Estimates of variance components due to parent-of-origin effects for body weight in Iran-Black sheep

- Maternal genetic effects are largely confounded with maternal parent- of-origin effects.
- The proportion of maternal parent-of-origin effect to the total phenotypic variance was high (12%-23%).
- Weaning weight and body weight at 6-month were significantly affected by maternal imprinting effect.
- Adding imprinting effects to the model can have beneficial effects on estimation of variance components for body weight.

In this study, variances due to differential expression of paternally and maternally imprinted genes for body weight of Iran Black sheep, a composite breed made by cross Chios rams with Iranian Baluchi ewes, were estimated. Data on body weight at birth (BW), weaning (WW), at the age of six months (W6), 9 months (W9) and one year (W12) were provided by the National Animal Breeding Center of Iran. Body weight records were obtained from a total of 5164 animals. The number of records used to estimate (co)variance components for BW, WW, W6, W9 and W12 were 4953, 3926, 3367, 2923 and 2704, respectively. Imprinting variances were estimated using the inverse of the gametic relationship matrix. Models were selected based on Bayesian information criterion. Estimates of variance for paternal imprinting effects, which induced changes in maternal gene expression, were approximately zero for all traits. Thus, adding paternal imprinting effects into model had no beneficial effects on our estimates. In contrast, maternal imprinting effects originating from paternal gene expression, had significant effects on body weight traits, and ranged from 12% (±6.6) to 23% (±5.2) of the total phenotypic variance. Interestingly, the best model for WW and W6 traits was achieved when maternal parent-of-origin effects were included in the model. Adding maternal parent-of-origin effects to the model resulted in a decrease in the estimates of heritability for all studied traits. Our results demonstrate the importance of including parent-of-origin effects into the model for the analysis of sheep body weight.