عنوان مقاله [English]
Evaluating the selection of improved Fars native fowl with the inbreeding assessment approach
Performance of native fowl can be improved by making changes in feeding, rearing and health issues. On the other hand, genetic improvement of these breeds can be achieved through the breeding programs such as selection, crossbreeding, or both. Selection programs may be time consuming, but implementing them will lead to a continued improvement (Padhi 2016). Considering the genetic diversity among the native fowl breeds of Iran, several breeding stations were established in different provinces of the country for the purpose of reproduction and genetic improvement of these breeds. Mating related animals in closed populations leading to accumulated inbreeding and reduced genetic diversity has destructive effects on additive genetic variance and phenotypic values (Falconer and Mackay 1996). Inbreeding is associated with an increase in homozygosity and usually decrease the fitness of individuals in the population, which is referred to as inbreeding depression (Ayroles et al. 2009). Inbreeding depression in domestic animals can cause reduced selection response and potential genetic gain in economic traits (Selvaggi et al. 2010). Since the improved Fars native fowls have been raised in a closed population and selected for some important economic traits during the successive generations, their inbreeding coefficients may increase and reduce the effectiveness of breeding programs. Therefore, it is of significant importance to monitor the inbreeding rate and its consequences on different traits.
The purpose of this study was to monitor the inbreeding rate and evaluate its possible effects on some important economic traits in improved Fars native fowl population using the pedigree information of 25 generations via different models.
Materials and methods:
Data of 63250 birds during the period 1369-1397 (25 generations) recorded in the breeding station of Fars native fowl were included in the study. Studied traits include body weight at hatch (BW1), body weight at 8 weeks of age (BW8), body weight at 12 weeks of age (BW12), age at sexual maturity (ASM), weight at sexual maturity (WSM), egg weight at 1st day of laying (EW1), egg number (EN) and average egg weight (AEW). Individual and maternal inbreeding coefficients of all birds estimated using the CFC program. Estimated inbreeding coefficients grouped into seven different categories of inbreeding: 0, 0 to 5%, 5 to 10%, 10 to 15%, 15 to 20%, 20 to 25% and 25 to 30%. Regression coefficients of studied traits on individual and maternal inbreeding percentage were estimated by Wombat software (May 2007) and restricted maximum likelihood (REML) method using six different models. Individual and maternal inbreeding coefficients were also included as a covariate in the model. In this study, among the six statistical models considered for each trait, finally, the appropriate model for each of them was selected through three methods of likelihood ratio test (LRT), Akaike’s information criterion (AIC) and Bayesian information criterion (BIC).
Results and discussion:
Pedigree analysis showed that 40184 birds were inbred and the mean of individual and maternal inbreeding was relatively low over 25 generations. The average individual and maternal inbreeding did not differ much over the generations. According to the results, the mean inbreeding for all birds was approximately equal to 2 % and in inbred birds was 4 %. From the fifth generation onwards, the average inbreeding (individual and maternal) of birds in the whole population was increasing. In the first four generations, inbreeding rate of population was estimated to be zero, which may be due to the unknown pedigree information in the first generations. Various studies have shown that accurate estimation of inbreeding is highly dependent on pedigree information. The results of a previous study on laying hen strains indicated that pedigree information influenced the inbreeding estimation in the first generations (Szwaczkowski et al. 2003). In another study, Cassell et al. (2003) reported that the use of incomplete pedigree in estimating the mean inbreeding reduces the mean inbreeding estimate and the variance of these estimates in cows. From the fifth to eighth generation, the rate of individual and maternal inbreeding was very small. Distribution of birds in different categories of inbreeding showed that 36.57 % (23066 birds) were non-inbred. Classifying birds into different inbreeding groups indicated that the highest number of inbred birds was in the inbreeding group 0 to 5 % (47.72 %) and 5 to 10 % (15.48 %). Although the number of inbred birds is high, but the amount of inbreeding coefficient is significantly low, reflecting careful planning of mating in the station. The study of Kamali et al. (2007) on the pedigree information of Fars native fowl (21245 birds) during eight generations demonstrated that the inbreeding rate is low. In addition, in the previous study on the improved Mazandaran native fowl during 26 generations, it was shown that the rate of inbreeding is relatively low, which is in accordance with the results observed in the present study (Ghorbani and Omrani 1399). According to the fitted models, model 5 for BW1, BW8 and BW12, model 6 for ASM and AEW, model 2 for WSM, model 4 for EN and EW1 considered as the most suitable model. Estimating the inbreeding depression in the studied traits revealed the most effect of inbreeding on the BW12, so that for every 1 % increase in individual inbreeding, BW12 is reduced by 2.14 grams. Also, for every one % increase in individual inbreeding, BW8 decreases by 1.07 grams. The highest effect of inbreeding was observed on BW8 and BW12, so that for every 1 % increase in individual inbreeding, BW12 decreases by 2.14 grams and BW8 decreases by 1.07 grams. ASM was significantly affected by inbreeding depression. ASM increased by 0.38 day per 1 % increase in inbreeding. The finding of earlier studies regarding the inbreeding effect on the ASM have shown that increased inbreeding does not have the same effect on different strains. For example, increase in inbreeding level results in increased ASM in the Leghorn (Sewalem et al. 1999) and decreased ASM in the New Hampshire (Szwaczkowski et al. 2003). Taken together, pedigree analysis showed that the depression effect of inbreeding on egg traits including EN, EW1 and AEW was negligible.
According to the results of pedigree analysis, inbreeding rate of improved Fars native fowl population is increasing at an acceptable level with a relatively gentle slope. In addition, depression caused by inbreeding in the population was fairly low. Since maintaining genetic diversity and keeping down the inbreeding rate in the station are considered as main factors in developing the breeding programs, it can be concluded that the implementation of breeding programs and selection of superior birds during the generations has gone in the right direction.