Improving pork quality is an industry-wide goal, but it also has been among the more difficult to achieve. Now by using cutting-edge technologies like gene mapping, genetic selection for pork quality may soon make large strides.
Max Rothschild, Iowa State University swine geneticist and U.S. pig genome coordinator, and his colleagues have discovered more than 100 new quantitative trait locci for pork quality. These QTL regions do not tell the exact location of a gene, but do point scientists in the right direction. Rothschild says that QTL regions are like finding where a city is located, but you have not yet located the actual house (gene) within the city.
In the past, selecting animals for pork quality traits was difficult because you had to slaughter the animal before you measure its lean quality. Naturally, that animal was no longer available to the breeding pool, but with the aid of genetic markers all that could change.
"If we know which genes control pork quality, we can get a DNA sample from a live animal and be able to make breeding decisions based on pork quality attributes," says Jack Dekkers, Iowa State University associate professor of animal breeding and genetics.
A major finding from this study is the discovery of new mutations of the Rn, or Napole, gene located on chromosome 15.
"The new Rn gene mutations are substantial because they are seen across several breeds," says Dekkers. " This broadens the scope of using the Rn gene marker. The effects on pH and water-hold capacity can be significant." This research indicates that genes affecting several traits, such as muscle pH, water-holding capacity and color, can be located in the same region, says Rothschild. Applications of the newly discovered QTLs could have significant impact on pork quality traits.
"Our data would suggest that some of these genes could raise pH by 0.1 – or for example, from 5.7 to 5.8," says Rothschild. "That's a sizable improvement." Although the Rn gene does stand out as an individual gene crucial to pork quality, the research shows that many genes are responsible for quality traits.
"Several regions of the chromosome show up as being important, which confirms that it is quantitative in nature," says Archie Clutter, DeKalb Choice Genetics. "There are several enes that show up and do not have a substantial effect on quality individually, but as a cumulative effect the results are significant." More work is needed on these findings, but applications of the research may find their way into breeding programs sooner than you think. Rothschild says genetic companies are already working on such applications. Each company must figure out how to incorporate the new knowledge into its breeding programs.
"Each region may have hundreds of genes, so we still have a lot of work to do to pinpoint those genes responsible for meat quality," says Anne Mann, PIC spokesperson. "Looking at genes in the equivalent regions of the human genome might be useful in further narrowing this list." " First, you need to verify that the affects associated with the findings are present in your target population. Then you need to look for markers that are more closely linked," says Clutter.
Dekkers warns against exclusively using information from gene markers for selection, because other undiscovered genes may also play a role in pork quality.
"Any new marker has to be thoroughly validated before it is used in our breeding program," says Mann. "Not only do we have to be sure of the size and economic relevance of the effect on the trait in question, we also need to ensure that selecting for the marker does not negatively affect other traits."
"That's always a possibility," says Dekkers. "There isn't a strong correlation between pork quality traits and growth traits, but you always have to be aware of potential negatives." The Rn gene mutations appear to have no negative effects, says Rothschild.
Another significant pork quality gene, the MC4R gene, varies greatly depending on which allele you look at. "One variation of the MC4R produces leaner hogs, but has a small negative effect on pH," says Rothschild. "The other allele encourages faster growth, which produces slightly fatter hogs but with better pH. So producers will need to select for their situation." Clutter echoes the sentiment that producers will have the ultimate say over how this research is applied.
"It is dictated by the customer," says Clutter. "Today pork quality represents a relatively small role in genetic selection, but we think it will increase. To some customers pork quality traits are less important, but to others they are very important." Mann says PIC's selection also is consumer-driven, with customers placing varying degrees of importance on meat quality. " Selection has always been a balancing act. Increasing the selection pressure on one trait, always means relaxing pressure on other traits," she says. "PIC uses a combination of quantitative genetics and marker genotypes in making selection decisions." The research also found 16 new QTLs for growth traits, which could lead to significant gains. Yet, it's not as significant a discovery as the QTLs for pork quality.
"Knowing the genes for growth traits is not as valuable as meat quality traits, because traditional means of genetic selection in this area have worked pretty well," says Dekkers.
Still, knowing those genes can give researchers and breeding companies information that result in quicker genetic advancements.
"If you add up all the genetic markers for one of the fat measurements it accounts for almost 25 percent of all the phenotypic variation of backfat," says Rothschild. " About 50 percent of the variation in backfat is genotypic, so genetic markers could be beneficial." He points out the QTLs discovered in this research could improve growth traits by 5 percent to 10 percent. Among the growth trait QTLs, MC4R is again cited as a gene that controls feed intake and fat composition.
Other applications from this study include a better understanding of gametic imprinting, says Dekkers. This means that some genes only have an effect if they were transmitted through the sire or, alternatively, through the dam. This has been found in humans and mice, and is now believed to exist in swine as well.
The study included more than 500 Berkshire/Yorkshire crossbred pigs, and was the first genetic study using Berkshires to look at pork quality traits. This is significant because previous studies have shown Berkshires to have favorable pork quality, but the genetic reasons had never been uncovered.
Ongoing research focuses on pooling these data with a similar study occurring at the University of Illinois. When the two data sets are combined, there will be a database of more than 1,200 animals with data on growth, backfat and meat quality traits, says Dekkers.
The National Pork Producers Council, Iowa Pork Producers Association and a consortium of genetics companies, including Babcock, Danbred, DeKalb, PIC, Shamrock and Seghers funded the original research. USDA has funded ongoing research through an Initiative for Future Agriculture and Food Systems grant. Results of this research are public and can be used by any company for use in marker-assisted breeding. PIC has commissioned a study to look deeper into the findings.
No one gene will be the savior of pork quality – or any other traits for that matter. But advances like the QTLs discovered in this study point researchers in the right direction and greatly expedite the rate of genetic improvements over traditional genetic selection methods.