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RESEARCHERS from Europe are pooling their resources in an attempt to
map the pig’s genome. Sixteen laboratories are involved in the project,
which is divided between Belgium, France, Denmark, Germany, the Netherlands,
Norway, Sweden and Britain. Chris Haley and Alan Archibald coordinate the
project from the Agricultural and Food Research Council’s Institute of Animal
Physiology and Genetics Research at Roslin, Edinburgh.

Pigs are better suited to genetic cartography than cattle or sheep and
their genome is recognisably similar to ours in size and organisation. ‘We
focused on the pig because it’s a nice animal to use from this point of
view. It has a short generation interval – a year – and we can produce fairly
large full sib families from it,’ says Haley. Another advantage is the appearance
of its chromosomes under the microscope. ‘The chromosomes are reasonably
easy to distinguish from one another,’ explains Haley, ‘and they have a
nice spread of sizes and morphologies – which again is not the case in sheep
or cattle.’

As a first step, researchers are looking for genetic markers – regions
of pig DNA which vary between individuals and which can be revealed by standard
biochemical techniques. Genetic markers are a source of much information.
If a marker is always inherited with a certain gene, for example, then gene
and marker are likely to be close neighbours on a chromosome. To aid these
studies, the Edinburgh team are producing a population of variable pigs
by crossing the European Large White with a Chinese breed called the Meishan.

In other laboratories, researchers are analysing the genetic fingerprints
of pigs – again in the search for suitable genetic markers. Elsewhere, teams
are studying the chemical makeup of the pig’s chromosomes by a procedure
called in situ hybridisation – a technique that reveals the whereabouts
of a particular sequence of DNA on an intact chromosome.

The project will benefit from similar work on the human genome, simply
because the two maps will cover the same basic mammalian terrain. ‘There
is a lot of homology with the human genome,’ says Haley. It is almost as
if the pig’s chromosomes are made by cutting human chromosomes into pieces
and reuniting the pieces in a different order. Genes that occupy adjacent
slots in the human genome are likely to do so in the pig.

Researchers expect the project to yield the harvest of new insights
into animal genetics. In particular, it will help them to understand the
basis of ‘quantitative traits’ – traits such as growth rate and fertility,
which are governed by a number of genes acting in concert. These genes,
many of which are of immense economic importance, have resisted analysis
by the methods of traditional genetics. ‘We don’t really know what the genes
which control variation in litter size, milk yield and so on are doing,’
says Haley. Once such genes are mapped, it should be easier to decipher
their identity and learn more about their function. This is where parallels
with the human gene map will prove most useful. Knowing the location of
an unidentified gene on the pig’s map will allow researchers to scan the
corresponding part of the human map and perhaps identify a likely candidate.

‘Gene mapping is going to be a very useful tool both for scientific
research and potentially for application in animal breeding over the next
couple of decades,’ says Haley. Although the first applications will be
in conventional breeding, gene mapping will also bring genetically engineered
bacon that bit nearer.