Genetic and genomic analyses of embryo production and embryo quality traits for Holstein donors in Canada

Assisted reproductive technologies, such as superovulation or ovum pickup and in vitro production (OPU-IVP) of embryos, are frequently used in the Canadian dairy industry to produce more offspring from elite females in the population. Using these procedures is expensive and there is a large variability in the number of embryos produced and the quality of these embryos. Even though technological improvements have helped to increase the number and quality of embryos produced, the genetic and genomic ways of improving these traits have yet to be assessed for the Canadian Holstein population. The main objectives of this study were, therefore, to perform genetic and genomic analyses for two embryo production traits and also to perform a genetic analysis for embryo quality. To achieve this, two very large datasets were provided by Holstein Canada. The first one contained all successful in vivo and in vitro procedures performed on Holstein donors across Canada since 1980, where the total number of embryos and the number of viable embryos for each procedure was known. In the second dataset more detailed information was available, such as the quality of the embryos produced. Linear models were developed to analyse the embryo production traits and different statistical approaches were used to analyze embryo quality. A genome-wide association study and a functional analysis were performed for embryo production traits, allowing the identification of regions significantly associated with the total number of embryos and the number of viable embryos and the creation of a list of key regulator genes. Results showed potential for genetic selection for both the total number of embryos and the number of viable embryos, with heritabilities of around 0.15 ± 0.01. Genetic correlations between the number of embryos produced using different procedures (in vivo and in vitro) suggested that a similar number of embryos should be expected from a donor regardless of the procedure used. Similarly, genetic correlations for the number of embryos produced by donors, both as a heifer and as a cow, indicated that a donor should produce a comparable number of embryos regardless of its status at embryo recovery. A main region on chromosome 11 of the bovine genome was found to be significantly associated with the number of embryos, indicating a potential regulatory role of this region on embryo production. For embryo quality, heritability estimates were much lower at around 0.04 ± <0.01, so that selection for this trait would not be as efficient. Overall, these findings are of interest for the Canadian dairy industry since they provide useful information for breeders that are interested in producing embryos from the elite donors in their herds or in the population using in vivo or in vitro procedures. The results from this project are also of interest for future research that could focus on finding causative mutations affecting embryo production traits, but also for research in reproductive physiology to adapt protocols based on the donors’ genotype. Finally, considering that the traits studied are related to reproduction, the findings from this project are also of interest for global reproduction of dairy cattle.