Embryo survival to accelerate genetic progress in dairy herds (review)

Continuity of genetic progress and the use of advanced technologies in the breeding of highly productive livestock are the distinctive features of modern dairy cattle breeding (G.R. Wiggans et al., 2017; B.V. Sanches et al., 2019). An example of Holstein cows of North American selection indicates the achievement of genetic changes (more than 56,0 %) in animals over 50 years (1963-2013), when milk yield doubled from 6619 kg to 12662 kg (A. Garcia-Ruiz et al., 2016). Along with this, genetic improvements aimed at higher milk yields have decreased the reproductive capacity and impaired health of cows (J. Kropp et al., 2014; L. Hyun-Joo et al., 2015, B. Fessenden et al., 2020) that is a global problem (E.S. Ribeiro et al., 2012; K.J. Perkel et al., 2015). High-yielding cows are 30-50 % susceptible to mastitis, metritis, lameness and other diseases (I. Cruz et al., 2021), and the average calving rate is about 40-50 % with 90-95 % fertilization (M.G. Diskin et al., 1980; P. Humblot, 2001). The embryonic period of cows which is up to 42-45 days of gestation (J. Peippo et al., 2011) is characterized by high (up to 40 %) embryonic mortality (D.C. Wathes, 1992; K.J. Perkel et al., 2015; P. Rani et al., 2018), the multifactorial etiology of which has not yet been elucidated. Loss of genetic potential (unborn bull sires, replacement heifers, mothers of bull sires, and embryo donor cows) slows down selection process in dairy herds (M. Ptaszynska, 2009). This review focuses on the genetic predisposition of the embryo to survival as one of the important factors determining the onset and development of pregnancy of dairy cows. Blastocysts retain the ability to survive in stressful conditions of in vivo or in vitro production after cryopreservation-thawing (J.L.M. Vasconcelos et al., 2011; C. Galli, 2017; H. Erdem et al., 2020) and bisection (microsurgical division of the embryo in half for two demi-embryos) (Y. Hashiyada, 2017). The information on embryo survivability becomes more genetically founded as candidate genes associated with high embryo competence to development are found (M.C. Summers and J.D. Biggers, 2003; A. El-Sayed et al., 2006). Molecular genetic technologies make it possible to study the entire set of genes that endow the blastocyst with the ability to develop sustainably (A.M. Zolini et al., 2020), as well as epigenetic changes of gene expression patterns before and after embryo implantation (A. Gad et al., 2012; P. Humblot, 2018). It will help to develop methods for marker-assessed diagnostics of embryonic disorders, to regulate embryonic genes expression, to elevate the pregnancy rate in cows possessing economically valuable traits and, finally, to accelerate genetic progress in dairy cattle populations.