Genetic components of litter size variability in sheep

Abstract

-Classical selection for increasing prolificacy in sheep leads to a concomitant increase in its variability, even though the objective of the breeder is to maximise the frequency of an intermediate litter size rather than the frequency of high litter sizes. For instance, in the Lacaune sheep breed raised in semi-intensive conditions, ewes lambing twins represent the economic optimum. Data for this breed, obtained from the national recording scheme, were analysed. Variance components were estimated in an infinitesimal model involving genes controlling the mean level as well as its environmental variability. Large heritability was found for the mean prolificacy, but a high potential for increasing the percentage of twins at lambing while reducing the environmental variability of prolificacy is also suspected. Quantification of the response to such a canalising selection was achieved.canalising selection / threshold trait / heterogeneous variances / litter size / sheep

Genetic heterogeneity of residual variance in broiler chickens

“…In previous studies, a stronger negative correlation (−0.62) was found by Sorensen and Waagepetersen [24] and positive correlations of 0.19 and 0.8 by SanCristobal-Gaudy et al [23] and Ros et al [20], respectively, although partly explicable as a scale effect.…”

“…Strong evidence for genetic variation in environmental variance also comes from direct analysis by Mackay and Lyman [16] of variance in bristle number among isogenic lines of Drosophila derived from natural populations, and in farmed species from more formal analyses than that presented here incorporating Bayesian models including genetically structured environmental variation for data on litter size of sheep [23], litter size of pigs [24] and body weight of snails [20]. Under the assumption that genetic segregation is unlikely to be the cause, the heterogeneity can be attributed to differences in response to environmental conditions.…”

“…If genetic variation in environmental variance can be incorporated into current selection tools, this could be of particular interest in threshold traits such as fecundity [23] and also where homogeneity of product is desirable, or even serve to harness a further source of variation.…”

“…Foulley and Quaas [4] discuss the estimation of source and magnitude of effect of heterogeneous variances within linear mixed models. There is evidence to suggest that residual heterogeneity is under genetic control, based on analysis of data on fecundity of sheep by SanCristobal-Gaudy et al [23], who used a log linear model to test the significance of additive genetic influence on the residual variance. Earlier and more direct evidence for genetically structured adaptation or environmental sensitivity is found in studies of genetic assimilation [27] canalisation [18] and environmental plasticity [26].…”

“…Variation in environmental sensitivity under genetic control might be explained by the assumption that genotypes at individual loci differ not only in their effect upon the mean but also on the variance of the trait [7,10,23]. Consequently, simple directional selection on the trait would change environmental variability [10].…”