Flow Cytometric Sorting of Mammalian Sperm for Predetermination of Sex

Abstract

The predetermination of sex is an appealing means of controlling the sex ratio in a selected population of production animals or wildlife. The sexing of mammalian sperm by flow cytometry for its use in artificial insemination (AI) or in vitro fertilization (IVF) has led to the birth of hundreds of thousands of offspring in numerous species, and remains the only scientifically proven method of selecting sex at the time of conception. Sperm sexing is made possible because of the difference in total DNA content between sperm carrying the larger X sex chromosome, and those carrying the smaller Y chromosome. To accomplish sorting, sperm are stained with a dye that allows the flow cytometer to detect the approximate 4% DNA difference between X and Y sperm, and separate them accordingly into two distinct populations with a purity of 90%. Sex sorted bull sperm are routinely cryopreserved and used for AI, with fertility retained at approximately 70–80% of that achieved with nonsorted sperm. A recent advance has allowed the sorting of previously cryopreserved sperm, with equal blastocyst formation in bovine IVF to nonsorted sperm, even after recryopreservation. With particular reference to dairy cattle, we discuss the advantages of sex determination, the method by which sperm are sexed, and the effects of the sorting process on sperm viability and fertility. The various avenues by which sexed sperm may be disseminated are discussed, as is the current status of sex-sorting sperm from cryopreserved semen.