Gene-mapping is a technology that will eventually allow seedstock producers to make rapid genetic changes in a variety of areas. It will consequently allow commercial producers and the industry as a whole to capitalize on that progress.
The potential to identify genes and use them to select for resistance to various swine diseases is many pork producers’ future hope.
Recently, a gene marker for E.coli F18 resistance was discovered, making long-awaited progress in disease resistance a burgeoning reality.
PIC has since introduced the first E.coli F18 resistant lines. It is the first commercial offering marketed as being resistant to a specific disease. E.coli F18 causes edema after weaning and can kill piglets or temper their growth.
The research identified the gene responsible for edema susceptibility as mapping to chromosome 6. The research confirmed that a variation in the FUT1 gene is the causative mutation for adhesion-resistant animals, says Marnie Mellencamp, with PIC.
In addition to the F18-resistance marker, there is a marker for E.coli K88, says Max Rothschild, Iowa State University swine geneticist and coordinator of the U.S. swine genome project. Scientists have known for nearly 25 years that it was controlled by genetics, but the marker wasn’t discovered until earlier this year.
A key chromosome for disease resistance is chromosome 7 where the Swine Leukocyte Antigen complex gene maps. This complex includes as many as 200 genes, many of which are critically involved in regulating immune and disease responses, says Joan Lunney with the USDA immunology and disease resistance lab in Beltsville, Md. Work over the last 25 years has proven that SLA genes are associated with vaccination responses and infection resistance.
One of the genes associated with resistance to Salmonella challenge in mice, has been mapped to the chromosome 15. Relationships between this gene and Salmonellosis in pigs is now being determined.
Other recent studies include those of German scientists who have identified regions in the swine genome, but not specific genes, associated with susceptibility to pseudorabies.
There also is a marker for a rare vitamin C deficiency, but since it is so rare it’s not a priority.
Genetic markers for disease resistance will eventually become a large part of swine genome efforts. There is a vast amount of research that’s looking at genetic susceptibility to certain diseases, but at this point more is known about production, reproduction and meat-quality traits.
“Everybody is looking at porcine reproductive and respiratory syndrome,” says Rothschild. “We know there are breed differences in how animals respond to PRRS, but we haven’t yet found any individual genes that affect PRRS.”
Lunney says there is an ongoing, nationwide PRRS initiative that has many scientists trying to identify PRRS susceptibility.
“For PRRS, we may know something about the receptor, but we’re not sure how alleles in that gene may affect other traits,” says Lunney. “So far, it does not appear that there are animals who are naturally fully resistant to PRRS.”
One of the reasons disease resistance lags behind other areas is because it is expensive to test. Every method requires that you either study the genetic makeup of sick animals, or introduce a disease challenge to a healthy herd, which of course is costly. There are three methods of testing disease resistance, says Rothschild.
1. “The first method is the candidate-gene approach, which means you look at genes that are associated with the disease response,” says Rothschild.
Researchers are looking at small pieces of genes from many chromosomes using the candidate-gene approach to determine if there are relationships between alleles or regions of the chromosome and disease resistance.
2. The second method is a genome scan conducted within a family usually produced by crossing lines. Rothschild says this is done by introducing a disease challenge to F2 animals in a three-generation family, then studying the genetic markers and how they relate to disease severity.
3. A third option involves using expression data taken from disease-stricken animals and compares them to tissues of healthy animals. “We then look at which genes are turned on in the sick animals,” says Rothschild.
Another approach is simply to select for overall immunity. Looking for animals that react against a virus more quickly may allow researchers to identify animals that overall are more resistant to several viral diseases and are more healthy and productive, says Lunney.
Also, animals that are more susceptible may be identified for elimination from the breeding herd or targeted early for vaccine or drug treatments.
There may be other benefits to this genetic research.
“This research can help make better vaccines,” says Lunney. “Determining how pigs respond differently to various vaccines would allow the development of new kinds of bio-therapeutic treatments and vaccines.”
Heritability for disease resistance is difficult to measure, and varies with each disease, but Rothschild pegs heritability between 5 percent and 20 percent, which is fairly low.
Selecting exclusively for a single trait, whether it’s disease resistance or any other production, reproductive, pork-quality or behavioral trait can be dangerous.
“Anytime you base selection on a single gene you have to make sure there are no negative effects,” says Rothschild. “At this point there has been no research to show any negative correlation between disease-resistance genes and other traits.”
Progress on the swine genome project is moving along well, says Rothschild. The genome hasn’t been sequenced yet, but about 5,000 to 6,000 genes and markers have been mapped. There are about 35,000 genes in the pig, but there can be multiple markers in a gene. There are bits of more than 100,000 markers in the swine genome database, there will be more than a million bits in a few years, says Rothschild. Parts of most every gene will be known in the next five years; and the pig is on the short list of animals that could be sequenced soon, he notes.
Researchers continue to make disease-resistance research a priority, and as more technologies become available more progress can be expected.
“Consider if producers could put aside losses from diseases, like fetal loss associated with PRRS,” says Lunney.
It certainly would be a welcome advancement.
Editor’s note: Part I in a series on the swine genome.