by Matthew Cobb
We have discussed the evolution of mammalian coat colour several times on this blog, including here, here and here. It still remains a mystery, mainly because although we know a lot about the patterns involved, and can guess about some of their adaptive advantages, their genetic bases are largely unknown. Using a mixture of classic pedigree studies and molecular genetics, a new paper in Genetics (abstract only unless you have a subscription) has examined the genetic bases of stripes and spots in the domestic cat.
Previous attempts to unravel the genetics of coat colour in domestic cats had come up with the following hypothesis, based on tracking coat patterns down the generations. The character(s) producing the classic tabby (like my cat Pepper) – D in the figure above – is/are recessive to all other forms (the is/are ambiguity is because we have (or had) no idea about the number of genes involved). The dominant form is the unmarked (Abyssinian) form (A), which is dominant over the spotted coat (B), which in turn is dominant over the striped coat (like my cat Ollie – C).
To see how many genes are actually involved in determining cat coat patterns, the researchers, led by Eduardo Eizirik, now at the Pontifíca Uniservidade Católica de Rio Grande do Sul (Porto Alegre, Brazil), set up a series of mating crosses. The figure below shows the way they studied the basis of the spotted and “ticked” (Abyssinian) variants. They crossed homozygous individuals (aa or AA, respectively), then “backcrossed” their heterozygous offspring (Aa – called the F1 generation), to each of the parental types (only aa in this example) and tried to make sense of what happened in the third generation (or backcross – this is the bottom line on the figure). This is essentially the same procedure used by Gregor Mendel with his peas, over 150 years ago, which led to the foundation of genetics.
The figure below shows what happened when they tried to work out what genes are controlling the “spotted” coat variant, by crossing it to the tabby version (top line). The offspring (F1) showed a range from stripes to spots to tabby blotches (not shown). The backcross animals sometimes showed complete stripes, including the “mackerel” variant (bottom left).
They then used “microsatellite” genetic markers – small sequences of DNA that can be used to track and identify regions of the genome that may be responsible for the character under study. They were able to identify at least three different genes responsible for coat patterns – Tabby, which has two alleles or versions (one producing the mackerel pattern, the other the blotched); Ticked, which has an Abyssinian and a non-Absyssinian version, and one or more genes that alter the mackerel stripes and may also produce the blotched pattern. Furthermore, Ticked can alter the wayTabby is expressed.
In other words, it’s complicated. Which is hardly surprising, in a way – if it was straightforward, cat breeders would have figured it out long ago. What this study has shown is that there are genes involved in coding colour and pattern, that they are not necessarily the same, and that they affect the way each others’ expression. Exactly how this happens – or indeed, what these genes actually do – is unknown. They have yet to be identified at the molecular level.
Intriguingly, some of these genes may have equivalents in other animals, and may help in precisely identifying the genes involved in cat coat colour. For example, a human gene that may be similar to Tabby may be involved in the rare Hermansky-Pudlak syndrome type 2, where patients have reduced skin pigmentation.
Once the genes involved in determining coat colour and pattern in cats have been identified, we will be able to have a stab at understanding how they do what they do what they do. It will also give us the opportunity to study these genes in the 37 felid species that are still roaming the planet. In turn, that may help us understand apparently simpler patterns, such as those seen in tapirs, raccoons and badgers.
Of course, just because different species have similar patterns, that doesn’t mean that they will necessarily use the same genes to produce them. Natural selection “cares” about the phenotype, not the genes that underlie that phenotype. There is more than one way to skin a cat.
[First posted over at the Z-letter.]