Genetics: Genetic markers help build a better pig faster

Wednesday, February 4, 2009

With DNA markers, researchers can greatly improve the accuracy of selecting for specific traits, such as meat quality, disease resistance and robustness of the animal

by SERGE POMMIER

When people ask me about the importance of genetic markers, they often bring up the halothane gene. Would greater focus on developing genetic markers help us discover more of these revolutionary genes? Would these markers transform genetic development and help us develop a super pig, one with improved meat quality, robustness, feed conversion and disease resistance?

In addressing these questions, the first thing I point out is that the halothane gene is one of the few genes which have a single, important impact on certain traits of production. In this case, it's an example of a defect which effects calcium regulation in muscle and has an impact on meat quality. Another example is the Belgian Blue beef breed, which has a gene involved in regulating myostatin production. Single genes with a major effect are therefore easily identified through measurements on the pig and "traditional" selection by any breeder. They are, however, relatively rare.

Classical selection (in any species) has been about selecting animals based on phenotypic animal measurements and assuming that we are concentrating the many beneficial genes which contribute to the performance we measure. However, in the case of most pig muscle development, growth and reproductive potential, there are many genes which affect the course of the expression of the phenotype.

The potential of markers is tremendous, especially with the development of faster and more cost-effective computers and programming as well as genome sequencing and analysis techniques that have evolved at a logarithmic pace. Today, researchers can identify markers in a fraction of the time it took 10 years ago. Quite simply, what we used to do in years we can now do in weeks.

Computers play an important role in marker development because of the need for a tremendous database of phenotypic traits. In reality, the candidate gene approach has been replaced by High Density Genotyping (HDG), the process of scanning the entire genome with thousands of genetic markers linked to different phenotypic traits.

With DNA markers, researchers can greatly improve the accuracy of selecting for specific traits, particularly with traits that are more difficult to measure and are lowly heritable in the live animal, such as meat quality, disease resistance and robustness of the animal.

One area where markers are having a great impact is increasing meat pH levels to optimize meat quality. Typically, normal meat pH ranges from 5.6 to 5.7. For example, with the aid of markers, genetic trends have been identified in Duroc sires which have helped increase pH to the 5.8 to 5.9 range.

There's also been work done to develop markers for PRRS resistance in populations which do not seem to be as negatively affected by PRRS contamination. Even in the same pen, some pigs do not react as negatively to PRRS contamination as others. Here, markers can be an effective means of identifying animals that have resistance and we could select for that trait.

Also of note is the key role markers are playing in developing robust pigs, defined as expressing a high production potential without problems across a wide range of environmental conditions.

So what results can markers deliver for producers? The use of markers included in the Estimation of Breeding Values increases the accuracy of the trait estimate and leads to an increase in the rate of genetic improvement. This improvement in accuracy can be 15 to 20 per cent higher in highly heritable, easily measured growth and carcass traits. For less heritable, more difficult to measure traits like reproduction and robustness, accuracy could be improved by up to 50 to 60 per cent.

Modern, intensive livestock production puts a great deal of pressure on animals and, in some cases, may limit the full expression of the genetic potential of the animals. Only the more robust are able to cope with these practices. But, with markers, we can realize significant genetic improvement in areas which have become some of the most important aspects of modern pig production. BP