Sex influence on genetic expressions of early growth in Afshari lambs
Abstract. Very little is known about the genetic aspects of sexual dimorphism of body weight in domestic sheep, and therefore this study was conducted to quantify the genetic basis of sexual dimorphism for early-growth-related traits in Afshari lambs. Traits evaluated included birth weight (BW), weaning weight (WW), and growth rate (GR) in male and female lambs. Male lambs were 6.6 % heavier at birth, had 14.4 % higher preweaning growth rates and were 16.0 % heavier at weaning compared to female lambs. Levels of sexual-size dimorphism (SSD), expressed as the ratio of male to female means, for BW, WW and GR were 1.07, 1.14 and 1.15, respectively, which indicated low levels of SSD in the traits studied. Fixed effects of year of birth and type of birth interacted with sex effects, with greater variability in birth and weaning weights among years and birth types in male lambs, suggesting greater environmental sensitivity in the males. Bivariate animal models and restricted maximum likelihood (REML) procedures were used to estimate phenotypic variances and their genetic and non-genetic components in male and female lambs. Estimates of the direct heritability (h2) and additive coefficient of variation (CVA) for BW were higher in males. However, for WW and GR, heritability estimates were higher in females. In contrast, whereas the contribution of maternal permanent environmental effects (c2) to variation of BW was higher in females, for WW and GR higher estimates of c2 were observed in males. Respective genetic and maternal permanent environmental correlations between records on males and females were 0.986 and 0.723 for BW, 0.995 and 0.983 for WW, and 0.995 and 0.966 for GR, indicating possible sexual dimorphism only for maternal effects on BW. Based on an approximate 95 % confidence interval, none of the observed differences in variance components between sexes differed from zero and none of the observed genetic or maternal correlations differed from 1.0, indicating no need or opportunity for sex-specific selection strategies.
