Heritability (measured at 25 degrees C) of two related measures of PFA were 13% and 21%, both of which differed significantly from zero. In contrast, heritability estimates for fluctuating asymmetry in the total number of anterior (0.7%) and transverse (2.4%) sternopleural bristles were smaller, not significant, and in quantitative agreement with previously published estimates. Heritabilities for bristle number (trait size) were considerably greater than that for any asymmetry measure. The experimental design controlled for the potentially confounding effects of common familial environment, and repeated testing revealed that PFA differences between lines were genetically stable for up to 16 generations in the laboratory at 25 degrees C. We performed line cross analysis between strains at the extremes of the PFA distribution (highest and lowest values); parental strains, F1, F1r (reciprocal), F2, backcross, and backcross reciprocal generations were represented. The inheritance of PFA was described best by additive and dominance effects localized to the X-chromosomes, whereas autosomal dominance effects were also detected. Epistatic, maternal, and cytoplasmic effects were not detected. The inheritance of trait size was notably more complex and involved significant autosomal additive, dominance, and epistatic effects; maternal dominance effects; and additive and dominance effects localized to the X-chromosomes. The additive genetic correlation between PFA and its associated measure of trait size was negative (-0.049), but not statistically significant, indicating that the loci contributing additive genetic effects to these traits are probably different. It is suggested that PFA may be a sensitive measure of developmental instability because PFA taps the ability of an organism to integrate interconnected developmental pathways.     

Phylogeography of isolated freshwater three-spined stickleback Gasterosteus aculeatus populations in the Adriatic Sea basin

Analyses of mitochondrial (mt) DNA and microsatellite variation were carried out to examine the relationships between 10 freshwater populations of three-spined sticklebacks Gasterosteus aculeatus along the eastern coast of the Adriatic Sea. Partial sequences of the mtDNA control region and cytochrome b gene, in addition to 15 microsatellite loci, were used to analyse populations from four isolated river catchments. Results uncovered an Adriatic lineage that was clearly divergent from the European lineage, and confirmed that the most divergent and ancient populations are located within the Adriatic lineage as compared with other European populations. Two northern Adriatic populations formed independent clades within the European mitochondrial lineage, suggesting different colonization histories of the different Adriatic populations. Nuclear marker analyses also indicated deep divergence between Adriatic and European populations, albeit with some discordance between the mtDNA phylogeny of the northern Adriatic populations, further highlighting the strong differentiation among the Adriatic populations. The southern populations within the Adriatic lineage were further organized into distinct clades corresponding to respective river catchments and sub-clades corresponding to river tributaries, reflecting a high degree of population structuring within a small geographic region, concurrent with suggestions of existence of several microrefugia within the Balkan Peninsula. The highly divergent clades and haplotypes unique to the southern Adriatic populations further suggest, in accordance with an earlier, more limited survey, that southern Adriatic populations represent an important reservoir for ancient genetic diversity of G. aculeatus.     

Mixture analysis of asymmetry: modelling directional asymmetry, antisymmetry and heterogeneity in fluctuating asymmetry

The occurrence of different forms of asymmetry complicates the analysis and interpretation of patterns in asymmetry. Furthermore, between-individual heterogeneity in developmental stability (DS) and thus fluctuating asymmetry (FA), is required to find relationships between DS and other factors. Separating directional asymmetry (DA) and antisymmetry (AS) from real FA and understanding between-individual heterogeneity in FA is therefore crucial in the analysis and interpretation of patterns in asymmetry. In this paper we introduce and explore mixture analysis to (i) identify FA, DA and AS from the distribution of the signed asymmetry, and (ii) to model and quantify between-individual heterogeneity in developmental stability and FA. In addition, we expand mixtures to the estimation of the proportion of variation in the unsigned FA that can be attributed to between-individual heterogeneity in the presumed underlying developmental stability (the so-called hypothetical repeatability). Finally, we construct weighted normal probability plots to investigate the assumption of underlying normality of the different components. We specifically show that (i) model selection based on the likelihood ratio test has the potential to yield models that incorporate nearly all heterogeneity in FA; (ii) mixtures appear to be a powerful and sensitive statistical technique to identify the different forms of asymmetry; (iii) restricted measurement accuracy and the occurrence of many zero observations results in an overestimation of the hypothetical repeatability on the basis of the model parameters; and (iv) as judged from the high correlation coefficients of the normal probability plots, the underlying normality assumption appears to hold for the empirical data we analysed. In conclusion, mixtures provide a useful statistical tool to study patterns in asymmetry.     

Sexual selection, stabilizing selection and fluctuating asymmetry in two populations of pupfish (Cyprinodon pecosensis)

Fluctuating asymmetry of morphological traits is thought to reflect the capacity of a genotype to produce an integrated, functional phenotype. I tested three predictions. (1) In a polygynous breeding system, under intense sexual selection on males, breeding males should show greater symmetry in bilaterally symmetrical traits than non-breeding males or females. (2) If these traits are under stabilizing selection, highly symmetrical individuals also should be modal phenotypes, thus near the mean value for that trait, whereas individuals with increased asymmetry should represent marginal phenotypes, near the extremes of the distribution for that trait. (3) Differences in the intensity of sexual selection should be reflected in differences in the degree of fluctuating asymmetry between sexes among populations. I examined the relationship between male breeding status and the degree of fluctuating asymmetry of four bilaterally symmetrical- traits, preorbital and preopercular pores and pectoral and pelvic fin rays, in two populations of Pecos pupfish which differed in the intensity of sexual selection. These traits do not function in male-male competition or female choice, thus are not directly affected by sexual selection. In Mirror Lake breeding males, as a group, were most symmetrical for all four traits, while non-breeding males and females showed higher levels of fluctuating asymmetry. Similarly, symmetrical individuals also represented modal phenotypes for four traits (breeding males), and for three traits (non-breeding males and females). These patterns were not seen in the Lake Francis population, where breeding males were as asymmetrical as non-breeding males and females, and the degree of fluctuating symmetry did not differ between modal and marginal phenotypes for any of the four traits. When ecological conditions favour intense sexual selection, either through female choice, male-male competition, or both, breeding males represent the most fit phenotypes. Thus sexual selection reinforces the effects of stabilizing selection on characters that do not function as secondary sexual traits. However, when sexual selection is relaxed, differences between sexes disappear.     

Genetic structuring in fluctuating populations

The effect of genetic drift in spatially distributed dispersal-linked and density-regulated populations is studied in a classical one-locus two-allele system. We analyse emergence of genetic differentiation assuming random drift only, where the noise-like variability is due to demographic stochasticity. We find emergence of clusters of sub-units with local allele fixation and persistence of both alleles in lengthy simulations. We demonstrate that local allele fixation (extending over a number of adjoining spatial sub-units) – without global loss of alleles – may occur when the carrying capacities of local patches are small, under a full range population dynamic regimes, when dispersal rate is small, and when redistribution (through dispersal) does not act as global mixer. These results are novel. The key to the observations is that drift is simultaneously influenced by distance-dependent dispersal, demographic stochasticity and autocorrelated population fluctuations due to delayed-density dependence. These are standard elements of contemporary population models in spatially structured context. With stable large populations, no stochasticity and dispersal limited to neighbours only, our model collapses to the stepping-stone model, while with dispersal being random and global, the model collapses to Wright's island model.