Heritability Of Fluctuating Asymmetry Research Articles

Between-family variation and quantitative genetics of developmental instability of long bones in rabbit foetuses

The genetic basis of developmental instability (DI) remains largely unknown as a result of its morphological expression, fluctuating asymmetry (FA), poorly reflecting DI, especially if few traits are studied. The typically low values of heritability of FA (h2FA) can be translated into higher values of DI (h2DI) by the hypothetical repeatability, yet leading to wide confidence intervals. Thus, high sample sizes and/or several traits are indispensible for reaching meaningful conclusions. To obtain more insights into quantitative genetic variation of DI, we investigated between-family variance in DI in six long bones of 1126 foetuses of the New Zealand white rabbit from a full-sib experiment. We applied different approaches to obtain genetic parameters for DI. Heritabilities and the coefficients of between-family variation (CVB) were calculated for six individual traits and composite indices. The results obtained, despite a likely upward bias as a result of maternal and non-additive effects, lend support to the presence of moderate additive genetic variance for DI. It is suggested that, in foetal traits, the environmental variance was minimal, leading to a high likelihood of detecting genetic variation in DI, thus creating an ideal model system for studying the genetic basis of DI. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 33–42.

THE QUANTITATIVE GENETICS OF FLUCTUATING ASYMMETRY: A COMPARISON OF TWO MODELS

The genetic basis of fluctuating asymmetry (FA), a measure of random deviations from perfect bilateral symmetry, has been the subject of much recent work. In this paper we compare two perspectives on the quantitative genetic analysis of FA and directional asymmetry (DA). We call these two approaches the character-state model and the environmental responsiveness model. In the former approach, the right and left sides are viewed as separate traits whose genetic coupling is manifested by the genetic correlation. This model leads to the relationship, h2DA = h2[(1 − rA)/(1 − rP)], where h2 is the heritability of each component trait (assumed to be the same), rA and rP are the genetic and phenotypic correlations between traits, respectively. Simulation shows that, under this model, the heritability of FA is considerably less than that of DA, except when heritabilities are very close to zero. The environmental responsiveness model permits genetic variance in FA even when the genetic correlation between traits is 1. Simulation shows that under this model the heritability of FA can be uncoupled from that of DA. The additive and nonadditive components of the component (right and left) traits, their DA and FA values are estimated using a diallel cross of seven inbred lines of the sand cricket, Gryllus firmus. Four leg measurements were made and both the individual DA and FA values and the compound measures DASUM and CFA estimated. The heritabilities of the compound measures are slightly larger than the individual estimates. Dominance variance is observed in the individual traits but predicted to be an even smaller component of the phenotypic variance than the additive genetic variance. The estimated values confirm this, although a previous study has demonstrated that dominance variance is present. Because the heritabilities of FA are generally larger than those of DA, which never exceed 0.02, the environmental responsiveness model is more consistent with the data than the character-state model. A review of other data suggests that both sources of variation might be found in some species.

Detecting genetic variation in developmental instability by artificial selection on fluctuating asymmetry

Abstract Fluctuating asymmetry (FA) is frequently used as a measure of developmental instability (DI). Assuming a genetic basis to DI, many have argued that FA may be a good indicator of genetic quality to potential mates and to human managers of populations. Unfortunately FA is a poor indicator of DI, making it very difficult to verify this assertion. A recent review of the literature suggests that previous studies of the inheritance of FA and DI using half-sib covariances and parent–offspring regression have been unable to put meaningful limits on the heritability of FA and DI because of the extremely low power of the experiments performed. In this study, we consider the power of artificial selection on FA as an alternative approach to studying the inheritance of FA and DI. Using simulations, we investigate the efficacy of selection for both increased and decreased FA for detecting genetic variation. We find that selection for increased FA has much more power to detect the presence of genetic variation than does selection for decreased FA. These results hold when realistic sample sizes are employed. Artificial selection for increased FA is currently the most powerful approach for the detection of genetic variation in DI.

Genetics of wing size asymmetry in Drosophila buzzatii

Abstract 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 (FAWL, FAWW) were low (0.037) but significantly different from zero, and the genetic correlation between FAWL and FAWW 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 FAWL and FAWW that combine multiplicatively to produce overdominance for FAWA, 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.

Fluctuating asymmetry of sexual and nonsexual traits in stalk-eyed flies: a poor indicator of developmental stress and genetic quality

Abstract It has been proposed that females use fluctuating asymmetry (FA) in sexual ornaments to assess male quality. FA of sexual traits is predicted to show greater sensitivity to stress than FA of nonsexual traits, and to be heritable. We used a half-sib mating design and manipulation of larval food environment to test these predictions on stalk-eyed flies, Cyrtodiopsis dalmanni, in which females prefer males with larger eyespans. We measured size and FA of eyestalks and of two nonsexually selected characters, wing length and width. We found no evidence of an increase in FA under larval food stress in any of the individual traits, although trait size decreased under stress. We combined FA across traits into a single composite index, and found that males reared in the most benign larval environment had significantly higher composite FA than males reared on other media. There was no such effect in females. Heritability of FA was not significantly different from zero in any of the traits, in any of the environments, although trait sizes showed high heritability. We conclude that FA in sexual and nonsexual traits is a poor indicator of developmental stress and genetic quality.

Genetic analysis of fluctuating asymmetry for skeletal characters in mice.

Fluctuating asymmetry (FA)--nondirectional variation between left and right sides of bilateral characters--has become increasingly used as a measure of developmental stability. In the comparison of stability levels among various populations, however, it often has been assumed that FA does not have a genetic basis (i.e., its heritability is zero). To test this hypothesis, heritabilities were estimated via maximum likelihood correlations of parents and offspring for FA in eight quasi-continuous skeletal characters (primarily presence/absence of various foramina) in randombred house mice. Genetic correlations between left and right sides for each of the characters also were calculated to test whether they were one, as would be predicted if differences between sides are purely environmental in origin. Heritabilities of FA in all eight characters were not significantly different from zero, as hypothesized, their mean value being very low (0.032). Similarly, genetic correlations between sides were not significantly different from one for most of the characters. It was concluded that for this specific population of mice, there was no detectable additive genetic variance for FA.

The heritability of fluctuating asymmetry and the genetic control of developmental stability.

Fluctuating asymmetry (FA) is the unsigned difference between the left and right sides for some trait on an organism with no average difference between the sides. Because FA is assumed to reflect differences in development but not in genetic effects, it is often used as a measure of the developmental stability of an organism. The genetic properties of developmental stability, such as the heritability and correlations among characters, are of some interest, because the evolutionary future of developmental stability in populations depend on these quantities. As such the heritability of FA has often been measured. This paper notes a substantial bias in the estimation of the heritability of developmental stability; FA is a trait with very low repeatability and as such is only a weak estimation of the developmental stability of an organism. The biases are correctable, though from a knowledge of the coefficient of variation of FA in the population being measured. This paper demonstrates why the heritabilities of fluctuating asymmetry are always so small in well designed experiments and suggests more accurate measures of the genetic properties of canalization. Furthermore, the correlation of FA of different traits in the same individuals is always low, which is inconsistent with the mechanisms proposed for the development of FA. Because of the low repeatability of FA, these correlations are expected to be low.