19 June 1998

Fitting genes for profit

Identifying useful genes and ensuring they are inherited in

progeny should help boost production. Emma Penny reports

on progress in mapping the pig genome

IDENTIFYING genes which affect economically important traits in pigs – such as the Halothane gene – should help to boost production and returns for pig producers.

Research at Edinburghs Roslin Research Institute is helping to identify which genes affect production, and is also looking at other sources of genes which could prove useful, as scientist Alan Archibald explains.

"A genome map is now widely accepted as an essential tool for studying the genetics of any organism, including livestock. We have been mapping the pig genome, trying to find areas which affect traits of economic and biological significance.

"Most of these useful traits – such as growth rate, fat levels, meat quality, reproductive performance and disease resistance – are quantitative, so the regions of the genome influencing these traits are called quantitative trait loci or QTL."

Three phases

Finding the areas on the genome which influence such traits is split into three phases. Developing mapping tools – or reagents – and building the map is the first phase, followed by using the reagents to identify QTLs, then, the most challenging area, isolating and characterising trait genes within a QTL.

This kind of development is rather like discovering which city a gene lives within, then eventually narrowing it down to which house number and street the gene is in, explains Dr Archibald.

"The Halothane gene is a good example of this type of development. We have known for 20-30 years where the gene is, but it is only eight years ago that we managed to identify exactly where the gene is and so develop a test for it."

Researchers have already produced a map of the pig genome, but Dr Archibald explains that it is rather like a schoolboy atlas in resolution. "You can use it to identify the coast, and major cities, but try driving by it and you are likely to find it difficult."

Work at Roslin is concentrating on mapping QTLs to give maps better resolution. "We are looking for associations between combinations of markers – which show up different genes and are identified using reagents – and looking at whether an animal is, for example, fat or thin."

An example of this is a gene which controls growth and fatness on chromosome four, says Dr Archibald. "There are two locations for growth on this chromosome that we know of. On one, the Large White grows faster than the Meishan, so if you want fast growth you want the Large White gene at that location."

That theory is now on test on 30-85kg pigs at Roslin. "We think that, for example, an animal with one copy of the Large White gene at chromosome four will grow 50g a day faster than one which has not got that gene. But if the animal has two copies of the Large White gene in the same chromosome, it should grow 100g a day faster than animals without the gene."

Also located on chromosome four is a region which contributes to whether a pig is fat or thin. "If a pig has the Meishan version of this gene its P2 backfat is likely to be 1.8mm greater than a pig without it. If it has two copies of the Meishan gene it will be 3.6mm fatter."

Chromosome four results from six other labs in Europe and the US are being analysed at Roslin. "This joint analysis should increase the degree of confidence with which we can identify useful genes."

The Meishan is also providing some interesting results for chromosome seven. "This appears to be a fat QTL, and a region of chromosome seven has a major effect on P2 backfat levels. This time, for every copy of the gene, Meishan pigs are up to 3.5mm less fat, so with two copies, it is substantially thinner.

"This proves the case that rare or exotic species can be a useful source of genes; you would expect the Large White to be substantially thinner than the Meishan, so it is a bit cryptic."

But Dr Archibald warns that more research is needed to see whether the gene would work in Large White pigs. "The effect of genes may be influenced by the context. For instance, a rugby player from the fourth XV might look big within his own team, but be quite small when compared to players in the other three teams. It depends on the circumstances."

Despite this caution, MAFF is funding a trial looking at back-crossing pigs with the Meishan version of the chromosome seven gene with purebred Large Whites.

"This should help define the boundaries of this QTL more carefully, and demonstrate that we can use marker technology to shift genes from one population to another. The Meishan gene has such an effect that it should be easily seen."

Other exotic breeds are also being examined for their genetic potential, and Dr Archibald is collaborating with Hungarian scientists to look at the Mangalica, a lard-producing pig which is almost a rare breed.

"We are looking at crossing the Mangalica with a Duroc. While the Mangalica has high backfat levels – ideal for lard production – the Duroc is perceived as having high intramuscular fat levels, contributing to good meat quality. We are hoping to find different regions within the genome which control these characteristics."

But while producers may look forward to genetically better animals, he says that it is down to breeding companies as to how quickly any research is taken into practice.

"We are at the stage with some QTLs that we were with the Halothane gene 20 years ago. Then, the Swiss and Germans used markers linked to the gene to screen against it, but it was not until the gene itself was identified eight years ago that UK breeders started to select against it."

GENOME MAPPING

&#8226 Identify useful traits.

&#8226 Exotics a source of useful genes.

&#8226 Some QTL technology ready for use.