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Cats and humans share similiar X and Y chromosomes
By Sharon Guynup


Researchers, acting as "genetic paleontologists," recently discovered that the X and Y chromosomes of cats and humans are remarkably alike, despite the fact that the two species haven't shared a common ancestor for about 90 million years—around the same time the human line diverged from goats, sheep, and cows. This similarity doesn't govern sexual behavior, as sexuality is not determined by genes found on X and Y chromosomes. But it may help scientists better understand male infertility, human genetics, and may even help preserve endangered cat species.

William Murphy, Stephen O'Brien, and their colleagues at the U.S. National Cancer Institute (NCI) examined 25 unique DNA sequences on the X chromosomes of cats, mice and human beings, and looked at eight genes on the Y chromosome—with startling results.

The order of these genes on the sex chromosomes of cats more closely resembles that of humans than of any other mammal studied so far, the team reports in a recent issue of Genome Research. Genes on X chromosomes of cats and people are lined up in the same way. This long-term chromosome stability is rare, showing no evidence of evolutionary rearrangements, says O'Brien. "It's like there was some master template that maintained the order of these genes on the X chromosome from the distant ancestors of these two mammalian orders."

Most X chromosomes are home to a variety of genes, including what Murphy calls "essential housekeeping genes," genes involved in basic cellular functions such as metabolism. This is why all mammals have the same genes on X chromosomes, but in various arrangements. Studies have shown that the same genes in mice, cows, goats and rats are arranged in different sequences.

The researchers also isolated a cluster of three genes on the Y, or male, chromosome in cats, mice and humans that have maintained identical order and spacing over the millennia. This conservation appears to be important for sperm production. When regions of the Y chromosome harboring those genes in cats are deleted, Murphy says, the males are infertile—a fact which may make the cat a useful model to study infertility. Of all seven genes the group studied on the Y chromosome, only one is in a different order in cats than it is in humans.

About half of the current X chromosome is like a fossil, having remained the same through the evolution of all mammals, even the relatively primitive marsupials. Genes on the remainder of the X in placental mammals, on the other hand, were more recently acquired from other chromosomes. Some researchers suggest that some genes cycled from regular chromosomes to the X chromosome and then to the Y chromosome. Once on the Y, they acquired a male-specific function or were lost.

Because the sex chromosomes of cats bear such a close similarity to those of humans, cats may come into their own as valuable surrogates for studying a range of human maladies. O'Brien says almost 200 human hereditary diseases also manifest in cats, including diabetes, Tay Sachs disease and hemophilia. Furthermore, certain viruses behave in cats much as they do in humans. Feline leukemia and FIV, the Feline Immunodeficiency Virus (which is very similar to HIV), rank high on this list.

The house cat belongs to a family that has rich diversity in nature with 37 species. Males of certain of its wild relatives, notably the Florida panther, cheetah and clouded leopard, suffer from poor reproductive ability. As their populations dwindled, these species underwent evolutionary bottlenecks, leading to inbreeding. As a result, they have low sperm counts with a high degree of structural abnormalities in the sperm. While this is bad news for the species, it may make these cats good candidates for infertility studies. Such research might benefit not only humans, but the cats as well. "Every species except the house cat is threatened or endangered," says O'Brien. Research on human infertility might turn up ways to help the endangered wild cats.

O'Brien hopes cats will get their own genome project. "I believe we will add the cat to the list. It, too, may be finished within the next decade, depending on funding" he says. Tracing our genetic ancestry will not only unearth details about our evolution, but may help us find the cures to our most debilitating human diseases.

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Murphy, W.J., Sun, S., Chen, Z., Pecon-Slattery, J. & O'Brien, S.J. Extensive conservation of sex chromosome organization between cat and human revealed by parallel radiation hybrid mapping. Genome Res 9, 1223-1230 (December 1999).

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