Effects of inbreeding depression for growth traits in Iranian sheep: A meta-analysis study

Introduction: Inbreeding depression indicates a decrease in mean phenotypic value with increase in inbreeding level (Doekes et al 2021). The conventional method of livestock selection, in which animals are selected based on estimated breeding values, leads to increase in genetic achievement but also in inbreeding rates (Selvaggi et al 2010). Inbreeding is caused by increased homozygosity, which reduces the expression of dominance effects. When dominance effects are favorable (i.e., when dominance is in the desired direction), reducing the expression of dominant effects leads to a decrease in the mean phenotypic value. The degree of inbreeding may vary among traits (Doekes et al 2021). Loss of diversity and consequent increase in homozygosity may lead to reduced production in inbred animals. Furthermore, decreased inbreeding in domestic animals can lead to reduced selection response and genetic potential gains in economic traits. Therefore, it is important to estimate the effect of inbreeding on these traits in order to determine the magnitude of changes associated with increased inbreeding (Selvaggi et al 2010). The rate of inbreeding should be limited to maintain the genetic diversity at an acceptable level which is necessary for proper response to environmental changes and selection in the future (Van Wy ketal 2009). The effect of inbreeding depression on economic traits have been studied in many research studies. The results of these studies are not reliable because they come from special experimental conditions and limited sample size (Roger St-Pierrer 2007). In many cases, even contradictory results are observed from similar published studies. In such cases, meta-analysis with the nature of aggregating the results of individual studies could be used as a tool to achieve an overall result with higher reliability (Roger St-Pierrer 2007). In present study, meta-analysis was used to investigate the effect of inbreeding depression on weight at different ages in Iranian sheep based on the results of individual studies. By increasing the sample size, it is possible to comment with greater confidence on the effect of inbreeding on growth traits in Iranian sheep.
Materials and methods: Information related to inbreeding depression on body weight of Iranian sheep at different ages in 31 papers published in scientific journals, was used. Studied traits were birth weight, weaning weight and weights at six, nine and twelve months of age. To access these articles, a comprehensive search was conducted in Persian and English databases and journals using the keywords inbreeding, inbreeding loss, production traits, and sheep in Persian and English. The extracted data consisted of sample size and the regression coefficient between rate of inbreeding and the average of studied traits called inbreeding depression. Studies with absence of these information were removed from list of studies. Each study consisted of author name, year of publication, journal title, sample size, race, and regression coefficient presenting inbreeding depression and standard error or standard deviation. Inbreeding depressions were expressed in the form of regression coefficient in studies. Regression coefficients were converted to correlation coefficients to be used in meta-analysis. Normally, studies with significant results take more attention and publish more easily. These kinds of studies with higher chance to be repeated are called publication bias and may confuse the researchers. Funnel plot is a tool for presenting variation of studies around average. It is assumed that effects of studies must be distributed randomly and symmetrically around meta-analysis effect. In the case of publication bias, studies have an asymmetric distribution around meta-analysis effect. Funnel plot was used to check the publication bias in this study.
Random effects model was applied for analyzing the collected data. The meta-analysis was performed using CMA software version 3. Using the I2 and Q indices, the heterogeneity among studies was examined. Values between 0 to 25% shows very small amounts of heterogenty, values between 25 to 50% presents small heterogenty, values between 50 to 75% considerable amount of heterogenty and values higher than 75% shows high rate of heterogeneity among studies.
Results and discussion: The range of I2 index for fixed model was from 82.83 to 98.54. According to high heterogeneity among studies, random effect model was used for meta-analysis. According to the funnel plot, the result of the meta-analysis was free of publication bias, and on the other hand, examining the funnel plot for growth traits in this study showed that using a fixed effect model to express the results was inappropriate. The estimated correlation coefficients for birth weight were -0.001 (with confidence interval of -0.11 to 0.009), for weaning weight was -0.02 (with confidence interval of -0.443 to 0.003), for weight at six months was -0.46 (with confidence interval of -0.99 to 0.007), for weight at nine months was -0.03% (with confidence interval of -0.76 to -0.011) and for weight at twelve months was -0.09 (with confidence interval of -13.13 to -0.041). According to the P value, the result obtained from the meta-analysis showed that the effect of inbreeding depression on birth weight, weaning weight and weight at six months of age were not significant, while it has highly significant on weights at nine and twelve months of age in Iranian sheep breeds (P < 0.01). This means that with a one percent increase in inbreeding, the average of weights at nine and twelve months of age decrease by 43 and 90 grams, respectively. Therefore, using appropriate mating strategies for Iranian sheep breeds are very important for controlling inbreeding depression at 9 and 12 months of age. Choosing appropriate mating strategies in the herd to control and reduce inbreeding is important and should be considered.