Gender preselection in mammals: an overview

Recent advances in the separation of X and Y chromosome bearing spermatozoa have led to the availability of a method (Beltsville Sperm Sexing Technology) to preselect the sex in several mammals. Progeny using this procedure have been produced in cattle, sheep, swine and laboratory animals. Mammalian sperm are inherently different in that the X sperm carries from 2.8 to 7.5% more DNA than the Y sperm. Individual sperm DNA can be determined and used as the differentiating characteristic with flow cytometry and cell sorting instrumentation especially modified to measure small amounts of DNA in sperm. The process utilizes the fluorochrome Hoechst 33342 to bind to the DNA. The relative DNA is measured by passing the living sperm through a laser beam and collecting the light energy from the individual sperm. Data is acquired and used to select the particular sperm for deflection into collection tubes. The proportions of sorted X and Y sperm in each tube can be validated by reanalyzing an aliquot for DNA content. This value is then used to predict the outcome of fertilization and subsequent gestation. The sorted sperm are used to inseminate eggs via in vitro fertilization (IVF) or by surgical insemination into the oviduct or the uterus of appropriate females. Sperm are sorted at the rate of 0.5 million per hour for most species with the expectation of 90% or greater of one sex or the other being born. Progeny in cattle using IVF have been produced at greater than 90% accuracy. Rabbits have produced greater than 90% females using this process. Progeny produced from pigs average 85% for one sex or the other. All progeny produced (N = or > 300) have exhibited completely normal morphological appearance and normal reproductive function. Because of the inability to obtain large numbers of sorted sperm in a short amount of time, the technologies use for regular artificial insemination would not be practical in most domestic species. This sexing technology however is very applicable where IVF, intrauterine or intratubal insemination are convenient means for producing offspring. In addition, the recent advent of ultrasound guided insemination in cattle may provide and opportunity to use this technology for much lower numbers of sperm per insemination than previously thought possible. Using less than 2 x 10 sorted X or Y sperm would move the technology one step closer to practicality.