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Australian Researchers Decipher Sheep Coat Color Genes

NEW YORK (GenomeWeb News) – New research is revealing the unexpectedly complex genetics behind coat color in domestic and wild sheep.
 
An Australian research duo compared the genome region containing a pigmentation gene called agouti in four different breeds of sheep. Their work, published online in Genome Research, uncovered tandem duplications in agouti and two nearby genes that lead to white coat color. In contrast, black sheep appear to have a single, silent copy of agouti. The finding is economically relevant, researchers say, since it could eventually lead to genetic tests that help select desired coat colors in sheep.

 

“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,” lead author Belinda Norris, a senior research scientist in the Livestock Industries division of Australia’s Commonwealth Scientific and Industrial Research Organisation, said in a statement.
 
Wild sheep generally have dark bodies and pale bellies. Over the years, though, sheep have been domesticated and selected to have a uniformly white coat. In these domestic herds, white coat color is dominant and determined by the wild-type agouti allele. Even so, some white sheep are recessive for black coats. As a result, black sheep — carrying the “non-agouti” allele — consistently turn up at low frequencies in populations of white sheep.
 
The agouti signaling protein, or ASIP, allele has also been implicated as a player in coat color in several other animals, including mice, domestic dogs, and horses. But the gene has other functions too, some better characterized than others. In mice, for example, ASIP deregulation has been linked to obesity, diabetes, and tumor growth. In humans, the gene is expressed in several tissues, though its function is unknown.
 
In an effort to determine how different ASIP alleles influence coat color in sheep, Norris and her co-author Vicki Whan, also at CSIRO, sequenced and compared genomic DNA in and around the ASIP gene region in Merino, Romanov, Texel, and Barbary sheep.
 
They found that rather than being determined by one mutation or polymorphism, sheep coat color is regulated by a combination of copy number variation and deregulated gene expression.
 
Specifically, they found that white sheep have a tandem duplication of ASIP and two nearby genes, the ITCH promoter and a neighboring gene called AHCY. Moreover, the researchers identified some white sheep with not just two, but three or four copies of the ASIP allele.
 
In contrast, the researchers found that black sheep have a non-duplicated agouti allele behind a silenced promoter that mutes its expression. Though the precise nature of the silencing is unclear, the researchers speculated that it stems from an as-yet unidentified mutation in the promoter.
 
They further confirmed the importance of the ASIP duplication in sheep coloring by examining an ancient breed of sheep called Barbary sheep, which have tan bodies and pale bellies. Consistent with the role of agouti copy number in coat color, the Barbary sheep have a single, expressed copy of the agouti gene.
 
To date, this sort of tandem duplication is a relatively unknown and unstudied source of molecular variation in livestock, the researchers noted. “This is the first sheep trait attributable to gene duplication,” Norris and Whan wrote. They speculated that non-allelic homologous recombination and gene conversion events in the duplicated regions could have a hand in the evolution of sheep pigmentation.
 
“Tandem duplication represents an under-investigated source of molecular variation in livestock species,” they wrote. “The duplication/deletion of the agouti gene is the first characterized example in sheep of the involvement of gene duplication in the creation of a genetic variation that contributes to a major breed and production trait — coat color phenotypes.”
 
In the future, the researchers noted, it may be possible to pinpoint how this duplication arose during sheep domestication and to apply this new understanding of sheep genetics to create a genetic coat color test. And, they added, the work may pave the way for a better understanding of the way copy number variation contributes to traits in other kinds of livestock as well.

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