Scientists, practitioners and technical experts from around the world gather every five years at the International Conference on Boar Semen Preservation. They've done so since 1985.

The fourth such conference was hosted by the USDA Agricultural Research Service in Beltsville, MD. These reproductive specialists gather to more formally exchange their latest research findings, most focusing on the principles of boar semen preservation, including new semen extenders and preservation methods.

Complete proceedings will be available. For more information check the USDA Web site: www.lpsi.barc.usda. gov/boarconf/index.htm.

Freezing Boar Semen Paul Watson of the Royal Veterinary College, London, U.K., discussed the effect of cooling on sperm capacitation (the change sperm undergo in the female reproductive tract that enables them to penetrate and fertilize an egg).

Boar semen that has been subjected to cryopreservation, or even cooling below 15 degrees C. (59 degrees F.), show many signs of being capacitated, he explained. Consequently, they show signs of reduced longevity, tend to undergo spontaneous acrosome reaction soon after rewarming, as if they have become capacitated.

The cooling process appears to accelerate sperm development more than fresh semen, therefore longevity in the female reproductive tract is reduced.

The researcher's challenge is to determine at what degree freezing/cooling mimics capacitation, and whether it can be prevented or reversed. They tackle this challenge by comparing sperm that have been incubated in a capacitating medium with those that have been cooled, then rewarmed. The capacitation similarities recorded in the comparison are temperature dependent, Watson said.

"The proportion of live spermatozoa showing a capacitation-like change was related inversely to the final temperature reached in the range from 0 degrees C. (32 degrees F.) to -24 degrees C. (11.2 degrees F.)." The capacitation effect was seen when rewarmed to 39 degrees C. (102.2 degrees F.), but could be prevented by rewarming to only 24 degrees C. (75.2 degrees F.). Not only does this indicate an enzyme-related response, but it offers some clues that it is possible to extend the viability of frozen semen.

"Our preliminary conclusions are that the cooling response indeed resembles capacitation and it does not represent bypassing the need for capacitation," Watson explained.

Emerging Technologies Lawrence Johnson with the Germplasm & Gamete Physiology Laboratory in Beltsville, MD, reported on the status of sperm sexing and the ability to alter the sex ratio of offspring. "The method is based on the flow cytometric separation of X- and Y-chromosome-bearing sperm based on X/Y DNA difference," Johnson explained.

The method involves treating sperm with a DNA binding fluorochrome and flow cytometrically sorting them into separate X and Y populations. These are used to surgically deposit the sorted sperm cells into various places in the reprod uctive tract - tubular, uterine, deep uterine, regular artificial insemination (AI) and in vitro fertilization - to produce sexed embryos for transfer.

Johnson reported that skewed sex ratios of 85-100% of one sex or the other have been repeatedly achieved in most species. "The method has been used worldwide to produce hundreds of morphologically normal animal offspring of the predicted sex," he noted.

Johnson explained that significant improvements of the sexing technology have been achieved in the past two years.

First, a new orienting nozzle improved sorting efficiency two- to three-fold. In effect, that means only about 25% of sperm were originally properly oriented to the laser beam which guides the sorting.

Secondly, a high-speed cell sorter was modified to accept the new nozzle adaptation for sorting sperm. The result of these modifications was an increase of sorted sperm production from 0.6 million to 12 million sperm per hour.

"Pigs of the desired and predicted sex have been born after surgical insemination and after transfer of sexed embryos produced from IVM-ET (in vitro maturation-embryo transfer)," Johnson said. "Conventional AI in the pig with sorted X or Y sperm is not yet feasible since the sperm numbers required to attain pregnancy are too great." But, he added, "Development of low sperm number insemination technology for the pig that would accommodate low numbers of sexed sperm is under development by several laboratories.

"Livestock reproductive efficiency can be advanced substantially through the use of the Beltsville Sperm Sexing Technology to separate X- from Y-chromosome bearing sperm. The early years of the next millennium will see it applied in practical settings across many species on a worldwide basis," Johnson predicted.

"Super Cool" Pigs "Cryopreservation of Swine Embryos: Farrowing Super-Cool Pigs" was the title of John Dobrinsky's presentation. Also of the USDA's Germplasm & Gamete Laboratory, he reported that although sperm can be successfully preserved, the ability to preserve oocytes (eggs before maturation) and embryos has been elusive until recently. Driving the research to preserve embryos is the increased efficiency of transmitting improved genetic potential and offers numerous applications, such as:

* transport of maternal germplasm (much as boar semen transport does today);

* quicker regeneration or expansion of new or existing genetic lines;

* the ability to increase genetic selection in nucleus herds;

* salvaging healthy stock from diseased herds;

* improve or eliminate quarantine conditions; and

* provide a method for international exporting or importing of potential breeding stock.

Transportation costs to place pigs in new breeding units in other regions of the world are high, risk of disease transmission remains a threat, and health regulations for import or export are difficult and involved. The ability to ship embryos instead of live animals would lower those costs considerably.

And, Dobrinsky noted, with the growing use of pigs in human biomedical research, embryos from valuable genetic or transgenic lines of pigs used as donors for xenotransplantation could be preserved for future use.

"Collectively, these technologies could be instrumental in the continuous production of animals of high genetic merit capable of having a significant impact on the improvement of the world swine population and human medicine," he explained.