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Gene technology
Gene maps fight resistance
by Ken Beh,
CSIRO
he development of detailed genetic maps
could help livestock producers select
superior animals for improved disease
resistance in the near future.
T
International collaboration has led to the
map of the sheep being one of the most
accurate and comprehensive available, with
about 1500 markers.
These gene maps are essential tools to locate
individual genes (markers) which affect traits of
economic importance such as fibre diameter or
disease resistance.
Gene maps will help researchers better
understand the nature of measurable variation
of such traits in individual animals.
Such knowledge can be used in the process
of ‘marker-assisted selection’, where breeding
decisions are based on information about the
selected animal’s marker genotype rather than
direct measurement of the actual trait.
While all traits could be selected for using
markers it is likely the technology will be
confined to those traits which are difficult to
measure through observation, and the most
important of these are traits associated with
disease resistance.
Economic importance
There are three diseases of sheep that are
economically important including footrot, fly
strike and worms. The most economically
important is internal parasites.
Current worm parasite control measures in
sheep are based on the strategic application of
drenches and rotational grazing. But frequent
drenching has led to the development of
drug-resistant parasites.
CSIRO
Researchers have developed detailed genetic
maps for all the common species of agricultural
livestock.
CSIRO scientists are looking for the genes
responsible for parasite resistance in sheep.
Genetic markers, see above, offer a simple
way to identify sheep carrying resistance
genes.
A more enduring and reliable method of
control would ideally harness the sheep’s
resistance mechanisms by selecting resistant
sheep in a selective breeding programme.
But resistance to parasites is a difficult and
time consuming trait to select for since
observable assessment requires exposing
breeding stock to parasites to ensure
expression of resistance genes.
Using genetic markers is one way to
overcome this problem.
Markers are
molecular tags or ‘sign posts’ for a particular
part of the genetic material and they offer a
simple way to identify sheep carrying
resistance genes.
The first step to finding suitable markers is
to look for the inheritance of specific individual
markers in sheep which show parasite
resistance measured as faecal egg counts after
artificial challenge in families of sheep.
The first stage has recently been completed
using six sheep sire families with 200 offspring
per sire. Animals were typed with 150
molecular tags scattered around the genetic
material (genome). Analysis of the results of
this initial genome scan indicated a number of
interesting regions in the sheep genome likely
to carry resistance genes.
These initial findings need to be confirmed
by further studies and estimates of the size of
effect and position of resistance genes need to
be refined before markers for these regions
could be used (in a commercial situation) as a
simple blood test for breeding stock.
Research to date has looked at six sheep sire
families with 200 offspring per sire.
Analysis of the results has indicated a
number of interesting regions in the sheep’s
genetic material which are likely to carry
parasite resistance genes.
FARMING AHEAD No. 100 - April 2000
In the longer term it is hoped that positional
information can be used to find the identity of
the actual genes involved in parasite resistance.
For more information contact Ken Beh,
CSIRO Animal Production on phone
(02) 9840 2941, fax (02) 9840 2940 or
email [email protected]
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