'Cat owners have long recognized that, unlike most mammals, domestic cats are uniformly uninterested in sweet-tasting foods. An early study in the 1970s reports the same indifference to sweets is also evident in wild cats such as lions, tigers, leopards, and jaguars.
"One possible explanation for this behavior is that felines are unable to detect sweet-tasting compounds like sugars and high intensity sweeteners because their sweet taste receptor is defective," comments Xia Li, PhD, a molecular geneticist at Monell and lead author of the current study. "An obvious place to look, therefore, is at the genes coding for the sweet-taste receptor."
The mammalian sweet receptor is composed of two protein subunits, known as T1R2 and T1R3. Each is coded for by a separate gene. In the new study, reported in the July 2005 inaugural issue of PLoS (Public Library of Science) Genetics, the researchers show a defect in the gene encoding the T1R2 protein in domestic cats.
The Monell researchers also detected the same gene defect in tiger and cheetah, suggesting that it is common to species throughout the cat family. "This type of gene is known as a pseudogene and is somewhat like a molecular fossil," says Li. "It presumably once coded a functional protein, but no longer does so."
"The non-functional sweet receptor provides a molecular explanation for why cats have no avidity for sweets," says senior author Joseph G. Brand, PhD, a Monell biophysicist. "Looking beyond this elegant explanation, one can contemplate the importance that this molecular change had on the evolution of the cat's carnivorous behavior."
"What we still don't know is - which came first: carnivorous behavior or the loss of the T1R2 protein?"