Last year I co-authored why so many domesticated mammals have floppy ears for The Conversation for The Conversation. Since then, a recent paper has tracked down changes in specific genes that are associated with domestication.
As a reminder, the hypothesis was that functional and genetic differences in a specific group of cells – the neural crest – during early development, arose through domestication.
Cells in the neural crest, which forms just under the newly formed neural tube (precursor of the spinal cord), gradually migrate to the upper body where their descendent cells help to shape the brain, face, head, neck, intestine, adrenal glands and skin. Minor defects of neural crest cells, such as slower migration, were hypothesised to unite the main features of domesticated animals: tameness, smaller jaws, curly tails and floppy ears.
Current evidence points to the domestication of wild cats between 5,000 and 9,000 years ago. In the late Stone Age, when we were developing our skills in farming of plants and animals, it is likely that wild cats were attracted by food scraps we left around. They were also likely to be attracted to vermin who fed on stored grain, which you can guess, may have led to a mutually beneficial relationship between cats and us. The ancient Egyptians were so grateful to cats for controlling mice and snakes and so much in admiration of their grace and poise that they elevated them to gods.
In the recent paper, researchers compared the gene sequences of wild cats with a number of different breeds of domesticated cats and the results support the domestication hypothesis. Using stringent criteria, they found over a hundred genetic regions that differed markedly between domesticated and wild cats.
Looking through these regions, they found genes related to neural crest survival and migration, fear, reward and pigmentation. Most are active in the brain. Fittingly, the gene responsible for the white pigmentation is called KIT, which is essential for the growth and migration of pigmented cells of the skin known as melanocytes. Genetic differences in the KIT gene often result in inefficient migration and maturation of melanocytes during development, resulting in white patches over the body. This condition is known as piebaldism and is seen in humans and in horses (such as pintos), dogs, birds, pigs, cows and even some snakes. In cats, various degrees of white spotting have been produced in the last 200 years by selective breeding. The paper highlights the Birman cat (below), which has quite a high proportion of white fur.
What makes the cat sequencing paper fascinating is how the researchers linked specific genes to specific functions of tissues and organs derived from the neural crest. In addition to taking a guess at a gene’s function from the structure of the protein it produces, they looked at data from animals such as mice and frogs which have been bred in the lab to have one or both copies of a genes either defective or missing altogether (so-called “knockouts”). Comparisons have also come from human medical conditions such as piebaldism, mentioned above, that have had their genetic defects sequenced.
The cat sequencing paper also compared cats to other carnivores such as dogs and analysed the genetic differences in a similar way. They identified gene variants that are likely to provide cats with a superior sense of night vision, hearing and sensing of sex hormones, or pheromones.
What the paper did not answer was why do cats NOT have floppy ears? Maybe not all the boxes of domestication syndrome need to be ticked for each animal. Or maybe floppy ears may have been selected against because it would impair cats’ acute sense of hearing?
What all this means is that we are further along the road to understanding how cats and other animals were domesticated. The information gathered from the hundreds of cat genomes sequenced means that we will be better able to understand and treat cats when they get sick. An understanding of early mammalian development will also help us understand our own development and disease predisposition.