Effect of Inbreeding Coefficient on Growth and Fitness Traits in a Closed Flock of Corriedale Sheep

The Corriedale sheep which is known for its good mutton conformation, excellent wool characteristics, relatively early maturity and having good range characteristics was imported in the state during sixties for use in improvement of local sheep at Mountain Research Centre for Sheep and Goat, Shuhama (MRCSG) which is one of the organized sheep farms found in the valley. Since the time of its establishment in late 1960’s, the animals of the station over the years were allowed to mate within the selected animals of the flock as there were very little transfer/ importation of exotic animals from another farms since the farm was established within the farm. This has led to apprehensions over the loss of genetic diversity through years.
       
The mating of related animals within a closed population reduces the genetic variability and increases the rate of inbreeding (Norberg and Sorensen, 2007; Barczak et al., 2009). Genetic diversity is central to the further genetic improvement, improving the fitness and resilience of the animals to the abiotic and biotic stresses due to climatic changes. Besides heterozygosity and allelic diversity, level of inbreeding coefficient is an important measure of the genetic diversity within a population. The latter is the consequence of the unavoidable mating between the closed relatives in a closed flock. Inbreeding is the probability that two alleles at any locus are identical by descent and occur when related individuals are mated to each other. The initial consequence of inbreeding is inbreeding depression, which reduces the performance in terms of growth, production, health, fertility and survival traits. The rates of inbreeding must be limited to maintain diversity at an acceptable level  (<6.25%) (Boon, 2014) so that genetic variation will ensure that future animals can respond to selection and changes in environment. The most straight-forward approaches for measuring inbreeding utilize the inbreeding coefficient “f” (Wright, 1922): defined as the probability that two alleles at a locus are identical by descent (IBD). For example, relatives can be deliberately mated to produce individuals of known f, which are then compared to outbred individuals from the same population. A similar approach is to use pedigree records to calculate f for all of the individuals in the population. Estimating inbreeding coefficient for most populations is difficult because it generally requires knowledge of the pedigree of the individuals, but in populations in which parents are known for several generations, good estimates of inbreeding coefficients have been obtained.

Data collection
 
Data sets spanning over a period of 49 years (1969 to 2017) were retrieved from birth registers and pedigree sheets, maintained at Mountain Research Centre for Sheep and Goat (MRCSG), F.V.Sc and A.H, SKUAST-K Shuhama. MRCSG is located at 33°53' latitude N and 74°37' longitude E, in district Srinagar at an altitude of 5300 ft. above mean sea level. Srinagar has a temperate climate. Winters are cool, with daytime average of 2.5°C (36.5°F) and temperatures below freezing at night, during January. Moderate to heavy snowfall occurs in winter. Summers are warm with a July daytime average of 24.1°C (75.4°F). The average annual rainfall is around 720 mm (28 in). Spring is the wettest season while autumn is the driest. The highest temperature reliably recorded is 38.3°C (100.9°F) and the lowest is -20.0°C (-4.0°F). Generally, the temperature ranges from -8°C during winter to 33.5°C during summers.
 
Management and feeding
 
The station follows semi-migratory, semi intensive mode of rearing. Animals were machine shorn twice a year spring and autumn. The first clip was harvested at age of six months. The flocks are provided timely veterinary healthcare by qualified veterinarians. The animals were dosed regularly against endoparasites and dipped twice a year, besides providing vaccination cover as per recommended schedule. As a routine practice ewes are put to mating after attaining the age of 18 months from late September to November after being flushed on nutrient rich pastures.

Traits under study
 
Litter size was recorded at the time of lambing whereas age at first lambing, total lambs born per ewe and survival rate were calculated separately. The mean, standard error and coefficient of variation (CV) of all fitness traits were computed statistically by SPSS software. Coefficient of variation was calculated by using the following formula:
                                       
Individual inbreeding coefficients for each animal was computed using all known relationships among animals by CFC software programme (Sargolzaei et al., 2006). On the basis of the inbred coefficient the animals were divided into five inbred classes. This was done based on the amount of inbreeding in that particular class (0, 0-6.25, 6.25-12.50, 12.50-25 and >25). Analysis was done using general linear model procedure (GLM) procedure of SAS (SAS, version 9.3, 2010). Different inbreeding coefficients levels, sex of animal and year of birth were used as fixed effects to analyze the data and following statistical model was used to serve the purpose.
                              
Where,
Y_ijkl is the l^threcord of the animal belonging to i^th period, j^th sex and k^th inbreeding class, F_k is the effect of k^th inbreeding class (F), m equals overall population mean, P_i fixed effect of i^th period, S_j is the fixed effect of j^th sex,  e_ijkl= Random error associated with each observation, NID (0, σ ²e). However, effect of sex was not used for analyzing age at first lambing (AFL).
       
The statistical significance of various fixed effects in the least squares model was determined by ‘F’ test. For significant effects, the differences between pairs of levels of effects of inbreed class were tested by Duncan’s multiple range test (DMRT) as modified by Kramer (1957).

Descriptive statistics
 
Descriptive statistics of growth and fitness traits is presented in Table 1. The coefficient of variations (CV %) of all traits under study were low for to high indicating that the traits had low to high variability. The highest CV% was observed for TLB (53.63%) whereas lowest CV% was observed for 9MW. The variability associated with different traits raveled there is scope for their improvement through selection. More or less similar CV% and mean for BW, 6MW, 12MW and AFL were also observed by Rather (2019) in Kashmir Merino sheep.
 

 
Inbreeding coefficients
 
Average inbreeding coefficient (F) during different periods and distribution of animals according to different level of inbreeding coefficient are presented in Table 2 and 3, respectively. Grouping of animals according to level of inbreeding coefficient for birthweight is represented in Fig 1.  The average inbreeding coefficient (F) of all animals was 18% and overall percentage of inbred animals over the periods was 43.3% (Table 2). Both the inbreeding coefficient (F) and percentage of inbred animals presented a more or less a linear increase over the periods. During the last period 80% of the animals were inbred. Similar increasing trend was observed by Mandal et al., (2005) in Muzaffarnagari Sheep and Rzewuska et al., (2005) in Booroola flock. The increasing inbreeding coefficient over the years may be attributed to closed nature of the research centre with no importation during the period of study, small population size and mating of inbred males and female individuals. The results of the present investigation were in consonance with the report of li et al., (2011) in local sheep. However lower estimates were reported by Eteqadi et al., (2014) in Guilan sheep, Mokhtari et al., (2015) in Moghani sheep, Naghavian et al., (2016) in shirvan Kordi sheep and Patiabadi et al., (2017) in Iranian shal sheep. Maximum number of animals were in the first class (F=0) and the minimum number of animals were in the fifth class (F>25) for all the traits under study.
 

 

 

 
Effect of inbreeding on growth and fitness traits
 
The average estimates of growth and fitness traits along with effect of inbreeding (F) on these traits in Corriedale sheep is presented in Table 5 and the regression of growth and fitness traits on inbreeding is presented in Table 4. Negative effect of increased inbreeding coefficients (F) was observed on all the traits under study (Table 5). The effects of inbreeding are high in a small population (Windig et al., 2018). Many authors have also reported negative effect of inbreeding on different growth and fitness traits (Van-wyk et al., 2009). However, the results of present study were contradictory to results of Rzewuska et al., (2005) in Booroola flock pertaining effect of inbreeding on fertility and other reproduction traits. The regression of most of the traits on F was negative except age at first lambing. Highly significant effect of increased inbreeding (F) was observed on all growth traits (except 9MW), AFL, TLB, SR, while as significant (p>0.05) and non-significant of F was observed on 9MW and LS, respectively (Table 5). All the significant effects with negative value indicated of significant inbreeding depression in these traits. Similar results were also reported by Wang et al., (2020) The positive value for AFL also corresponds to inbreeding depression in AFL. Significant detrimental effect of inbreeding on body weight traits from birth to yearling stage were also observed by Mandal et al., (2005) in Muzaffarnagari sheep. The Linear regression indicated that an increase of 1% of inbreeding coefficient  was associated with a change of -0.016 kg, -0.077kg, -0.079 kg, -0.12 kg, -0.13 kg, 5.384 days, -0.11, -0.05 and -0.07% in BW, WW, 6MW, 9MW, 12MW AFL, LS, TLB, SR, respectively (Table 4). Among the significant effects, the first level with zero inbreeding was significantly different from other classes with inbreeding levels from 0-6.25, 6.25-12.5, 12.5-25 to >25 per cent except birth weight which did not differ from 0-6.25 level of inbreeding. However, lowest values of growth and fitness traits (except AFL) were found in inbreeding levels of >25 per cent. The regression coefficient estimate for birth weight was (-0.016 kg %) is comparable to reported estimates in literature for different sheep breeds (Van Wyk et al., 1993; Boujenane and Chami, 1997; Analla et al., 1998; Mandal et al., (2005); Rzewuska et al., (2005). The effect of lamb inbreeding on WW (-0.048 kg/%) was in consonance with, Mandal et al., (2005) in in Muzaffarnagari sheep. However, a lower estimate was found by Boujenane and Chami (1997). Significant negative effect on 6, 9 and 12 months body weight was also observed by Dorostkar et al., (2012) in Iranian Moghanisheep, Gowane et al., (2014) in Malpura sheep, Naghavian et al., (2016) in Shirva Kordi sheep and Venkataramanan et al., (2016) in Nilagiri sheep. The significant deleterious effect of increased inbreeding was observed on all fitness traits in the present study. However, Rashidi et al., (2014) in Markhoz Goats and Patiabadi et al., (2017) in Iranian sheep observed non-significant deleterious effect on fitness traits. Increase in level of inbreeding results in decrease of all fitness traits under study except AFL which present undesirable increase.


 

The present study revealed that levels of inbreeding in a closed flock of the Corriedale sheep have increased over the period. There was a constant trend in accumulation of inbreeding over the years with a marked increase in the number of inbred animals. Increased inbreeding depression in lambs significantly reduced body weights at all ages from birth to yearling body weight. Litter size, total lambs born and survival rate also got reduced with increased inbreeding. However, increase in average age at first lambing was observed. As the increased level of inbreeding of lambs had detrimental effect on body weight and fitness traits in closed Corriedale flock with small population hence introduction of variability through importation of Corriedale germplasm with uses of modern ICT bossed tool for data management is highly recommended to avoided deleterious negative effect of inbreeding.

The authors declare that there is no conflict of interest.

The Authors are thankful to In-charge and staff members of the farm of MRCSG, Farm, F.V.SC, Shuhama, SKUAST-K, for providing necessary facilities and support for the present study.