Coat color of wild and domestic animals is a critical trait that has
significant biological and economic impact. Researchers have now
identified the genetic basis for black coat color, and white, in a
breed of domestic sheep.
In the wild, mammalian coat color is essential for camouflage and
plays a role in social behavior. Coat color also strongly influences
the animals we choose to breed both as livestock and as pets.
Understanding the genetic determinants of coat color in livestock
species such as sheep, specifically bred for their coat color, is
critical for improving efficient selection of the desired trait.
Classical genetics has associated alternative forms, or alleles, of
the agouti signaling protein gene (ASIP) with coat color variation in a
number of mammals including mice, rats, dogs, cats, pigs, and sheep.
However, most research has been focused on the mouse, with little
understood about the genetic basis for coat color in economically
important livestock species such as sheep.
The wild-type coat color of sheep is typically dark-bodied with a
pale belly, however sheep raisers have strongly selected for a
uniformly white coat domestic sheep. A problem for the sheep industry
is a recessive black "non-agouti" allele of the ASIP gene carried by
white sheep that cannot be distinguished within the flock, resulting in
black coat color at a low, but persistent frequency. Determining the
exact genetic differences at the ASIP locus could assist in efficient
selection for white coat color.
Scientists at the CSIRO Queensland Bioscience Precinct in Australia
have now taken this step and identified the molecular mechanisms
underlying white and black coat color in domestic sheep. The
researchers investigated the genetic architecture of the ASIP gene in
several sheep breeds by sequencing the ASIP locus and measuring gene
expression. "Surprisingly what we found was in fact that the genetic
cause of domestic white and black sheep involves a novel tandem
duplication affecting the ovine agouti gene and two other neighboring
genes, AHCY and ITCH," explains Dr. Belinda Norris, lead author of the
"We discovered a novel mechanism in which the dominant white sheep
is caused by the ubiquitous expression of a duplicated agouti coding
sequence located immediately downstream of a duplicated ITCH gene
promoter region." It was found that recessive black sheep harbor only
poorly expressed non-duplicated agouti alleles, likely a result of a
defective single-copy ancestral agouti gene promoter. The researchers
also studied the ASIP locus in Barbary sheep, an ancient species
exhibiting a tan body and pale belly. They confirmed in this ancient
sheep that expression of a single-copy agouti gene determines coat
color patterning, similarly to findings previously described in mice
Norris notes that this work will aid in the development of gene copy
number detection and analysis methods in the mapping and association of
heritable traits in livestock animals. For sheep raisers, this could
ultimately mean a genetic test that would identify carriers of the
black non-agouti allele. Furthermore, these findings will help to
unravel the events leading to the domestication of sheep, and future
work may be able to pinpoint when the dominant Agouti mutation
occurred, and whether it occurred as single or multiple events.
Scientists from the Commonwealth Scientific and Industrial Research
Organisation (CSIRO) Queensland Bioscience Precinct (St. Lucia,
Australia) contributed to this study.
This work was supported by the Australian Sheep Industry Cooperative Research Centre.
Journal reference: Norris et al. A gene duplication affecting expression of the ovine ASIP gene is responsible for white and black sheep. Genome Research, 2008; DOI: 10.1101/gr.072090.107
Cold Spring Harbor Laboratory. July 2008.