Lucky Thirteen (For Cats, Anyhow)
As few as 13 genes may separate domesticated cats from their wild ancestors.
Posted Nov 17, 2014
The sequencing of the domestic cat’s genome continues to shed light on how domestication may have come about. Earlier this year, a mainly Russian research team, guided by veteran molecular biologist Stephen O’Brien, made the first comparison between the nuclear DNA of domestic cats and a single individual from the closely related European wildcat Felis silvestris silvestris (by the name of Sylvester, natch), but the result was buried deep in the Supplemental data and attracted little attention. In a new paper in the Proceedings of the National Academy of Sciences, Wesley Warren and colleagues not only report an enhanced level of resolution of the domestic cat genome, they also make the first ever comparisons between domestic cats and their closest wild relative, the Middle Eastern/North African wildcat Felis silvestris lybica. From these analyses they have already been able to draw valuable conclusions about the evolution of the Felidae, the domestication of our pet cats, and a mutation that characterises one of today’s popular breeds (the Birman): given the richness and extent of the data, there will undoubtedly be much more to come.
Those who blithely assume that dogs’ noses outperform those of cats will have their prejudices confirmed by the slightly smaller number of functional olfactory receptor genes (Or) - around 700 in domestic cats and tigers, more than 800 in dogs - though how the extra couple of hundred or so actually benefit dogs is still open to conjecture. However, the greater power of the cat’s vomeronasal organ - the structure above the roof of the mouth that “tastes” social odours - has been confirmed, with 21 functional vomeronasal receptor (V1r) genes in domestic cats, compared to the dog’s 8 (both down slightly on previous estimates). While it is still unclear how the number of receptor genes translates either into acuity or into discrimination between different odours, this difference is consistent with the domestic cat’s origins as a solitary, territorial predator that communicated mainly by scent, contrasting with the dog’s descent from the wolf with its sophisticated repertoire of visual and auditory signals.
Owners may find that some of their cat’s more puzzling behaviour makes more sense once they realise just how sensitive their vomeronasal organs (VNO) are. For example, when one cat disappears from the house, the remaining cat(s) often continue to search for their missing companion for days or even weeks afterwards. This could apply equally to a mother cat whose kittens have just been homed, or a cat whose feline companion has been killed in a traffic accident. Understandably, owners usually interpret such restless behaviour as a symptom of grief, but cats are probably incapable of such a complex emotion (one that doesn’t emerge in children until they are at least four years old). Instead, they may be trying to resolve a paradox: they can no longer see or hear the cat or kitten that’s gone, and cannot comprehend its disappearance, but they can still detect its scent-marks around the house (cats constantly mark the core of their territory with glands on the sides of their faces). Lacking a VNO of our own, and having such a weak sense of smell compared to the cat’s, it is understandable that we owners don’t appreciate just how much importance cats place on the information that they glean from traces of odour. However, with time these scent-marks gradually fade away, and once they have dropped below the threshold of even the cat’s VNO, the memory of the missing companion is no longer triggered, the paradox is resolved, and the “grieving” behaviour ceases.
Detailed analysis of our cats’ DNA is not only helping us understand their looks and their behaviour, it is also beginning to reveal how they may have become domesticated. Although the subspecies from which the domestic cat is derived, Felis silvestris lybica, still inhabits the Middle East, it has proved well-nigh impossible to make comprehensive observations of its behaviour. However, all agree that these wildcats are both solitary and difficult to tame, so our pets’ ability to form affectionate relationships with humans and also (to a certain extent) with each other are both presumably consequences of domestication. The new study identifies just 12 or 13 genes that differ consistently between wildcats and domestic cats: since domestic and wild differ little in appearance, it is likely that the products of these genes are responsible for those very changes in behaviour that have facilitated domestication. Supporting this idea, the regions of the chromosomes where these differences occur turn out to play important roles in the development of the nervous system, the formation of memories, and emotional responses. More specifically, several affect the early development of the nervous system via the neural crest cells, consistent with a recently-proposed general theory of mammalian domestication. Others enhance the rate of recombination between chromosomes, speeding up the rate at which new combinations of genes appear, and potentially helping to account for the extraordinarily short time (in evolutionary terms) that the wildcat turned into today’s affectionate pet.