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• Transmission Ratio Distortion patterns within Pura Raza Española horse population were studied. • This distortion can be influenced by various factors during gamete or embryonic development. • The study involved 126 394 trios genotyped using a panel of 17 microsatellite markers. • Regions with Transmission Ratio Distortion included 24 annotated genes. • Detected genes could potentially lead to reproductive health issues. Transmission Ratio Distortion (TRD) is a genetic phenomenon widely demonstrated in several livestock species, but barely in equine species. The TRD occurs when certain genotypes are over- or under-represented in the offspring of a particular mating and can be caused by a variety of factors during gamete formation or during embryonic development. For this study, 126 394 trios consisting of a stallion, mare, and offspring were genotyped using a panel of 17 neutral microsatellite markers recommended by the International Society for Animal Genetics for paternity tests and individual identification. The number of alleles available for each marker ranges from 13 to 18, been 268 the total number of alleles investigated. The TRDscan v.2.0 software was used with the biallelic procedure to identify regions with distorted segregation ratios. After completing the analysis, a total of 12 alleles (out of 11 microsatellites) were identified with decisive evidence for genotypic TRD; 3 and 9 with additive and heterosis patterns, respectively. In addition, 19 alleles (out of 10 microsatellites) were identified displaying allelic TRD. Among them, 14 and 5 were parent-unspecific and stallion-mare-specific TRD. Out of the TRD regions, 24 genes were identified and annotated, predominantly associated with cholesterol metabolism and homeostasis. These genes are often linked to non-specific symptoms like impaired fertility, stunted growth, and compromised overall health. The results suggest a significant impact on the inheritance of certain genetic traits in horses. Further analysis and validation are needed to better understand the TRD impact before the potential implementation in the horse breeding programme strategies. [ABSTRACT FROM AUTHOR]

Abstract: Spanish heavy horses play a very important role in keeping up ecologically important rural areas. The interest in their conservation and characterization has increased over the last few years. The aim of this work is to contribute to the characterization of the endangered Spanish heavy horse populations in order to obtain useful information to implement conservation strategies for these genetic stocks. A total of 426 horses from six Spanish heavy horse populations corresponding to the four main heavy horse breeds (Hispano-Breton (HB) – with most animals located in three areas: Burgos (HBbu), León (HBle) and Palencia (HBpa) – Jaca Navarra (JN), Burguete (B) and the Cavall Pirinenc Català (CPC)) were analysed using 22 body measurements, 10 indices calculated from the body measurements and 16 microsatellite markers. From a morphological point of view, all the populations are clearly differentiated, except for the HBpa female population, which is located between the HBbu and JN populations. The HBle has greater body size than the other populations analysed, especially when compared to the JN. The genetic analysis provides us with information about the history of the analysed breeds. However, the most important point borne out by this analysis is the clear identification of different genetic backgrounds within the Spanish heavy horse populations. The overall information given in this work leads us to consider that the genetic scenario of the Spanish heavy horses is more likely to be due to ‘ancestrally’ different genetic backgrounds. Spanish heavy horse breeds resulted from the crosses between native mares and foreign stallions. The CPC population was the first in which this introgression occurred and the use of foreign stallions became more common. This work presents results based on neutral genetic variation, but also within- and between-population differences in morphological traits that have undergone artificial or natural (adaptive) selection. This information should be taken into account in future conservation strategies in order to contribute towards the efficiency of conservation measures. In this sense, the main problems are the small population size and genetic degeneration, with the consequent future loss of diversity. Therefore, the most immediate and effective conservation priorities would be (1) to avoid inbreeding within populations, (2) to increase the population size, and (3) to facilitate genetic exchange among the populations. [Copyright &y& Elsevier]