Mutations produce black house cats, jaguars. (Feline Finding).

A symbol of bad luck for others, the black cat may have had good luck itself. Researchers have identified gene mutations that produce the inky coats in house cats and jaguars, and the scientists speculate that some of these mutations protected the black felines from an epidemic long ago.

The mutations occur in two genes previously implicated in coat color in animals ranging from mice to sheep. One gene encodes a protein called agouti, which normally signals skin cells called melanocytes to produce a reddish-yellow pigment. The second gene encodes a switch flipped by agouti, a cell-surface protein called melanocortin-1 receptor (MC1R). If agouti binds to this receptor, melanocytes make the red-yellow pigment. If functional agouti isn't present, however, another signal latches onto the receptor, and the cells make a black-brown pigment.

Eduardo Eizirik and Stephen O'Brien of the National Cancer Institute in Frederick, Md., and their colleagues discovered that domesticated cats with solid-black fur have a characteristic mutation in the gene for agouti, one that shortens the protein so much that it's not functional. The researchers also found that black jaguars have a mutation in the gene for MC1R, This defect, a deletion within the gene's DNA sequence, isn't found in tawny jaguars with typical rosette markings.

Similarly, dark-brown jaguarundis, felines native to Central and South America, have their own distinct mutation in the gene for MC1R, the investigators report in the March 4 Current Biology. The dark jaguarundi is actually more prevalent than its reddish-hued relatives, notes O'Brien.

While a black coat offers felines camouflage at night, O'Brien suggests another reason that the dark fur may have arisen. Several years ago, his team discovered that many people are protected from the virus that causes AIDS because they have a mutation in the gene encoding a cell-surface protein that the virus uses to infect cells. Since MC1R resembles that protein, O'Brien speculates that thousands of years ago, an infectious agent that exploits the receptor caused an epidemic in jaguars and jaguarundi. If so, cats with mutations in the receptor's gene, the dark ones, would have had a better chance of resisting infection. That past survival advantage could explain why black felines are so prevalent today in certain species, he says.

Leslie Lyons, a cat geneticist at the University of California, Davis, notes that the gene for agouti has also been implicated in obesity in mice. Variations in the gene may predispose cats to weight problems, she suggests. "What do these genes do other than make a cat black?" Lyon wonders.