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Black Cats and Genomics Cross Paths
By Adam Marcus

Gene mutations are the reason black cats are black, according to new research. The mutations appear to have arisen independently over time and are relatively common in the cat species they affect. Interestingly, they are not all alike. For instance, the gene that makes jaguars black is different from the one that gives domestic kittens black fur.

Black fur clearly looks like something evolution has preserved, says Stephen O'Brien, an expert in cat genetics at the US National Cancer Institute's Laboratory of Genomic Diversity, in Frederick, Maryland, and a co-author of the study.

For nocturnal big cats like jaguars and panthers, black fur might help them blend in better with their surroundings, making them less visible to prey and potential predators. But O'Brien says he doesn't believe camouflage can fully account for the prevalence of dark fur in cats, especially big ones.

The average jaguar doesn't fear much, says O’Brien, whose study appears in Current Biology. “It's a killing machine that has evolved to do just that.”

O’Brien offers another possibility: the mutations that make cats dark also provide them with a survival advantage, such as protection from infections.

O'Brien and his colleagues looked for genes that cause dark pelts, or melanism, in three cat species: domestic felines, the jaguar and a South American wildcat called the jaguarundi.

In jaguars and jaguarundis, dark pigment is linked to two different mutations in the same gene, MC1R. MC1R belongs to a family of genes that code for proteins called (seven-helix) transmembrane receptors that stud the surface of cells. In humans, certain viruses, including HIV, use these receptors to break into cells and hijack their DNA. People with two mutant copies of the (seven-helix) transmembrane receptor CCR5 are much less likely to become infected with HIV than those with normal versions of the receptor.

Presumably, O'Brien says, some microbes that attack cats may find mutant receptors in those animals less inviting, too. To test that theory, researchers are considering a study of whether black cats are immune to feline HIV.

MC1R is also a pigment gene in humans. People with red hair and pale skin have mutations that reduce the effect of the gene. The researchers believe black fur in big cats reflects overactive MC1R.

The mutation that causes black fur in domestic cats involves a different gene, called ASIP. Since five other cat species also have dark coats, but not as a result of the three known gene mutations, at least one other pigment mutation must affect felines.

The latest research is part of a larger effort at NCI to learn more about human disease by studying cat genes.

Eduardo Eizirik, an NCI researcher who led the study, says scientists have so far found about 260 inherited gene mutations that affect cats. Scientists have looked at ten of those in depth, and all have counterparts in humans.

Greg Barsh, a geneticist at Stanford University School of Medicine, who studies pigment in mice, calls the latest study "fascinating."

Investigations of skin color genes in mice have led to insights about the origins of obesity, neurodegenerative diseases, and basic cell biology, Barsh says. "It's really interesting when one finds examples of genetic changes in pigmentation in a natural population," he adds.

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Eizirik, E. et al. Molecular genetics and evolution of melanism in the cat family. Curr Biol 13, 448-453 (March 4, 2003) .

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