Genetic diversity and demographic structure of the endangered Sorraia horse breed assessed through pedigree analysis

Abstract

The complete pedigree information included in the Sorraia horse studbook was analyzed to investigate on the breed's genetic variability and demographic structure and ascertain the status of the current population. The dataset included 653 animals registered since the breed's foundation, in 1937, until 2006. In some cases, only a reference population consisting of 206 living animals was considered for the calculations. The inbreeding coefficients (F) and the genetic contribution of each founder to the population's genetic pool were assessed and parameters such as average relatedness (AR) and genetic importance of the studs were, for the first time, computed for this breed. The average generation interval found for the whole pedigree was 7.94 yr. Consistent with previous results, low levels of genetic diversity were found for this breed. The unbalanced use of animals for reproduction during most part of the breed's history are reflected by the low values of effective number of founders (7.46) and effective number of ancestors (4) characterizing the whole population, with only 15 animals explaining its overall genetic variability. Extremely high values were reported for the F and AR coefficients and mean levels of 26.99% and 46.26% were observed. The rate of inbreeding per generation (5.2%) exceeds the 1% (0.01) maximum limit defined by FAO. Taking into account only the living population, roughly 97% of the animals showed F≥25% (some of them exhibiting values above 60%), and there were no cases with AR values lower than 50%. The average F, AR and mean kinship coefficients computed for this group were 36.90%, 55.11% and 0.34, respectively. Indicating high losses of genetic diversity through generations, great differences were found for the genetic contribution of each founder to the current genetic pool and some of these animals are no longer represented. In the more recent years, an improvement of the population's parameters can, although, be observed, especially as a consequence of the conservation strategies that are being carried out by the Breeders Association. The number of births registered in the studbook strongly increased in the last two decades and the average values of F and AR tended to stabilize and even decrease in the recent past. The results make clear that an efficient conservation plan is essential to insure the maintenance of this singular breed. Appropriate management measures must continue to be implemented and improved as a way to preserve the genetic diversity still present in the population.