Feline Melanism: Adaptive and Evolutionary Significance
Melanism (dark coat coloration) is a common polymorphism observed in many animals. This remarkable feature has been described in as many as 13 out of 37 felid species (Schneider et al, 2015). It is still widely speculated exactly why feline melanism evolved in the first place and what kind of adaptive and evolutionary significance it has for our favorite furriends.
A 2003 study by Eduardo Eizirik, an evolutionary geneticist at the US National Cancer Institute in Maryland, emphasizes that there must be at least four independent genetic origins for melanism in the cat family. The recurrent evolution of the trait and its preservation in the feline population indicate its potential adaptive importance and raises a question: does melanism have an evolutionary advantage for cats? A 2015 study by Schneider supports the claim by demonstrating that "some "black cats" are black not by chance, but by selection for a mutation that provides increased fitness".
The evolution of feline melanism
We know that in domestic cats the solid coloration is determined by a recessive allele (ASIP-∆2) of the agouti signaling protein gene (ASIP) (this allele is often referred to as non-agouti). Only recessive homozygotes for this gene will result in solid coloration. The coloration itself is influenced by other genes involved in the formation of the coat color. The black coloration is a product of a dominant allele B of the feline primary gene for coat color (B/b/b1). Eizirik and his colleagues mapped, cloned and sequenced the ASIP gene to characterize the mutation coding for melanism in domestic cats. Furthermore, they also analyzed another gene called melanocortin 1 receptor (MC1R), which is associated with dark coloration in wild cats (jaguars and jaguarundis). They found two independent deletions implicated in melanism in jaguars and jaguarundis.
The ASIP gene was mapped to chromosome A3, and MC1R to chromosome E2. Their positions in the feline genome correspond to the position of homologous human genes in the human genome. The mutation in the ASIP gene associated with black coloration in domestic cats results from the ASIP-∆2 allele. The study emphasizes that this mutation actually causes complete loss of the gene's function resulting in a dysfunctional protein. This loss-of-function, therefore, is what leads to melanism in domestic cats. It was also discovered that it’s inherited recessively. All the studied black cats were homozygotes for the allele, while all the non-black studied cats had at least one wild-type allele. This allele was not found among the studied wildcats.
In jaguars, an allele of the MC1R gene, MC1R-∆15, was associated with black coloration and it was found to be inherited dominantly. All black jaguars had at least one copy of the MC1R-∆15 sequence. The second allele of this gene, MC1R-∆24, was also identified and associated with black coloration in the jaguarundis. This mutation arose independently from MC1R-∆15 and it found to be semi-dominant. Interestingly, in this study, none of these three mutations are associated with melanism in other felid species. This lead to the conclusion that there could be at least 4 independent origins of the trait. Mutations in MC1R gene, however, result in a gain of function and lead to melanism (Eizirik et al, 2003).
A 2015 study by Schneider and her colleagues examined three closely-related Leopardus cats (pampas cat, kodkod and Geoffroy's cat) and found similar mutations. "In both the pampas cat and the kodkod, mutations in ASIP were identified that are predicted to cause a loss-of-function, and homozygosity for these mutations was completely associated with melanism. […] In Geoffroy’s cat, we identified an MC1R mutation as the likely cause of melanism." - (Schneider et al, in "Recurrent Evolution of Melanism in South American Felids, 2015).
Evolutionary significance of feline melanism
The results of the studies described above lead to an inevitable question. Why did this trait evolve multiple times throughout feline history and does it serve an adaptive purpose? While it can be speculated that this is an adaptation to a nocturnal lifestyle for many different species of cat, scientists now believe that melanism could prove to have a more significant contribution to feline survival benefit. For example, the identified mutations are actually found in the same family of genes as those involved in the human response to human diseases (such as response to HIV). This indicates that there is a possibility that black cats could carry resistance to certain diseases. Eizirik told the NewScientist: "There are parallels with humans. [The gene mutation system found] does have parallels with other systems potentially involved in disease."
Before we can be certain about the evolutionary and adaptive significance of coat coloration of cats, there is still a lot more we must learn about the evolution of feline melanism and the natural selection that has shaped the feline color variation to what it is today. The data currently available gives prominence to the important role of melanism, and further research, especially with an extent to other species, may reveal more. Learning more about the magnificent felines dressed in black could lead to key discoveries in human sciences as well.
Sources and further reading:
1. Eizirik E, Yuhki N, Johnson WE, Menotty-Raymond M, Hannah SS, O'Brien SJ (2003). Molecular Genetics and Evolution of Melanism in the Cat Family. Current Biology 13: 448-453.
2. Schneider A, Henegar C, Day K, Absher D, et al (2015). Recurrent Evolution of Melanism in South American Felids. PLOS Genetics 11(4): e1005126