Wikipedia

Melanism

Melanismis thecongenitalexcess ofmelaninin an organism resulting in darkpigment.

Melanistic blackeastern grey squirrel(Sciurus carolinensis)
Melanisticguinea pigs(Cavia porcellus) are rare, and are used in rituals by Andeancuranderos.[1]

Pseudomelanism, also called abundism, is another variant of pigmentation, identifiable by dark spots or enlarged stripes, which cover a large part of the body of the animal, making it appear melanistic.[2]

The morbid deposition of black matter, often of a malignant character causing pigmentedtumors,is calledmelanosis.[3]

Adaptation

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A melanisticEuropean adder(Vipera berus) compared to a normal-colored adder

Melanism related to the process ofadaptationis called adaptive. Most commonly, dark individuals becomefitterto survive and reproduce in their environment as they are better camouflaged. This makes some species less conspicuous to predators, while others, such asleopards,use it as a foraging advantage during night hunting.[4]Typically, adaptive melanism isheritable:Adominantallele,which is entirely or nearly entirely expressed in thephenotype,is responsible for the excessive amount of melanin. By contrast, adaptive melanism associated with Batesian mimicry inZelandoperla fenestratastonefliesis controlled by arecessive alleleat the ebony locus.[5][6]

Adaptive melanism has been shown to occur in a variety of animals, including mammals such assquirrels,manycatsandcanids,andcoral snakes.Adaptive melanism can lead to the creation ofmorphs,a notable example being thepeppered moth,whose evolutionary history in theUnited Kingdomis offered as a classic instructional tool for teaching the principles ofnatural selection.[7]A more replicated example of human-induced shifts inmelanismhas arisen from repeated selection against melanicZelandoperla fenestratastonefly phenotypes following widespreaddeforestation in New Zealand.[5][6]


Industrial melanism

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Main article:Industrial melanism

Industrial melanism is anevolutionaryeffect in insects such as the peppered moth,Biston betulariain areas subject toindustrial pollution.Darker pigmented individuals are favored bynatural selection,apparently because they are bettercamouflagedagainst polluted backgrounds. When pollution was later reduced, lighter forms regained the advantage and melanism became less frequent.[8][9][10][11][12][13]Other explanations have been proposed, such as that the melanin pigment enhances function of immune defences,[14]or a thermal advantage from the darker coloration.[15][16][17]

In cats

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See also:Black cat
Melanistic and normally coloured jaguars

Melanistic coat coloration occurs as a commonpolymorphismin 11 of 37felidspecies and reaches high population frequency in some cases but never achieves completefixation.Theblack panther,a melanisticleopard,is common in theequatorial rainforestofMalayaand thetropical rainforeston the slopes of some African mountains, such asMount Kenya.Theservalalso has melanistic forms in certain areas ofEast Africa.In thejaguarundi,coloration varies from dark brown and gray to light reddish. Melanic forms ofjaguarare common in certain parts ofSouth America.[18]In 1938 and 1940, two melanisticbobcatswere trapped alive in sub-tropicalFlorida.[19]

Pseudomelanism on acheetah
Melanistic eastern gray squirrel

In 2003, thedominantmode of inheritance of melanism in jaguars was confirmed by performingphenotype-transmission analysis in a 116-individual captivepedigree.Melanistic animals were found to carry at least one copy of a mutantMC1Rsequenceallele,bearing a 15-base pairinframe deletion. Ten unrelated melanistic jaguars were eitherhomozygousorheterozygousfor this allele. A 24-base pair deletion causes the incompletely dominant allele for melanism in the jaguarundi. Sequencing of theagouti signalling peptidein theagouti genecoding region revealed a 2-base pair deletion in blackdomestic cats.These variants were absent in melanistic individuals ofGeoffroy's cat,oncilla,pampas catandAsian golden cat,suggesting that melanism arose independently at least four times in the cat family.[20]

Melanism in leopards is inherited as aMendelian,monogenicrecessive traitrelative to the spotted form. Pairings of black animals have a significantly smaller litter size than other possible pairings.[21]Between January 1996 and March 2009,Indochinese leopardswere photographed at 16 sites in theMalay Peninsulain a sampling effort of more than 1000 trap nights. Of 445 photographs of melanistic leopards, 410 were taken south of theKra Isthmus,where the non-melanistic morph was never photographed. These data suggest the near fixation of the dark allele in the region. The expected time to fixation of this recessive allele due togenetic driftalone ranged from about 1,100 years to about 100,000 years.[22] Melanism in leopards has been hypothesized to be causally associated with a selective advantage for ambush.[23]Other theories are that genes for melanism in felines may provide resistance to viral infections, or a high-altitude adaptation, since black fur absorbs more light for warmth.[24]

In birds

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White Silkie rooster
Black Silkie rooster

The chicken breedsSilkieandAyam Cemanicommonly exhibit this trait.Ayam Cemaniis an uncommon and relatively modern breed of chicken from Indonesia. They have a dominant gene that causes hyperpigmentation (Fibromelanosis), making the chicken entirely black; including feathers, beak, and internal organs.

In April 2015, an extremely rare blackflamingowas spotted on the Mediterranean island ofCyprus.[25]

In amphibians

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Thealpine salamander,Salamandra atra,has one subspecies (S. atra atra) that is completely black.[26]The pigment comes from a specific cell called a melanophore, which produce the compound melanin.[27][28]

There are four other subspecies of this salamander,[29]and they have varying levels of melanin pigmentation.[28][30][31]The subspecies have yellow spots in different concentrations or proportions. Thepigment-producing cellsthat contribute to the yellow spots of some sub-species are called xanthophores.[30]It appears that the fully-black phenotypes do not ever develop these xanthophores.[31]Alpine salamanders produce a toxin from their skin, and both fully melanistic, black salamanders and spotted individuals produce the compound.[32]

Studies done that traced DNA histories have suggested that the original alpine salamander phenotype was black with some yellow spots, meaning that the fully black color evolved over time and was thus selected for over many generations.[31]

In humans

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Melanism, meaning a mutation that results in completely dark skin, does not exist in humans. In humans, the amount of melanin is determined by three dominant alleles (AABBCC), and different ethnicities have varying amounts.[33]Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmfulultraviolet radiation.The same ultraviolet radiation is essential for the synthesis ofvitamin Din skin, so lighter colored skin – less melanin – is an adaptation related to the prehistoric movement of humans away from equatorial regions, as there is less exposure to sunlight at higher latitudes. People from parts of Africa,South Asia,Southeast Asia,and Australia may have very dark skin, but this is not melanism.

Peutz–Jeghers syndrome

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Main article:Peutz–Jeghers syndrome

This raregenetic disorderis characterized by the development ofmaculeswithhyperpigmentationon the lips andoral mucosa(melanosis), as well as benign polyps in the gastrointestinal tract.[34]

Socio-politics

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Further information:Melanin theory

The termmelanismhas been used onUsenet,internet forumsandblogsto mean anAfrican-Americansocial movementholding that dark-skinned humans are the original people from which those of otherskin colororiginate. The termmelanismhas been used in this context as early as the mid-1990s[35]and was promoted by someAfrocentrists,such asFrances Cress Welsing.

See also

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References

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Bibliography

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