Published in Agri-Food Research in Ontario
June, 1996

Stressed Pigs Get Better Fitting Genes

John P. Gibson
Centre for Genetic Improvement of Livestock
Animal & Poultry Science, University of Guelph

The modern pig has been bred to be faster growing, leaner, more feed efficient and to put pork on the dinner plate at a lower cost than pigs 30 years ago. This has led to substantial benefits in terms of a healthier product with less waste and a lower use of natural resources (see Vol. 16, No. 4). During the early 1970's, however, it became clear that these improvements were being made at the price of an increased frequency of pale, soft and watery meat, and an increased frequency of pigs with a peculiar sensitivity to stress.

Early family studies suggested that these effects might be caused by a single gene, but were not conclusive. Then, quite by chance, it was discovered that a few pigs anaesthetized with the gas halothane would rapidly become stiff and rigid while most pigs remained perfectly relaxed. Studies of this effect showed that it was controlled by a single gene, that it was pigs carrying two copies of a mutated form of this gene that turned rigid under halothane and anaesthetization, and that these same pigs were stress susceptible and had pale, soft, watery meat. Later research showed that these same pigs were also faster growing and were leaner. There was also a suggestion, but it could not be proved, that pigs which carried one copy of the mutant form of the gene were slightly faster growing and slightly leaner than normal pigs, even though they did not show any increased stress susceptibility and were not sensitive to the gas halothane. It was probably because of these positive effects that the mutant gene had increased in frequency in the populations so rapidly. Having discovered the gene, many pig breeders wanted to eliminate it or, at the very least, stop it from increasing further in their populations. The halothane test was, however, far from perfect since it only detected those pigs that had two copies of the mutant form while the carriers with one copy (which were much more frequent in the population) remained undetected.

Aliases of the Pig Gene

By the late 80's, several groups around the world suspected that the ryanodine receptor gene (see box) might be the cause of a rare form of sensitivity to halothane in humans. Dr. Hugh McLennan of the University of Toronto, a pioneer in this field, got together with Dr. Peter O'Brien of the University of Guelph to see if the same gene might be involved in halothane sensitivity in pigs. Luck was on their side, and it was almost immediately shown that one particular mutation was the cause of halothane sensitivity in local pigs. It was rapidly shown afterwards that this same mutation causes halothane sensitivity in all breeds of pig worldwide. This allowed the development and patenting of the HAL-1483â test for pigs which carry the stress susceptibility gene (Figure 1). The availability of this test has made it possible to study the effects of this gene in much more detail. For example, as part of the Ontario Pork Carcass Appraisal Project, some 3,000 Ontario pigs with detailed information on growth, carcass composition and carcass quality are being tested for this gene. The results will provide accurate estimates of the effects of this gene on a wide range of commercially imported traits. On a direct practical level, the availability of this test has allowed pig breeders worldwide to detect and eliminate carriers of this gene in their population, thereby eliminating the stress susceptibility syndrome and improving the welfare of commercial pigs, as well as making substantial improvements in meat quality.

Acknowledgement: Research funded by Ontario Swine Improvement, Ontario Pork Producers Marketing Board and OMAFRA. Current testing for this gene in our experiments is in collaboration with VITA-TECH Canada Inc. HAL-1483â is a registered trademark of Innovations Foundation, Toronto.