Over the past 40 yr, litter size has
improved due to management changes, use of superior
dam lines, and through crossbreeding. Selection
for litter size has been reasonably successful in
mice, but the response in
pigs has been extremely variable.
The development of genome maps offers the opportunity
to identify individual genes controlling
reproduction. More recently, associations with genetic variation
at the pig estrogen receptor (ESR) locus have
been discovered. Therefore, the objectives of this research
was to evaluate the effect of the ESR locus on litter
size in a large sample of pigs from four commercial
lines and to measure pleiotropic effects on growth and
carcass traits. The effect of the estrogen receptor (ESR) locus on
litter size was demonstrated in a large sample of pigs
from four commercial lines in which the B allele was
present. It was found that the potential economic value from marker-assisted
selection using the B ESR allele is considerable.
In addition to the positive effects of the ESR B allele
for litter size, a small favorable response was detected
in backfat thickness associated with a small decrease
in average daily feed intake but no effect on growth
rate. Therefore, selection for prolificacy based in part
on ESR genotype should not adversely affect growth
and carcass traits. The only negative effect was a
slight reduction in teat number. The use of this
genotype offers a clear example of economically useful
marker-assisted selection in pigs.