Sexed semen in animal husbandry (review)

The development of biotechnology in dairy cattle breeding is aimed at intensifying the selection process, increasing productivity and economic efficiency of reproduction. One of the key technologies that allows for a targeted influence on the sex of the offspring and the quality of the herd is the use of sexed sperm. This technology is especially relevant in the dairy direction, where the birth of heifers is of crucial importance for the formation of replacement young stock and enhancing genetic progress (G.E. Seidel Jr., 2003; E.A. Oleksievich, 2020). The first successful technology for determining the sex of sperm was based on flow cytometry (B.L. Gledhill et al., 1982) which made it possible to separate sperm based on the content of DNA stained with the fluorescent dye Hoechst 33342 (Garner D.L., 2009). Later, improved methods SexedULTRA™ and Sexcel™ were developed, aimed at reducing damaging factors during the sorting process (G.E. Seidel et al., 2014; R.W. Lenz et al., 2017). Despite the high accuracy of the technology, sperm separation by sex has a negative impact on the morphology and physiological characteristics of sperm: DNA fragmentation increases, ATP content and motility decrease (A.I. Abilov et al., 2021; M.I. Dunin, 2010). It was also found that the dye and laser cause phototoxic and thermal damage, disrupting mitochondrial activity in sperm (D. Rath et al., 2013). The fertility of sexed sperm varies significantly and depends on a number of factors: the physiological state of the animal, the frequency of insemination, the accuracy of ovulation synchronization, housing conditions and personnel qualifications. In heifers, with a correctly selected insemination scheme and compliance with the technology, the pregnancy rate can reach 55-60 % (G.E. Seidel Jr. et al., 1999; S. Diers et al., 2020), while in dairy cows it usually fluctuates between 21-45 % (D.L. Garner et al., 2013; K. Oikawa et al., 2019). In addition, in recent years, alternative methods for sperm separation have been developed. Among them are the immunological approach based on the difference in surface proteins of X- and Y-spermatozoa (E. Barsuren et al., 2018); a method using gold nanoparticles and magnetic sorting; proteomic analysis of sperm membranes to identify sex markers (J. Quelhas et al., 2021). These methods are at the experimental stage, but show potential for improving accuracy and reducing the negative consequences of sorting. The use of sexed sperm in combination with embryo transfer programs expands the possibilities of targeted reproduction, especially when using highly productive donors. However, during in vitro production, a decrease in the percentage of cleavage and the yield of high-quality blastocysts was noted (R.D. Wilson et al., 2005). Nevertheless, recent developments make it possible to successfully use embryo transfer schemes using sexed semen, especially with strict adherence to technological regulations (N.A. Zinovieva, 2020; A. Velazquez et al., 2020). Thus, despite the high cost, sensitivity to storage conditions and reduced fertility rates, sexed semen technology remains in demand and promising in dairy cattle breeding. It allows for effective management of the herd's genetic potential and acceleration of selection (D.J. Cottle et al., 2018; M. Osada et al., 2019). Further research in the field of improving sorting technology, increasing fertility and integration with other reproductive methods is of considerable interest to both the scientific community and enterprises.