BLACK COATS
(MELANISTIC POLYMORPHISM)

Melanism (black colouration of the coat) is very common in cats. 11 out of 37 species of cat have melanistic variants. In some species, like the jaguar (P. onca) and leopard (P. pardus) melanistic variants are especially common, though the incidence never reaches 100%.

Several genes have been found to influence coat colour in a variety of mammals; these include agouti signalling protein (ASIP) and the melanocortin-1 receptor (MC1R). ASIP, if present, signals for light coloured coats. Both copies of ASIP must be defective or missing for a melanistic coat – this is known as recessive inheritance. Mutation of a single copy of MC1R, however, leads to melanism – this is called dominant inheritance.

The most common form of inheritance reported in the jaguar is dominant. A group in the National Institutes of Health (NIH) in Maryland, USA sequenced the MC1R gene in a 116 individual (captive) pedigree. They found a deletion of 15 nucleotides (the building blocks of DNA) within the MC1R gene in melanistic jaguars, compared to normal (wild-type) individuals. This version (allele) of the gene is called MC1R-delta15. They confirmed this finding in ten unrelated black jaguars and 36 normal jaguars in the wild – all ten black panthers had at least one copy of MC1R-delta15 and none of the wild-types did.

CTG GAG ACG GCC GTC ATG CTG CTG CTG GAG GCA GGC GCC CTG GCC F. catus (domestic cat) 

CTG GAG ACG GCC GTC ATG CTG CTG CTG GAG GCG GGC ACC CTG GCC P. onca

CTG GAG ACG GCC GTC ATG CTG CTG --- --- --- --- --- ACG GCC P. onca (melanistic jaguar)

Each group of three nucleotides (A, T, G or C above - the basic code of DNA) codes for one amino acid (the basic building blocks of proteins). Proteins perform all the actions of a cell and can be thought of as acting out the code of DNA. As 15 nucleotides are deleted in this mutation, the final protein is missing five amino acids (15/3=5). This seriously harms the ability of the MCR1 protein to do its job in the cell.

V-L-E-T-A-V-M-L-L-L-E-A-G-A-L-A

five amino acid deletion (in frame)

The same group have reported evidence that melanism in domestic cats (which is normally recessive) is related to a mutation in agouti signalling protein (ASIP).

The persistence of melanistic variants of jaguars and leopards, but not lions, is probably explained by a difference in habitat. The jungle cats may well have an advantage when coloured black, but the plains-living lion would stick out like a sore thumb! The tiger's traditional range was very varied, from thick jungle to open savannah. It seems that melanism does not give the tiger a sufficient advantage for the mutation to persist in jungle-living groups. There are multiple unconfirmed reports of such animals but, if they are real, they are probably isolated individuals rather than large pedigrees.