Melanism

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Melanistic black eastern grey squirrel (Sciurus carolinensis) Melsquir.jpg
Melanistic black eastern grey squirrel (Sciurus carolinensis)
Melanistic guinea pigs (Cavia porcellus) are rare, and are used in rituals by Andean curanderos. Cavia porcellus-Licorice.jpg
Melanistic guinea pigs (Cavia porcellus) are rare, and are used in rituals by Andean curanderos .

Melanism is the congenital excess of melanin in an organism resulting in dark pigment.

Contents

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 pigmented tumors, is called melanosis. [3]

Adaptation

A melanistic European adder (Vipera berus) compared to a normal-colored adder Hugorme.jpg
A melanistic European adder (Vipera berus) compared to a normal-colored adder

Melanism related to the process of adaptation is called adaptive. Most commonly, dark individuals become fitter to survive and reproduce in their environment as they are better camouflaged. This makes some species less conspicuous to predators, while others, such as leopards, use it as a foraging advantage during night hunting. [4] Typically, adaptive melanism is heritable: A dominant allele, which is entirely or nearly entirely expressed in the phenotype, is responsible for the excessive amount of melanin. By contrast, adaptive melanism associated with Batesian mimicry in Zelandoperla fenestrata stoneflies is controlled by a recessive allele at the ebony locus. [5] [6]

Adaptive melanism has been shown to occur in a variety of animals, including mammals such as squirrels, many cats and canids, and coral snakes. Adaptive melanism can lead to the creation of morphs, a notable example being the peppered moth, whose evolutionary history in the United Kingdom is offered as a classic instructional tool for teaching the principles of natural selection. [7] A more replicated example of human-induced shifts in melanism has arisen from repeated selection against melanic Zelandoperla fenestrata stonefly phenotypes following widespread deforestation in New Zealand. [5] [6]


Industrial melanism

Industrial melanism is an evolutionary effect in insects such as the peppered moth, Biston betularia in areas subject to industrial pollution. Darker pigmented individuals are favored by natural selection, apparently because they are better camouflaged against 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

Melanistic and normally coloured jaguars Melanism in Panthera Onca.jpg
Melanistic and normally coloured jaguars

Melanistic coat coloration occurs as a common polymorphism in 11 of 37 felid species and reaches high population frequency in some cases but never achieves complete fixation. The black panther, a melanistic leopard, is common in the equatorial rainforest of Malaya and the tropical rainforest on the slopes of some African mountains, such as Mount Kenya. The serval also has melanistic forms in certain areas of East Africa. In the jaguarundi, coloration varies from dark brown and gray to light reddish. Melanic forms of jaguar are common in certain parts of South America. [18] In 1938 and 1940, two melanistic bobcats were trapped alive in sub-tropical Florida. [19]

Pseudomelanism on a cheetah Acinonyx jubatus King Cheetah.jpg
Pseudomelanism on a cheetah
Melanistic eastern gray squirrel Melanistic eastern gray squirrel 2.jpg
Melanistic eastern gray squirrel

In 2003, the dominant mode of inheritance of melanism in jaguars was confirmed by performing phenotype-transmission analysis in a 116-individual captive pedigree. Melanistic animals were found to carry at least one copy of a mutant MC1R sequence allele, bearing a 15-base pair inframe deletion. Ten unrelated melanistic jaguars were either homozygous or heterozygous for this allele. A 24-base pair deletion causes the incompletely dominant allele for melanism in the jaguarundi. Sequencing of the agouti signalling peptide in the agouti gene coding region revealed a 2-base pair deletion in black domestic cats. These variants were absent in melanistic individuals of Geoffroy's cat, oncilla, pampas cat and Asian golden cat, suggesting that melanism arose independently at least four times in the cat family. [20]

Melanism in leopards is inherited as a Mendelian, monogenic recessive trait relative 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 leopards were photographed at 16 sites in the Malay Peninsula in a sampling effort of more than 1000 trap nights. Of 445 photographs of melanistic leopards, 410 were taken south of the Kra 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 to genetic drift alone 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

White Silkie rooster Poule Soie gris perle.jpg
White Silkie rooster
Black Silkie rooster Coq poule soie noir.png
Black Silkie rooster

The chicken breeds Silkie and Ayam Cemani commonly exhibit this trait. Ayam Cemani is 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 black flamingo was spotted on the Mediterranean island of Cyprus. [25]

In amphibians

The alpine 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. The pigment-producing cells that 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

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 harmful ultraviolet radiation. The same ultraviolet radiation is essential for the synthesis of vitamin D in 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, Australia, Papua New Guinea, Fiji, Vanuatu, New Caledonia, and the Solomon Islands may have very dark skin, but this is not melanism.

Peutz–Jeghers syndrome

This rare genetic disorder is characterized by the development of macules with hyperpigmentation on the lips and oral mucosa (melanosis), as well as benign polyps in the gastrointestinal tract. [34]

Socio-politics

The term melanism has been used on Usenet, internet forums and blogs to mean an African-American social movement holding that dark-skinned humans are the original people from which those of other skin color originate. The term melanism has been used in this context as early as the mid-1990s [35] and was promoted by some Afrocentrists, such as Frances Cress Welsing.

See also

Related Research Articles

<span class="mw-page-title-main">Peppered moth</span> Species of moth

The peppered moth is a temperate species of night-flying moth. It is mostly found in the northern hemisphere in places like Asia, Europe and North America. Peppered moth evolution is an example of population genetics and natural selection.

<span class="mw-page-title-main">Human skin color</span>

Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigmentation, which is the result of genetics, exposure to the sun, disorders, or some combination thereof. Differences across populations evolved through natural selection or sexual selection, because of social norms and differences in environment, as well as regulations of the biochemical effects of ultraviolet radiation penetrating the skin.

<span class="mw-page-title-main">Black panther</span> Variant of leopard and jaguar

A black panther is the melanistic colour variant of the leopard and the jaguar. Black panthers of both species have excess black pigments, but their typical rosettes are also present. They have been documented mostly in tropical forests, with black leopards in Africa and Asia, and black jaguars in South America. Melanism is caused by a recessive allele in the leopard, and by a dominant allele in the jaguar.

Ecological genetics is the study of genetics in natural populations. It combines ecology, evolution, and genetics to understand the processes behind adaptation. It is virtually synonymous with the field of molecular ecology.

<span class="mw-page-title-main">Cat coat genetics</span> Genetics responsible for the appearance of a cats fur

Cat coat genetics determine the coloration, pattern, length, and texture of feline fur. The variations among cat coats are physical properties and should not be confused with cat breeds. A cat may display the coat of a certain breed without actually being that breed. For example, a Neva Masquerade could wear point coloration, the stereotypical coat of a Siamese.

<span class="mw-page-title-main">Peppered moth evolution</span> Significance of the peppered moth in evolutionary biology

The evolution of the peppered moth is an evolutionary instance of directional colour change in the moth population as a consequence of air pollution during the Industrial Revolution. The frequency of dark-coloured moths increased at that time, an example of industrial melanism. Later, when pollution was reduced in response to clean air legislation, the light-coloured form again predominated. Industrial melanism in the peppered moth was an early test of Charles Darwin's natural selection in action, and it remains a classic example in the teaching of evolution. In 1978, Sewall Wright described it as "the clearest case in which a conspicuous evolutionary process has actually been observed."

<span class="mw-page-title-main">Plumage</span> Layer of feathers that covers a bird

Plumage is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, there can be different colour morphs. The placement of feathers on a bird is not haphazard but rather emerges in organized, overlapping rows and groups, and these feather tracts are known by standardized names.

<span class="mw-page-title-main">Alpine salamander</span> Species of amphibian

The alpine salamander is a black salamander that can be found in the French Alps, and through the mountainous range in Europe. It is a member of the genus Salamandra. Their species name, atra, may be derived from the Latin ater, meaning dull black. The salamanders' coloration has evolved over time, as some species are completely monochrome black and others have yellow spotting and marks. Their life expectancy is at least 10 years. There are four subspecies of the alpine salamander, with varied distribution and physical coloration. Unlike other salamanders, whose larvae are developed in water, the alpine salamander and its subspecies are a fully terrestrial species in life and gestation. They give birth to live young.

<span class="mw-page-title-main">Equine coat color genetics</span> Genetics behind the equine coat color

Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. Bay is the most common color of horse, followed by black and chestnut. A change at the agouti locus is capable of turning bay to black, while a mutation at the extension locus can turn bay or black to chestnut.

<span class="mw-page-title-main">Industrial melanism</span> Evolutionary effect

Industrial melanism is an evolutionary effect prominent in several arthropods, where dark pigmentation (melanism) has evolved in an environment affected by industrial pollution, including sulphur dioxide gas and dark soot deposits. Sulphur dioxide kills lichens, leaving tree bark bare where in clean areas it is boldly patterned, while soot darkens bark and other surfaces. Darker pigmented individuals have a higher fitness in those areas as their camouflage matches the polluted background better; they are thus favoured by natural selection. This change, extensively studied by Bernard Kettlewell (1907–1979), is a popular teaching example in Darwinian evolution, providing evidence for natural selection. Kettlewell's results have been challenged by zoologists, creationists and the journalist Judith Hooper, but later researchers have upheld Kettlewell's findings.

<span class="mw-page-title-main">White horse</span> Horse coat color

A white horse is born predominantly white and stays white throughout its life. A white horse has mostly pink skin under its hair coat, and may have brown, blue, or hazel eyes. "True white" horses, especially those that carry one of the dominant white (W) genes, are rare. Most horses that are commonly referred to as "white" are actually "gray" horses whose hair coats are completely white. Gray horses may be born of any color and their hairs gradually turn white as time goes by and take on a white appearance. Nearly all gray horses have dark skin, except under any white markings present at birth. Skin color is the most common method for an observer to distinguish between mature white and gray horses.

<span class="mw-page-title-main">Smoker's melanosis</span> Medical condition

Smoker's melanosis is seen with the naked eye as a brown to black pigmentation of the oral tissue i.e. the gums, cheeks or palate as well as in larynx. It is most often seen in the lower labial gingiva of tobacco users. Most easily it is found in Caucasians, due to their lack of a genetically caused melanin pigmentation.

<span class="mw-page-title-main">Amelanism</span> Pigmentation abnormality

Amelanism is a pigmentation abnormality characterized by the lack of pigments called melanins, commonly associated with a genetic loss of tyrosinase function. Amelanism can affect fish, amphibians, reptiles, birds, and mammals including humans. The appearance of an amelanistic animal depends on the remaining non-melanin pigments. The opposite of amelanism is melanism, a higher percentage of melanin.

Oral pigmentation is asymptomatic and does not usually cause any alteration to the texture or thickness of the affected area. The colour can be uniform or speckled and can appear solitary or as multiple lesions. Depending on the site, depth, and quantity of pigment, the appearance can vary considerably.

<span class="mw-page-title-main">Melanocortin 1 receptor</span> Protein controlling mammalian coloration

The melanocortin 1 receptor (MC1R), also known as melanocyte-stimulating hormone receptor (MSHR), melanin-activating peptide receptor, or melanotropin receptor, is a G protein–coupled receptor that binds to a class of pituitary peptide hormones known as the melanocortins, which include adrenocorticotropic hormone (ACTH) and the different forms of melanocyte-stimulating hormone (MSH). It is coupled to Gαs and upregulates levels of cAMP by activating adenylyl cyclase in cells expressing this receptor. It is normally expressed in skin and melanocytes, and to a lesser degree in periaqueductal gray matter, astrocytes and leukocytes. In skin cancer, MC1R is highly expressed in melanomas but not carcinomas.

<span class="mw-page-title-main">Dark skin</span> Human skin color

Dark skin is a type of human skin color that is rich in melanin pigments. People with dark skin are often referred to as black people, although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.

<span class="mw-page-title-main">Albinism</span> Disorder causing lack of pigmentation

Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and reddish pink or blue eyes. Individuals with the condition are referred to as albinos.

The agouti gene, the Agouti-signaling protein (ASIP) is responsible for variations in color in many species. Agouti works with extension to regulate the color of melanin which is produced in hairs. The agouti protein causes red to yellow pheomelanin to be produced, while the competing molecule α-MSH signals production of brown to black eumelanin. In wildtype mice, alternating cycles of agouti and α-MSH production cause agouti coloration. Each hair has bands of yellow which grew during agouti production, and black which grew during α-MSH production. Wildtype mice also have light-colored bellies. The hairs there are a creamy color the whole length because the agouti protein was produced the whole time the hairs were growing.

Abdominal pigmentation in Drosophila melanogaster is a morphologically simple but highly variable trait that often has adaptive significance. Pigmentation has extensively been studied in Drosophila melanogaster. It has been used as a model for understanding the development and evolution of morphological phenotypes.

<i>Zelandoperla fenestrata</i> Species of stonefly

Zelandoperla fenestrata is a widespread endemic New Zealand ‘long-tailed’ stonefly that is often abundant in high-gradient stony streams, from near sea-level up to alpine elevations. The species name 'fenestrata' refers to the window-like rectangular patterning visible on the wings of non-melanic adult specimens.

References

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  15. ↑ Mikkola, K., Albrecht, A., 1988. The melanism of Adalia-bipunctata around the Gulf of Finland as an industrial phenomenon (Coleoptera, Coccinellidae). Annales Zoologici Fennici 25:177–85.
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  17. ↑ De Jong, P. W., Verhoog, M. D., Brakefield, P. M., 1992. Sperm competition and melanic polymorphism in the 2-spot ladybird, Adalla bipunctata (Coleoptera, Coccinellidae). Journal of Heredity 70:172–178.
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  25. ↑ Krol, Charlotte (2015-04-09). "Rare black flamingo spotted in Cyprus". The Telegraph. Archived from the original on 2015-04-25. Retrieved 2015-05-16.
  26. ↑ "Salamandre noire". www.karch.ch. Retrieved 2022-11-30.
  27. ↑ Trevisan, Pierluigi; Pederzoli, Aurora; Barozzi, Giancarlo (October 1991). "Pigmentary System of the Adult Alpine Salamander Salamandra atra atra (Laur., 1768)". Pigment Cell Research. 4 (4): 151–157. doi:10.1111/j.1600-0749.1991.tb00432.x. ISSN   0893-5785. PMID   1816547.
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  30. 1 2 Burgon, James D.; Vieites, David R.; Jacobs, Arne; Weidt, Stefan K.; Gunter, Helen M.; Steinfartz, Sebastian; Burgess, Karl; Mable, Barbara K.; Elmer, Kathryn R. (April 2020). "Functional colour genes and signals of selection in colour-polymorphic salamanders". Molecular Ecology. 29 (7): 1284–1299. Bibcode:2020MolEc..29.1284B. doi: 10.1111/mec.15411 . hdl: 10261/234500 . ISSN   0962-1083. PMID   32159878. S2CID   212664862.
  31. 1 2 3 Bonato, Lucio; Steinfartz, Sebastian (2005-01-01). "Evolution of the melanistic colour in the Alpine salamander Salamandra atra as revealed by a new subspecies from the Venetian Prealps". Italian Journal of Zoology. 72 (3): 253–260. doi:10.1080/11250000509356680. ISSN   1125-0003. S2CID   83504324.
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Bibliography

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