Genes and the DNA that makes them up form an intimate link between the present and the past. In the present they govern many aspects of our bodies and minds; how they do so has been sculpted by the adaptations of our ancestors to changing circumstances.
In the case of domestic animals, one of those changes is the move from a wild lifestyle to one intertwined with humans.
The genome (total DNA) of the domestic cat (Felis silvestris catus), published this month in the Proceedings of the National Academy of Sciences by Michael Montague and colleagues, reveals some of these clues of the past and shows how the cat has adapted to a domestic lifestyle.
Animals were domesticated for various reasons, be it for food, work or other usefulness. The cat was probably initially chosen because of its usefulness in catching vermin. However, this is sometimes disputed; some think that cats were spontaneously attracted to the vermin found near grain stores.
Similarly, it is thought that the dog also had no original domestic purpose and was not actively taken from the wild by humans, but rather started living closer to human settlements, where it could feed on the plentiful waste.
Not surprisingly to anyone who has ever spent time with one, cats are considered to be only “semi-domesticated”, not because of their famously aloof manner but because their food and breeding are not fully controlled by humans.
A typical characteristic of domestic animals, be it farm livestock or household pets, is their tameness. Domestic cats are tame compared to wildcats (Felis silvestris). They also have coat colours and patterns not usually seen in the wild relative.
Positive selection – the favouring of particular traits – leaves recognisable traces in the genome. Selection comes about when individuals with a particular trait are disproportionately successful at leaving offspring, thus passing on the genes responsible for the trait in question. This either happens by natural selection in the wild or by selective breeding on the farm. Eventually, the less favourable gene vanishes from the population and the remaining favoured gene harbours traces of accelerated evolution.
These traces can be searched for in order to determine which exact genetic changes were selected during the domestication process. In the case of the cat, these traces were found in several genes, including some genes active in the nervous system. These genetic changes are thought to be responsible for behavioural changes, including reduced fear (such as reduced fear of humans) and the ability to learn associated with food rewards.
Ultrasonic hearing
Compared with dogs, cats (including big cats such as lions and tigers) rely more on their hearing and less on smell. Cats can detect ultrasonic communications and slight movements of prey. This is mirrored in their DNA, where genes associated with hearing show the traces of positive selection. By contrast, cats have fewer olfactory (smell) genes than dogs.
As well as behavioural characteristics, domestic cats are distinguished by their coat colour and patterns. They can be pure white, have white patches, or white just on the paws (“gloving”), none of which is common in the wild.
Pure white fur in cats is caused by a mutation in a gene, which completely halts the pigmentation of the fur, masking the presence of all other fur pigment genes. A small number of other genes are responsible for all the other coat variations.
Piebald spotting has been linked to a gene called Kit, which is active in melanocytes (pigment-producing cells). In their paper, Montague and colleagues showed that the “gloving” appearance of Burmese cats is due to a version of this same gene.
The unusual colouration of Siamese and Burmese cats – which is limited to the extremities or “points” (tail, legs, ears and snout) – is due to particularly interesting genetics. It is caused by a temperature- sensitive version of a gene responsible for the first step in pigment production. This gene is shut down in the warmer parts of the body, so the pigmentation of the fur only occurs in the colder extremities.
The characteristics of cats have originated from both natural and unnatural selection, which have left clear traces on their genome. This genetic evolution shaped their DNA. When we sequence the genome we are getting a read-out of the natural history of cats, recorded in their genes.
Aoife McLysaght is a professor of genetics in Trinity College Dublin, where she leads a research group focusing on identifying and interpreting the evolutionary patterns in animal genomes