Sperm sorter ensures sex-linked litters.

Sperm sorter ensures sex-linked litters

Scientists have an improved method for sorting male-producing (Y chromosome) and female-producing (X chromosome) sperm cells in animals. Using the technique, they have produced rabbit litters with 80 to 90 percent one gender or the other. The technique shows promise for farmers raising beef cattle, swine and dairy cows, who have economic incentives to raise greater numbers of either males or females.

Diary farmers want to limit the number of newborn males--for obvious reasons. But beef producers prefer males because they grow faster, and swine farmers seek a preponderance of litter-producing females.

Previous work by Lawrence A. Johnson of the USDA's Agricultural Research Service in Beltsville, Md., yielded a machine that could sort male and female chinchilla sperm, but only after the sperm's tails had been removed (SN: 5/18/85, p.310). Johnson started with chinchillas because male-producing sperm from these animals pack a whopping 7 percent more DNA in their microscopic heads than do female-producing sperm -- a difference easily detected by the experimental device, which sorts cells on the basis of DNA quantity.

But male- and female-producing sperm in most livestock differ in DNA content by only about 3 percent. The newest sorter easily recognizes that subtle difference while leaving sperm cells their tails, and it does so at a rate of about 300,000 cells per hour. That's almost twice the rate of Johnson's previous models but still too slow to produce economically the millions of sperm required for each insemination in a typical farm animal. Nevertheless, the process may prove valuable for the growing number of animal breeders who use in vitro fertilization and embryo transfer techniques, which require far fewer sperm than natural insemination does.

The $250,000 sorter measures the amount of laser light reflected from individual sperm soaked in a DNA-staining dye, then sorts them according to light intensity. Since rabbit sperm heads are flat, Johnson had to devise a way to keep them all oriented in the same plane while they flow through the sorter, minimizing random variations in reflected light due simply to differences in sperm-head orientation. The new sorter does this by creating a ribbon-like flow of fluid that sends the flat-headed sperm to their sex-segregated destinations like manta rays skimming the ocean bottom in single file.

Rabbits inseminated with sorted sperm have about double the pregnancy failure rate of rabbits inseminated with unsorted sperm -- a reflection, perhaps, of DNA damage from the dye or laser, Johnson says. Ongoing experiments seek to find a cell-surface marker that might serve as a less damaging dye-binding site than the DNA itself.