Estimating effects of inbreeding and individual increase in inbreeding on growth traits of Moghani sheep
IntroductionThe unavoidable mating of related animals in closed populations leads to accumulation of inbreeding and decreased genetic diversity. The loss of diversity and an increase in homozygosity may result in decreased productions and fitness of inbred animals. It is apparent that different breeds as well as different traits vary in their response to inbreeding (Mackinnon 2003). It is important to account for the effects of inbreeding in populations undergoing selection to properly adjust the breeding program for the potential reduction in performance. The objectives of this study were to estimate the inbreeding trend and inbreeding depression on growth traits of Iranian Moghani sheep.
Materials and methodsPedigree records on the research flock of the Iranian Moghani sheep kept at the Jafarabad Breeding Station from 1990 to 2016 were used for analysis. The pedigree completeness index (PCI) proposed by MacCluer et al. (1983) was used to describe the degree of completeness of pedigree. In addition, for each individual, the number of complete generations equivalent (CGE) was computed as the sum of (1/2)n, where n is the number of generations separating the individual to each known ancestor (Maignel et al. 1996). Inbreeding coefficient for all animals were estimated using the method of Meuwissen and Luo (1992). Average inbreeding coefficients per year were computed and annual increases in inbreeding were estimated by linear regression over time. Individual increase in inbreeding coefficients (ΔFi) was calculated according to the methodology described by González-Recio et al. (2007) and modified by Gutiérrez et al. (2009) as ∆Fi = 1− √1−Fit i −1 , where Fi is the inbreeding coefficient of individual i and ti is the number of known equivalent generations for this individual. The analyzed traits for estimating inbreeding depression were birth weight (BW), 3-month weight (3MW), 6-month weight (6MW), 9-month weight (9MW), and yearling weight (YW). For estimating the inbreeding depression, only animals with PCI > 0.6 were kept in all analyses. All animals were grouped into three classes: first class included non-inbred animals (F=0); second class included animals with 0<F≤0.10, and third class included animals with F>0.10. Then inbreeding included in the model as the categorical fixed effects. Also, in a separate analysis, inbreeding and individual increase in inbreeding were included in the model as a linear covariate.
Results and discussionProportion of animals with F=0, 0<F≤0.10 and F>0.10 in all population were 78%, 21%, and 1%, respectively. These proportions in animals with PCI>0.6 were 23%, 74% and 3%, respectively. Average inbreeding of all and inbred indivduals within the animals with PCI>0.6 were 1.00% and 1.32%, respectively. The rate of inbreeding is more important than estimated level of inbreeding in genetic management of the population (Falconer and Mackay 1996; Bijma 2000). The evolution of the mean inbreeding, pedigree completeness index (PCI), and complete generations equivalent (CGE) of animals across years of birth are given in Figure 1. Trend of inbreeding was more coincident with PCI than CGE. Such a discrepancies in the behavior of two index of pedigree completeness could be attributed to the differences in the computation methods. From the linear regressions estimated, the rate of inbreeding was 0.03% (P < 0.01) per year equal to 0.12% per generation in the studied period. This estimated rate of inbreeding was less than the critical levels (1% per generation) recommended by FAO (1998) and Bijma (2000). The estimated rate of inbreeding for Moghani sheep in this study was similar to those reported by Hossein-zadeh (2012) and Gholambabaeian et al. (2012) for this breed. Also it was close to values reported for Iranian LoriBakhtiari, Sangsari and Zandi sheep (Almasi et al. 2014; Rashedi Dehsahraei et al. 2013; Sheikhlou et al. 2017). Nevertheless, it was less than the estimates for the Iranian Karakul, Iran-Black and Baluchi sheep (Bahri et al. 2015; Mokhtari et al., 2014; Tahmoorespur and Sheikhlou 2011). This differences in inbreeding rate could be attributed to the differences in the pedigree completeness of animals and also to the low proportion of matings between relatives and better control of inbreeding in Moghani sheep. Table 3 shows the least square means for body weight traits in different inbreeding classes of animals. The mean BW and 3MW of animals in the second class was lower than non-inbred animals (P<0.05). However, means of the BW and 3MW of animals in third class were higher than the second class. In other words, some of the heavy animals were between the highly inbred animals. Similar to our results, Prod’Homme and Lauvergne (1993) reported increased prolificacy with increasing inbreeding. They concluded that the positive effects of selection and improved management on prolificacy were larger than the negative effect of inbreeding. However, it should be kept in mind that the low proportion of animals in the third class of inbreeding in this study may also have contributed to these results. According to the Table 3, means of the 6MW, 9MW and YW have decreased with increasing in inbreeding, but the differences between inbreeding classes were not significant. The regression coefficients of BW, 3MW, 6MW, 9MW and YW on inbreeding were 0.006, -0.023, -0.051, -0.017 and -0.119 and on individual increase in inbreeding were 0.022, -0.044, -0.185, -0.111 and -0.326, respectively and were not significant (P>0.05). Converting individual increase in inbreeding coefficients to the equivalent inbreeding for an animal with an average depth of pedigree resulted in inbreeding depression of 6, -13, -55, -33, and -96 gram in BW, 3MW, 6MW, 9MW and YW, respectively.
ConclusionConsidering the results obtained in this study, large proportion of animals in this flock with good pedigree completeness were inbred. Thus, implementation of methods to control inbreeding in this flock are suggested to achieve desired genetic gain with minimum loos of genetic diversity in the future. It seems that preweaning traits (BW and 3MW) is more affected by inbreeding rather than other growth traits in this breed. In this study the pattern of the changes in inbreeding depression due to using individual increase in inbreeding in the model were not the same across the different body weight traits.
Journal of Animal Science Research, Volume:28 Issue:3, 2018
81 - 96
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