Sperm sort: on the road to sex selection.

Dairy farmers want female calves; beef farmers want males. A method to give them, and sheep and hog farmers, their preferences could save the U.S. livestock industries almost $700 million annually, says Lawrence Johnson of the USDA Agricultural Research Service in Beltsville, Md. Johnson and his colleagues report a technique to sort X-chromosome-bearing sperm, for female offspring, from Y-chromosome-bearing sperm, for male offspring. Such "sexed" sperm could eventually be used for artificial insemination of animals. The development of this method for farm use is expected to take several years, but it is already being used to measure sperm ratios.

Several techniques have been proposed previously to select X-bearing or Y-bearing sperm, and some semen now on the market for livestock insemination carries claims of being enriched in sperm of one type of the other. In a paper to be published in the JOURNAL OF ANIMAL SCIENCE, Johnson, along with Dan Pinkel and Bart Gledhill, both of Lawrence Livermore National Laboratories in Livermore, Calif., report tests of five current techniques for enriching semen in sperm carrying an X or a Y chromosome. These methods include the albumin gradient technique, which has been applied to human sperm (SN: 3/3/79, p. 135).

"Commercial 'sexed semen' now on the market is probably no more reliable than asking mother nature to yield offspring than one sex or the other," Johnson says. "We see no significant difference from 50 percent of each type [of sperm] for any of these enrichment techniques."

The evaluation and sorting method used by Johnson has grown out of work at the Livermore laboratory. It takes advantage of the greater amount of DNA in sperm containing an X chromosome rather than a Y chromosome. For counting or sorting, the sperm membranes and tails are stripped off and the naked sperm heads are stained so that their DNA fluoresces when exposed to a laser beam. The sperm containing an X chromosome glow slightly more brightly than those with a Y chromosome. As the cells move in a flowing stream of liquid past a fluorescence sensor, the machine counts the number of each type. A similar approach has been used to distinguish individual chromosomes and to determine the location of genes (SN: 2/23/85, p. 120).

To sort sperm cells, the system can give a droplet, containing a single sperm head, a positive or a negative charge depending on whether the sperm contains an X or a Y chromosome. The droplet than passes through an electrical field that separates the oppositely charged droplets, so that the sperm cells fall into one of two collecting tubes.

The difference in DNA content for sperm of most livestock is quite small--less than 4 percent for sperm of a bull, boar or ram. So, for their first experiments, Johnson and his colleagues used sperm of the chinchilla, a rodent raised commercially for its fur. There is more than a 7 percent difference in DNA content between chinchilla sperm bearing X and Y chromosomes. These sperm have been sorted by the fluorescence technique into samples containing 85 percent X-bearing or 85 percent Y-bearing sperm. Preliminary experiments injecting the sorted sperm into hamster eggs demonstrate that the DNA remains intact throughout this treatment. More recently, ram sperm has been sorted into samples containing 90 percent X-bearing or Y-bearing sperm.

The technique must now be modified in several ways to become useful for artifical insemination--for example, intact sperm, rather than stripped sperm heads, must be used. In addition, a speedier method is required. The current machine sorts 50 cells per second, but farmers routinely use 10 million sperm to inseminate a cow. Still, the USDA says that farmers may finally be on the road to controlling the sex of live-stock offspring.