THE GENETIC BASIS OF DEVELOPMENTAL STABILITY INAPIS MELLIFERA:HETEROZYGOSITY VERSUS GENIC BALANCE

Building a new research framework for social evolution: intralocus caste antagonism

The breeding and non‐breeding ‘castes’ of eusocial insects provide a striking example of role‐specific selection, where each caste maximises fitness through different morphological, behavioural and physiological trait values. Typically, queens are long‐lived egg‐layers, while workers are short‐lived, largely sterile foragers. Remarkably, the two castes are nevertheless produced by the same genome. The existence of inter‐caste genetic correlations is a neglected consequence of this shared genome, potentially hindering the evolution of caste dimorphism: alleles that increase the productivity of queens may decrease the productivity of workers and vice versa, such that each caste is prevented from reaching optimal trait values. A likely consequence of this ‘intralocus caste antagonism’ should be the maintenance of genetic variation for fitness and maladaptation within castes (termed ‘caste load’), analogous to the result of intralocus sexual antagonism. The aim of this review is to create a research framework for understanding caste antagonism, drawing in part upon conceptual similarities with sexual antagonism. By reviewing both the social insect and sexual antagonism literature, we highlight the current empirical evidence for caste antagonism, discuss social systems of interest, how antagonism might be resolved, and challenges for future research. We also introduce the idea that sexual and caste antagonism could interact, creating a three‐way antagonism over gene expression. This includes unpacking the implications of haplodiploidy for the outcome of this complex interaction.

The relationship between asymmetry, size and unusual venation in honey bees (Apis mellifera)

Despite the fact that symmetry is common in nature, it is rarely perfect. Because there is a wide range of phenotypes which differs from the average one, the asymmetry should increase along with deviation. Therefore, the aim of this study was to assess the level of asymmetry in normal individuals as well as in phenodeviants categorized as minor or major based on abnormalities in forewing venation in honey bees. Shape fluctuating asymmetry (FA) was lower in normal individuals and minor phenodeviants compared with major phenodeviants, whereas the former two categories were comparable in drones. In workers and queens, there were not significant differences in FA shape between categories. FA size was significantly lower in normal individuals compared with major phenodeviant drones and higher compared with minor phenodeviant workers. In queens, there were no significant differences between categories. The correlation between FA shape and FA size was significantly positive in drones, and insignificant in workers and queens. Moreover, a considerable level of directional asymmetry was found as the right wing was constantly bigger than the left one. Surprisingly, normal individuals were significantly smaller than minor phenodeviants in queens and drones, and they were comparable with major phenodeviants in all castes. The correlation between wing size and wing asymmetry was negative, indicating that smaller individuals were more asymmetrical. The high proportion of phenodeviants in drones compared with workers and queens confirmed their large variability. Thus, the results of the present study showed that minor phenodeviants were not always intermediate as might have been expected.

Developmental instability in incipient colonies of social insects

Social insect colonies can provide homeostatic conditions that buffer the incidence of environmental fluctuations on individuals, which have contributed to their ecological success. Coptotermes (Isoptera: Rhinotermitidae) is a highly invasive termite genus and several species have important economic impact in many areas of the world. Mature Coptotermes colonies with millions of individuals can provide optimal environmental condition and nurturing capacity for the developing brood. However, it was previously suggested that contrary to mature colonies, incipient colonies may be exposed to critical stress, which may explain for the low success rate of establishment within the first year of the life of a termite colony. We here investigated the stress imposed on individuals of incipient colonies by comparing the developmental instability of individuals between incipient and mature colonies of two Coptotermes species, C. formosanus Shiraki and C. gestroi (Wasmann). We assessed the developmental instability by measuring the asymmetry of morphological traits from the head capsule of the soldier caste. Soldiers from incipient colonies of both species displayed strong asymmetrical traits in comparison to soldiers from mature colonies. We suggest that homeostatic conditions for optimal development are reached as the colony matures, and confirmed that the incipient colony remains a critical bottleneck where individuals are exposed to high developmental stress.

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.

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.

Male behavioural maturation rate responds to selection on pollen hoarding in honeybees

Division of labour in social insect colonies relies on behavioural functional differentiation (specialization) of individuals with similar genomes. However, individual behavioural traits do not evolve independently of each other (behavioural syndromes). A prime example is the suite of behavioural differences in honeybee workers that has evolved in response to bidirectional selection on pollen hoarding of honeybee colonies (pollen-hoarding syndrome). More generally, these differences reflect functional differentiation between nectar and pollen foragers. We demonstrate here that this pollen-hoarding syndrome extends to drones. Similar to what has been shown in workers, drones from the high-pollen-hoarding strain had a higher locomotion activity after emergence, and they initiated flight earlier than did males derived from the low-pollen-hoarding strain, with hybrids intermediate. However, these two behavioural traits were unlinked at the individual level. We also found that social environment (the colony) affects the age at which drones initiate flight. The indirect selection responses of male behaviour suggest that male and worker evolution are not independent and may constrain each other's evolution. Furthermore, we identified three distinct peaks in the probability of flight initiation over the course of the experiment and a decreased phenotypic variability in the 'hybrid' males, contrary to quantitative genetic expectations.

The effects of rearing temperature on developmental stability and learning and memory in the honey bee, Apis mellifera

Honey bee workers maintain the brood nest of their colony within a narrow temperature range of 34.5+/-1.5 degrees C, implying that there are significant fitness costs if brood is reared outside the normal range. However, the effects of abnormal incubation temperatures are subtle and not well documented. Here we show that short-term learning and memory abilities of adult workers are affected by the temperature they experienced during pupal development. In contrast, long-term learning and memory is not significantly affected by rearing temperature. Furthermore, we could detect no effects of incubation temperature on fluctuating asymmetry, as a measure of developmental stability, in workers, queens or drones. We conclude that the most important consequence of abnormal rearing temperatures are subtle neural deficiencies affecting short-term memory rather than physical abnormalities.

Developmental stability in Brassica cretica: the effect of crossing distance on fluctuating asymmetry in cotyledon morphology

In the present investigation of Brassica cretica, a wild relative of the cultivated cabbage, B. oleracea, we performed an extensive crossing experiment, involving self-pollinations, random outcrosses within populations and hybridizations between populations or species, to evaluate the relationship between crossing distance and developmental stability, estimated as the absolute difference between the right and left lobe of the cotyledons. The frequency distribution of the right-minus-left scores had a narrower peak than expected for normally-distributed data, but there was no directional asymmetry or antisymmetry. Despite evidence for inbreeding depression in seedling biomass and cotyledon size, the type of cross had negligible influence on cotyledon asymmetry. Separate analyses of between-population hybrids revealed differences among progenies from different pairs of populations and a tendency for the F1 hybrid means to decrease with the geographic distance separating the parent populations, but only for the two size variables. Based on these and other observations, we propose that the degree of cotyledon asymmetry is unrelated to genome-wide characteristics, such as the level of heterozygosity and genomic co-adaptation, and that cotyledon asymmetry is unrelated to the level of genetic stress experienced by each individual. Hence, there is no reason to consider measures of asymmetry as more sensitive indicators of genetic health than conventional fitness variables.

Fluctuating asymmetry as an indicator of fitness: can we bridge the gap between studies?

There is growing evidence from both experimental and non-experimental studies that fluctuating asymmetry does not consistently index stress or fitness. The widely held--yet poorly substantiated--belief that fluctuating asymmetry can act as a universal measure of developmental stability and predictor of stress-mediated changes in fitness, therefore staggers. Yet attempts to understand why the reported relationships between fluctuating asymmetry, stress and fitness are so heterogeneous--i.e. whether the associations are truly weak or non-existent or whether they become confounded during different stages of the analytical pathways remain surprisingly scarce. Hence, we attempt to disentangle these causes, by reviewing the various statistical and conceptual factors that are suspected to confound potential relationships between fluctuating asymmetry, stress and fitness. Two main categories of factors are discerned: those associated with the estimation of developmental stability through fluctuating asymmetry and those associated with the effects of genotype and environment on developmental stability. Next, we describe a series of statistical tools that have recently been developed to help reduce this noise. We argue that the current lack of a theoretical framework that predicts if and when relationships with developmental stability can be expected, urges for further theoretical and empirical research, such as on the genetic architecture of developmental stability in stressed populations. If the underlying developmental mechanisms are better understood, statistical patterns of asymmetry variation may become a biologically meaningful tool.

Tarsal asymmetry, trait size, and extreme phenotypes in a sexually size-dimorphic water boatman Callicorixa vulnerata

Fluctuating asymmetry (FA) in morphological traits can vary with the size of characters and the mode of selection acting on them. In a previous study, mid-leg tarsal FA (tarsal spine number and tarsal length) in the water boatman Callicorixa vulnerata was inversely related to fitness in both sexes. The mid-legs of water boatmen are used to cling to bottom substrate during underwater feeding and if under stabilizing selection, extreme phenotypes are predicted to exhibit elevated FA. In this study, it is shown that water boatmen with large or small mid-leg tarsal traits tend to have higher levels of tarsal FA than modal phenotypes, possibly because of increased homozygosity, poor genomic balance, or greater stress during development in extreme phenotypes. This relationship suggests selection against asymmetry per se will indirectly impose stabilizing selection on trait size. Regression analyses revealed that the relationship between FA and trait size differed between the sexes and was best described by a U-shaped distribution in females but by a relatively flat, negative linear association in males. These results indicate possible directional selection on male tarsal traits. Alternatively, they suggest associations between FA and trait size do not always reflect the mode of selection acting on a trait.

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.

Developmental instability and working memory ability in children: a magnetic resonance spectroscopy investigation

This study of children (ages 7 through 12) wishes to determine (a) whether variation in frontal lobe brain chemistry, determined from proton magnetic resonance spectroscopy (1H-MRS), is related to performance on a working memory task in children, and (b) whether developmental instability (DI; the imprecise expression of the genetic plan for development due to several known genetic and environmental effects) underlies phenotypic variation in brain chemistry. 1H-MRS assessed neurometabolites in a right frontal white matter voxel. The Visual Two-Back test assessed working memory. A composite measure of DI was created from measures of minor physical anomalies, fluctuating asymmetry of body characteristics, and fluctuating asymmetry of dermatoglyphic features. Greater DI strongly predicted lower concentrations of creatine-phosphocreatine (Cre) and choline-containing compounds, whereas Cre and N-acetyl-aspartate positively correlated with working memory skills. Working memory skills thus seem related to frontal lobe energy metabolism, which in turn is related to DI.

Is There a Genetic Basis for Fluctuating Asymmetry and Does it Predict Fitness in the Plant Lotus corniculatus Grown in Different Environmental Conditions?

Fluctuating asymmetry (FA) is considered to be a good measure of developmental stability. We measured the asymmetry of leaves and flowers of 16 different genotypes of Lotus corniculatus grown in four different experimental environments to estimate the plasticity or developmental stability of asymmetry itself. We found that an index of FA (absolute difference between size of left and right sides, corrected for trait size) differed significantly across environments, with the treatment CO2+/N+ inducing the greatest FA for both flowers and leaves. Genotypes did not differ in FAs. Individual plants showed significantly different FAs only for flowers. At the individual level, we found no significant relationship between flower FA and fitness. Previous work indicates that change in asymmetry in a poor or perturbing environment versus a good environment could reflect the intrinsic quality of a particular genotype. However, in our experiment, genotype effect was significant only for change in asymmetry of leaves, and this last trait was not significantly correlated with our fitness estimate for each genotype in either the most or the least perturbing environment.

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).

Evidence for organism-wide asymmetry in five bird species of a fragmented afrotropical forest

Empirical evidence for between-trait correlation in fluctuating asymmetry (FA) at the individual level is generally lacking or contradictory. Yet the assumption of organism-wide asymmetry, estimated by the asymmetry of any given trait, is inherent to most studies that use FA as a measure of developmental stability (DS). A commonly assumed reason for this weak between-trait correlation is the low repeatability of individual, single-trait asymmetry. In this paper we describe high repeatability and significant between-trait correlation in population- and individual-level FA in five afrotropical bird species inhabiting a fragmented cloud forest. Absence of anti-symmetry and of between-trait correlation in signed FA levels permits us to translate the observed patterns into the presumed underlying DS, using the concept of hypothetical repeatability. This correction, which has not been applied before in this context, proved adequate as it yielded correlations comparable to those found at the population level.

No bilateral asymmetry in wild-caught, endangered Poeciliopsis o. occidentalis (Gila topminnows)

A well-known example of a positive association between the level of genetic variation and fitness in endangered species is the studies in Gila topminnow. The work of Vrijenhoek and his colleagues showed lower values for four fitness correlates in laboratory-raised fish from a population that was monomorphic for all 25 allozyme loci examined (Monkey Spring) than for fish from a population that was heterozygous for two of the allozyme loci (Sharp Spring). Here, bilateral asymmetry in wild-caught fish from these sites is examined to determine if the same environmental stressor (or one with similar effects) was present in natural populations of Gila topminnows. There were no differences for all three traits, lateral-line scales, pectoral-fin rays and pelvic-fin rays, previously found to be significantly different between Monkey Spring and Sharp Spring. This, coupled with our earlier finding that fish raised in our laboratory (where there is low mortality) had low bilateral asymmetry, supports the hypothesis that some unknown, and perhaps unnatural, environmental factor in the Vrijenhoek laboratory was responsible for the differences observed in bilateral asymmetry between Monkey Spring and Sharp Spring.

Stress and asymmetry during arrested development of the Australian sheep blowfly

The dieldrin and diazinon resistance systems of the Australian sheep blowfly (Lucilia cuprina) have been used previously to relate stress, departures from bilateral symmetry, developmental stability and relative fitness. These systems are now used to consider stress and asymmetry in a developmental context. Larval to adult development is shown to be significantly impaired after arrested development at 8 degrees C, however the asymmetry score of adults of a given genotype is similar after arrested or continuous development. Selection against dieldrin-resistant and unmodified diazinon-resistant genotypes occurs during arrested development because greater proportions of these genotypes pupae at 8 degrees C than do susceptible or modified diazinon-resistant genotypes. Pre-pupae of all genotypes complete development equally successfully when transferred from 8 degrees C to 27 degrees C. Adults fail to emerge when pupae formed at 8 degrees C undergo this temperature transition. Temperature-shift experiments show the asymmetry score is determined between pre-pupal and pupal stages of the life cycle. This stage occurs at 27 degrees C in arrested and continuously developing cultures providing an explanation for the independence of stress, selective mortality during developmental arrest and asymmetry score. The results emphasize the need for genetic, environmental and developmental data before an asymmetry phenotype can be directly related to developmental stability and relative fitness.

Developmental stability and environmental stress in Salmo salar (Atlantic salmon)

We have studied the developmental stability (measured as fluctuating asymmetry of five meristic characters) of three populations of Atlantic salmon Salmo salar (rivers Imsa, Lone and Ogna, western Norway). All three populations were both sampled in the wild, and hatched and reared in a common environment in a hatchery (with water from the river Imsa) from fertilization until smoltification. Both the Imsa and Lone hatchery populations have been sea-ranched in the Imsa for 10 years, whereas the Ogna populations is novel to the hatchery environment. Individual biochemical heterozygosity was scored at 50 loci, of which 11 were polymorphic. There was no correlation between biochemical heterozygosity and fluctuating asymmetry at the individual level, neither when tested within groups nor when tested between groups. There were no differences in fluctuating asymmetry between wild and hatchery Imsa and Lone fish, indicating that the hatchery environment did not disrupt early developmental homeostatic processes. However, the Ogna hatchery fish had significantly elevated levels of fluctuating asymmetry compared to the wild Ogna fish, indicating that the hatchery environment was hostile. The Ogna hatchery fish also had significantly higher fluctuating asymmetry than the Imsa hatchery and the Lone hatchery fish. Maladaptation to the hatchery environment is the most likely explanation for the increased asymmetry in river Ogna fish.

Genotype, environment and the asymmetry phenotype. Dieldrin-resistance in Lucilia cuprina (the Australian sheep blowfly)

Dieldrin-resistant (Rdl/Rdl and Rdl/+) and susceptible (+/+) phenotypes of Lucilia cuprina were scored for departures from bilateral symmetry for bristle characters after development at different temperatures, larval densities or concentrations of dieldrin. The asymmetry phenotype of resistant flies was dominant and independent of developmental temperature and larval density. The asymmetry of susceptibles increased for temperatures and larval densities above and below standard rearing conditions. A positive correlation was observed between asymmetry score and dieldrin concentration for all genotypes. The susceptible phenotype did not attain the asymmetry score of resistant in any environment. Resistant phenotypes showed an antisymmetric pattern in each environment; fluctuating asymmetry was observed for susceptibles. The relevance of the results of genetic and general or specific environmental stresses to estimates of developmental perturbation is discussed.

Fluctuating asymmetry does not increase with moderate inbreeding in Drosophila melanogaster

Fluctuating asymmetry, the unsigned difference between character values on the left and right sides of an individual, is often thought to be highly correlated with the heterozygosity of individuals or populations. A large sample of Drosophila melanogaster individuals with an inbreeding coefficient of F = 0.25 was derived from a laboratory population and compared to a sample of outbred individuals for the fluctuating asymmetry of sternopleural bristle number. Inbred flies were not more asymmetric than outbred flies. There was no evidence for heritability of fluctuating asymmetry, as measured by variance among full-sib lines. Fluctuating asymmetry may not be a reliable measure of the degree of inbreeding at the relatively low levels found in most animal populations and should be used with caution in the management of endangered species.