Scientists have pinpointed the genetic mutation that causes lambs to develop large and muscular rumps-a trait known as callipyge, from the Greek for 'beautiful buttocks.' The discovery ends a search that has lasted a decade and marks the beginning of an effort to explain how the mutation causes its dramatic effect. The trait first appeared in a sheep born in Oklahoma in 1983. After the ram's distinct features emerged, he was named Solid Gold and saved from slaughter. Although the hefty haunches initially seemed valuable, breeders have tried to eliminate the mutation because the meat in callipyge animals is not tender.

Callipyge sheep 1 and 3 are descendents of Solid Gold. Sheep 2 and 4 are normal.



Now, with the mutation known, breeders can identify animals that carry the variant even though they appear normal. The trait only occurs in lambs that inherit the callipyge mutation from the father; the gene from the mother must be normal. Lambs born with two copies of the mutation appear normal. The effect is known as 'polar overdominance.' The callipyge mutation occurs in a previously unidentified gene, now called CLPG1. It seems to have an effect on the activity of nearby genes. Specifically, the mutation may cause genes involved in tissue development to be activated, or expressed, more (or less) than normal. "My current thinking is that this mutation is influencing the expression of several genes in the area," says Brad A. Freking of the US Meat Animal Research Service in Clay Center, Nebraska, who led the study. His group and others are investigating the biology of callipyge to better understand meat quality, muscle and fat in sheep. The callipyge mutation is an example of genomic 'imprinting': Imprinted genes are regulated differently in the body depending on which parent they are inherited from. An imprinted gene inherited from the father may be silent while the copy from the mother is active. Often, imprinted genes are turned on during specific periods of development.

Callipyge sheep (front) and normal (back).



In humans, imprinted genes are associated with forms of mental retardation and developmental disorders. Cells have two copies of most genes, and a second copy can usually serve as a replacement for a defective copy. But if something goes wrong with one copy of an imprinted gene, the second copy may not be a replacement. Humans, mice, and cattle all share similar versions of the region containing the CLPG1 gene. The similarity indicates that this region has biological significance, the researchers write in Genome Research. Now, studies of the region in humans can go forward. In recent years, the callipyge region had been localized to a small stretch of chromosome 18. Freking and his colleagues screened a diverse population of sheep including callipyge and normal sheep. They identified 600 DNA variants on chromosome 18 and pinpointed the responsible mutation. "The genetic mutation is specific to the descendents of Solid Gold," says Freking. Callipyge sheep have very little fat in their hindquarters. The mutation may 'stunt' fat cells from growing, somehow causing food to be converted into muscle rather than fat, according to Randy L. Jirtle of Duke University Medical Center in Durham, North Carolina, and a member of the research team. See related GNN articles

»The Legacy of Solid Gold

»Human imprinted gene lies within non-imprinted gene . . .

Freking, B.A. et al. Identification of the single base change causing the callipyge muscle hypertrophy phenotype, the only known example of polar overdominance in mammals. Genome Res 12, 1496-1506 (October 2002).

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