September 21, 2012
How Cheetahs And Tabby Cats Got Their Distinctive Markings
April Flowers for redOrbit.com - Your Universe Online
Cat enthusiasts can tell you all about the myriad of coat patterns that can be found on domestic cats and their larger, wild cousins. From cheetahs to tigers to snow leopards, wild cats have distinctive patterns of light and dark fur, forming spots, stripes, swirls, blotches, marbles and rosettes.
A research team led by Stanford University has unlocked the biological secret behind a rare, striped cheetah found only in sub-Saharan Africa using feral cats in Northern California. This study, published in the journal Science, is the first to identify a molecular basis of coat patterning in mammals.
What they found was that both the feral domestic cats in California and the African cheetah share a biological mechanism responsible for the elegant stripes of the tabby cat and the cheetah's spot. Dramatic changes to these patterns occur when this pathway is disrupted, leaving the tabby with swirled patches of color, and the cheetah with thick, dark lines down its back.
"Mutation of a single gene causes stripes to become blotches, and spots to become stripes," said Greg Barsh, MD, PhD, emeritus professor of genetics and of pediatrics at Stanford and an investigator at the HudsonAlpha Institute.
The cheetah variation with dark stripes down its back is known as a King cheetah, which was first found in 1926 in what was then southern Rhodesia (modern-day Zimbabwe). Biologists believed it to be a separate species until 1939. In 1981, the species debate was settled once and for all when two king cheetahs were born at the Ann van Dyk Cheetah Centre in De Wildt, Africa. A pair of female siblings both mated with the same wild-caught male from the Transvaal area (where most king cheetahs had been recorded), and both gave birth to a litter with one king cheetah among the babies.
The domesticated cat has not been as lucky with names for the coat patterns. The striped tabby is known as a mackerel, and the swirled pattern is called "blotched."
The team, which included members from the National Cancer Institute, the HudsonAlpha Institute for Biotechnology and the Sichuan Key Laboratory of Conversation Biology on Endangered Wildlife, has spent decades investigating how traditional laboratory animals develop specific coat colors. Previous work included identifying a variety of biologically important pathways that control more than just hair or skin color, but which have been linked to brain degeneration, anemia and bone marrow failure as well. Laboratory mice, however, don´t display the pattern variation seen in many mammals.
"We were motivated by a basic question," said Barsh of the turn to the study of big (and little) cats. "How do periodic patterns like stripes and spots in mammals arise? What generates them? How are they maintained? What is their biological and evolutionary significance? It's kind of surprising how little is known. Until now, there's been no obvious biological explanation for cheetah spots or the stripes on tigers, zebras or even the ordinary house cat."
The study, set to be published in the September 21 issue of Science, relied heavily on DNA samples from feral cats in Northern California captures for sterilization and release. They also used tissue samples provided by the City of Huntsville Animal Services group, and small skin biopsies and blood samples from captive and wild South African and Namibian cheetahs. The hinge, however, for the whole project was the recent availability of the whole-genome sequence of the domestic cat. Marilyn Menotti-Raymond, of the National Cancer Institute, focuses her research on the genomic analysis of the domestic cat in order to understand many human diseases.
"The Laboratory of Genomic Diversity at the National Cancer Institute has long championed the cat as an animal model of human disease," said Menotti-Raymond. "Studying color variation in cats provides the opportunity to uncover new principles of gene action and interaction that may have unexpected applications to understanding developmental and morphologic variation in natural populations, including humans."
Discovery of genetic pathways and mechanisms is the foundation for understanding the blueprint encoded in any genome, including our own. Studies with other animals, such as fruit flies and roundworms, have revealed principles that govern how cancer cells live and die.
"Uncovering new biologic principles in animals that are more closely related to humans, like cats, dogs and laboratory mice, may reveal unexpected insights with far-reaching implications for human biology and disease," Barsh added.
Genetic mutations that cause the blotched tabby markings became apparent when the team compared gene sequences of feral cats with different patterns. They named the affected gene Taqpep. 58 out of 58 blotched tabbies had a mutation in each of its two copies of Taqpep, while 51 out of 51 mackerel tabbies had at least one unmutated gene.
Taqpep operates by encoding a protease normally found in the cell membrane, but that can be cleaved to allow it to diffuse outside the cell. The ability to float freely and interact with external molecules is a key component of a principle called reaction diffusion. Famous computer scientist Alan Turing proposed this principle in 1952 as a way to explain how periodic patterns can arise out of seeming randomness.
"Turing realized that, under specific conditions, diffusible 'activator' and 'inhibitor' molecules can self-organize into a variety of periodic patterns," said Barsh. "We are excited about the idea that Taqpep might be an entry point to understand if, and how, reaction-diffusion mechanisms can explain 'how the leopard got its spots.'"
After the team understood Taqpep's role in creating tabby stripes, and analyzing its role in pattern creation in more than 350 other cats of 24 distinct breeds, they questioned if it might play a similar role in the patterns of wild and captive cheetahs. Blood samples were obtained from a king cheetah named Kgosi, a resident of a wildcat education and conservation program in Northern California. Kgosi was also found to have a mutation in Taqpep.
The next step was to contact Ann Van Dyk who runs the Ann van Dyk Cheetah Centre in South Africa from which all captive king cheetahs, including Kgosi, originate. Van Dyk was the first to realize that the king cheetah pattern was a recessive gene mutation through her meticulous breeding records. Van Dyk collected DNA samples from all of her cheetahs, allowing confirmation that a Taqpep mutation is responsible for the king cheetah pattern.
Mammals aren't the only animals to have patterned fur or skin. Fish, salamanders, and some invertebrates have stripes and spots as well, but with one essential difference. As a non-mammal grows to adulthood, they add stripes or spots. Mammals, conversely, keep the same number of stripes or spots by increasing the surface area of the contrasting colors.
"Somehow, cells in the black stripes know they are in a black stripe and remember that fact throughout the organism's life," said Barsh. "We were curious about what's happening at the boundary between light and dark stripes and spots. How do these spots know to grow with an animal?"
What they found, after testing fetal cat skin seven weeks after gestation, is that the patterns only begin to arise when the hair begins to grow. There are no apparent differences between the cells themselves only in the color of hair they produce, suggesting that the changes in color are due to differences in the levels of expression of certain genes within those cells.
One team member, Lewis Hong, used a technique he calls EDGE to identify changes in gene expression levels between black and yellow areas of cheetah skin. Many of the differences he found had to do with a pathway influencing the expression of a gene called Edn3. Teammate Dr. Kelly McGowan found that Edn3 mRNA was produced at the base of the follicles making the black hairs.
Further testing, in collaboration with a group at Florida International University, created a yellow-colored laboratory mouse that had been engineered to express Edn3. The coat of these mice was much darker than the unmodified mice.
The team hypothesizes that expression of Taqpep is required to establish a pattern of stripes or spots in early feline development. Edn3 further carries this out as the hair grows.
Not all cats are patterned, of course. Some big cats like African lions or mountain lions are distinctive for their lack of color variation, even though their cubs are striped. Some domestic cats that do not have stripes and spots, like Himalayans and Abyssinians, have a surprisingly common rate of Taqpep mutation.
"We know there's a mutation that suppresses pattern formation in some cats," said Barsh. "We'd like to investigate that mechanism as well."