Albinism
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.[1][2] Individuals with the condition are referred to as albinos.
Varied use and interpretation of the terms mean that written reports of albinistic animals can be difficult to verify. Albinism can reduce the survivability of an animal; for example, it has been suggested that albino alligators have an average survival span of only 24 hours due to the lack of protection from UV radiation and their lack of camouflage to avoid predators.
In plants, albinism is characterised by partial or complete loss of chlorophyll pigments and incomplete differentiation of chloroplast membranes. Albinism in plants interferes with photosynthesis, which can reduce survivability.[9] Some plant variations may have white flowers or other parts. However, these plants are not totally devoid of chlorophyll. Terms associated with this phenomenon are "hypochromia" and "albiflora".[10]
Biological colouration
This section needs additional citations for verification. (April 2023) |
Animal colouration
Animals can appear coloured due to two mechanisms, pigments and
Pigments
Many animal body-parts, such as
- xanthophores (yellow): contain yellow pigments in the forms of carotenoids
- erythrophores (red): contain reddish pigments such as carotenoids and pteridine
- melanophores (black/brown): contain black and brown pigments such as the melanins
- cyanophores(blue): limited taxonomic range but found in some fish and amphibians
Structural colours
Animals can also appear coloured due to
- leucophores (reflective white): found in some fish, utilize crystalline purines(often guanine) to produce a reflective, shiny, white colour.
Plant colouration
The primary function of pigments in plants is photosynthesis, which uses the green pigment chlorophyll along with several red and yellow pigments including porphyrins, carotenoids, anthocyanins and betalains.
Definition
Definitions of albinism vary and are inconsistent. While they are clear and precise for humans and other mammals, this is because the majority of mammals have only one pigment, melanin. Many animals have pigments other than melanin, and some also have structural colours. Some definitions of albinism, whilst taking most taxa into account, ignore others. So, "a person or animal with very pale skin, white hair or fur, and pink eyes caused by a medical condition that they were born with"[13] and "a person or animal with white skin and hair and pink eyes"[14] do not include feathers, scales or cuticles of birds, fish and invertebrates, nor do they include plants. Some definitions are too broad to be of much use, e.g. "an animal or plant with a marked deficiency in pigmentation".[15]
Other definitions of albinism encompass most of the major animal taxa, but ignore the several other pigments that non-mammalian animals have and also structural colouration. For example, "Absence of the pigment melanin in the eyes, skin, hair, scales, or feathers."[16] refers only to the pigment melanin.
Because of the various uses of different terms applied to colouration, some authors have indicated that the colour of the eyes is the defining characteristic of albinism, e.g. "This leads to a good diagnostic feature with which to distinguish
The term "partial albino" is sometimes used in the literature. However, it has been stated that "A common misnomer is 'partial albino' – this is not possible since albinism affects the whole plumage of a bird, not just part"[17] and the definition of albinism precludes the possibility of "partial albinism" in which a mostly white bird shows some form of melanin pigmentation. "It is simply impossible, just like being 'partially pregnant' ".[18] Conditions that are commonly termed "partial albino" include neural crest disorders such as piebaldism, Waardenburg syndrome, or other depigmentation conditions such as vitiligo. These conditions result from fundamentally different causes to the seven types of oculocutaneous albinism that have been identified in humans (and confirmed in some other animals) and the use of the term "partial albino" is therefore misleading.[6]
One definition states that "albinism, (from the Latin albus, meaning "white"), hereditary condition characterized by the absence of pigment in the eyes, skin, hair, scales, or feathers",[19] however, this does not encompass invertebrates, nor does it include plants. Furthermore, it could be interpreted that "...absence of pigment..." does not include an absence of structural colours.
The lack of clarity about the term is furthered when the name of an animal includes the term "albino" although the animals (clearly) do not have the condition. For example, the albino gaur has this name because it is ash-grey whereas other gaur are almost black.
A clear definition appears to be – "Congenital absence of any pigmentation or colouration in a person, animal, or plant, resulting in white hair and pink eyes in mammals."[2] Whilst this does not state specifically that non-mammalian albino animals (or plants) are white, this can be inferred from "...absence of any pigmentation or colouration..." Due to the varied use and interpretation of the term "albino", written reports of albinistic organisms can often not be verified.
Mechanism and frequency
Melanin is an organic pigment that produces most of the colour seen in mammals. Depending on how it is created, melanin comes in two colour ranges,
Most forms of albinism follow a recessive pattern of inheritance.[21] However, this is not always the case. In palomino horses, genes coding for coat whiteness are dominant, and this is also true for several arctic mammals who possess dominant white colors which are pseudo-albinistic. However, these mammals differ from truly recessive albinos in that they still produce tyrosinase, and have normal eye pigmentation.[22] In Japan, research has identified a dominant albinism in the rainbow trout.[23][24]
Albinism occurs throughout the animal kingdom. The condition is most commonly seen in birds, reptiles and amphibians, but more rarely seen in mammals and other taxa. It is often difficult to explain occasional occurrences, especially when only one documented incidence has occurred, such as only one albino gorilla and one albino koala.[25] In mammals, albinism occurs once in every 10,000 births, but in birds, the rate is once in every 1,764 births.[26]
Some species, such as white peacocks, swans and geese, are not believed to be true albinos, as they do not have red eyes, rather, their colouration is suggested to be the expression of a white fur or feather gene, not a lack of melanin.[27]
Consequences
Melanin has several functions in most mammals and other animals; these are disrupted by albinism.
Abnormal eye development and appearance
Melanin functions in the normal development of various parts of the eye, including the iris, retina, eye muscles, and optic nerve. The absence of melanin results in abnormal development of eyes and leads to problems with focusing, and depth perception. The eyes of albino animals appear red because the colour of the
In all albino mammals studied, the centre of the retina is under-developed and there is a deficit of rod cells, which detect light levels; the central ganglion cell density is approximately 25% below normal (except for the eastern gray squirrel). In nearly all mammals, the overwhelming majority of photoreceptors are rods rather than cones, which detect color. Albinism specifically affects the rod cells, but the number and distribution of the cones is unaffected. In contrast, the retinas of birds are cone rich, meaning that the vision of albino birds is affected less than albino mammals.[29]
Reduced protection from sunlight in albino creatures
Melanin protects the skin from
Survival value
Animals with albinism may lack protective camouflage in some environments, and are therefore less able to conceal themselves from their predators or prey. However, in other environments albino animals are less likely to be killed, and selection may favor them.[30] The lower survival rate of animals with albinism in certain environments has been documented,[31][32] however, it has been stated that in studies where animals had many places to hide, predators captured albino and normally coloured animals at the same rate.[20] Furthermore, in certain species, albino animals may be excluded from families or other groups,[26] or rejected as mates.[20] On the other hand, there is also evidence for positive selection for albinism in some animals,[33] as well as cultural selection favoring albino people in some human societies, which results in the increase of genes associated with albinism.[34] The founder effect may explain why some animal populations become selected for albinism.[35]
The novelty of albino animals has occasionally led to their protection by groups such as the Albino Squirrel Preservation Society. They have also been protected in studies on their ecology, sociology and behaviour.[36]
Reduced viability
Studies on
Hearing disorders
Pigmentation disorders such as albinism are occasionally associated with hearing impairments in mice, rats, guinea pigs and cats.[38]
In mammals
Artificially selected
Intentionally bred albinistic strains of some animal species are commonly used as
Many individual albino mammals are in captivity and were caught while young. However, it is worth noting that the survival prospects of these creatures into adulthood outside of captivity are uncertain. [citation needed] It is doubtful whether these individuals would have survived to become adults without the protection and care they receive in captivity.[41]
Naturally occurring
It has been claimed that "Squirrels are the only known albino mammal to survive successfully in the wild".
A 2012 survey of the literature reported that in India, there were several records of albino mammals including the
Albino macaques have been reported in several occasions including a toque macaque (M. sinica), rhesus macaque (M. mulatta), and bonnet macaque (M. radiata).[42]
Albinism was observed in
A study on albinistic prairie voles (Microtus ochrogaster) found that albinism in this species conferred an advantage for the males compared to the wild-type; albino males had higher mount frequencies than wild-type males. In addition, the albinos had greater differential fertilizing capacity.[44]
Albinism can also occur in marsupials and monotremes, such as echidnas, kangaroos, koalas, possums, wallabies and wombats.
Marine mammals
The costs of albinism for marine mammals may include reduced heat absorption in colder waters, poor camouflage from predators, increased sensitivity to sunlight, and impaired visual communication. Despite the costs, some individuals do reach adult age and breeding status.[45]
Albino dolphins were first sighted in the Gulf of Mexico in 1962. Since 1994, three further individuals have been seen. These tend to be pink in colour due to blood vessels showing through the blubber and unpigmented skin.
A report published in 2008 stated that in marine mammals, "anomalously white" individuals have been reported for 21 cetacean species and seven
Whales and dolphins also may appear white if extensively scarred, or covered with a fungus, such as
Famous albino mammals
Famous albino mammals include
In birds
The most important pigments that determine plumage colouration in birds are melanines and carotenoids. The latter are ingested in food and transformed into colour pigments by enzymes. Aberrations in this pigmentation are mostly caused by food deficiencies and usually do not have a genetic basis. Well-known examples are
An albino bird has a white beak, white plumage, non-coloured skin, white talons and pink or red eyes. Albinism is only seen in about 1 of every 1,800 birds. The two most common species of albino birds are the
In one study, albinism in birds has been categorised according to the extent of pigment absence.[49]
- Total albinism – a simultaneous complete absence of melanin from the eyes, skin, and feathers. This is the rarest form. Only 7% of 1,847 cases of avian albinism examined was this type.
- Incomplete albinism – when melanin is not simultaneously absent from the eyes, skin and feathers.
- Imperfect albinism – when melanin is reduced in the eyes, skin and feathers.
- Partial albinism – when albinism is localized to certain areas of the body.
However, it has been argued that the definition of albinism precludes the possibility of "partial albinism" in which a mostly white bird shows some form of melanin pigmentation. "It is simply impossible, just like being 'partially pregnant'.[18]
In fish
As with other animals, it has been stated that for fish to be properly described as "albino", they must have a white body and pink or red eyes.[37]
Artificially selected
Zebrafish have three types of chromatophores—iridophores, melanophores, and xanthophores—which produce silver, black, and yellow pigmentation respectively. Zebrafish that lack iridophores are known as roy mutants, those that lack melanophores as albino mutants, and those which lack both melanophores and iridophores are ruby mutants. The gross eye morphology, feeding and swimming behaviours between wild-type and albino zebrafish were indistinguishable, except under dim or bright light or low contrast.[50]
In mammals, albinism is occasionally associated with hearing impairments. However, when tested, there was no differences in responses between wild-type and albinistic European wels catfish (
Naturally occurring
There are several reports of total albinism in both freshwater and marine fish, however, frequently captured albino fish are only reported in aquarium magazines and local newspapers.[51]
The incidence of albinism can be artificially increased in fish by exposing the eggs to heavy metals (e.g. arsenic, cadmium, copper, mercury, selenium, zinc).[52]
In the wild, albinism is reasonably common in the
In Actinopterygii
Albino and normally pigmented channel catfish (Ictalurus punctatus) differ in their characteristics. Normal individuals of this species are superior to albinos in body weight and total length. Albinos crossed with other albinos require 11 days longer to spawn and produce smaller egg masses. These masses contain eggs of lighter weight with poorer hatchability than crosses of normal fish. The albino fish have lower survival rates than normal fish but dress-out percentages are nearly equal.[54]
Some wild
In Chondrichthyes
In the class
Albinistic individuals of the following shark species have been reported:[57][58][59]
- Basking shark (Cetorhinus maximus) – two cases
- Broadnose sevengill shark (Notorynchus cepedianus)
- Great white shark (Carcharodon carcharias)
- Grey smooth-hound(Mustelus californicus) – two cases
- Japanese topeshark (Hemitriakis japanica)
- Japanese wobbegong (Orectolobus japonicus)
- Java shark(Carcharhinus amboinensis)
- Leopard shark (Triakis semifasciata)
- Narrownose smooth-hound (Mustelus schmitti)
- Pigeye shark (Carcharhinus amboinensis)
- Porbeagle (Lamna nasus)
- Scalloped hammerhead shark(Sphyrna lewini)
- Spiny dogfish (Squalus acanthias)
- Tawny nurse shark (Nebrius ferigineus)
- Tiger shark (Galeocerdo cuvier) – embryo
- Whale shark (Rhincodon typus)
- Whitespotted bamboo shark (Chiloscyllium plagiosum)
- Zebra shark (Stegostoma fasciatum)
A study published in 2006 reported albinistic individuals of the following ray species:[58]
- Bat ray (Myliobatis californica)
- Common skate (Raja batis)
- Common stingray (Dasyatis pastinaca)
- Common torpedo (Torpedo torpedo)
- Cownose ray (Rhinoptera bonasus)
- Giant electric ray(Narcine entemedor)
- Giant oceanic manta ray (Manta birostris)
- Ocellate spot skate (Okamejei kenojei)
- Southern stingray (Dasyatis americana)
- Thornback ray (Raja clavata)
- Cuckoo ray(Raja naevus)
In Hyperoartia
There are several reports of albino lampreys and it has been estimated that this occurs at a frequency of one in 100,000 normal individuals.[37]
In Sarcopterygii
Albinism in the
In reptiles
Many reptiles labeled as albino are, in fact, not completely lacking in all colour pigments. They are actually amelanistic, not albino. Reptiles often possess at least two pigments. Among the most common are xanthin (yellow) and erythrin (red). An amelanistic reptile therefore, may still have pale yellow, orange, or red pigmentation.
The
In snakes, partial absence of pigment is more common than absolute albinism. For snakes that are usually patterned in colours, they appear as a faint blue, peach or yellowish. In these cases, there has been a genetic mutation in the melanin and pigment delivery. The appearance comes from the inability for full colours to be present, such as black, red, brown and others. The eyes of an albino snake are typically red or pink. Albino snakes can remain in the sunlight for several hours with minimal harm. Corn snakes and snakes of larger types, such as a boa or diamondback snakes, are the most commonly affected by albinism often appearing to be a pinkish or yellowish colour.[25]
Albino tortoises and turtles are uncommon;
In 2012, an albino
There are three known "albino" strains of leopard gecko, however, breeders state that albinos are generally recognized by their lack of black pigment and having red eyes is not a requirement to be considered an albino. These three strains are called "tremper albinos", "rainwater albinos" and "bell albinos".[67]
In amphibians
As with reptiles, many amphibians labeled as albino are, in fact, not completely lacking in all colour pigments. They are actually amelanistic, not albino. Amphibians have six types of chromatophore in their skin, i.e. melanophores, xanthophores, erythrophores, leucophores, cyanophores and iridophores.[68] An amelanistic amphibian therefore, may still have various pigmentation.
The incidence of albinism in frogs, salamanders, and newts is relatively higher than other taxa. It has been estimated that one in four hundred of these animals is albino. When albino tadpoles hatch, they are almost transparent. This may help camouflage them initially, however, after two weeks, when their hindlegs begin to emerge, they become milky white. A survey in 2001 found hundreds of tiny albino plains leopard frogs, but when the researcher returned a few months later, not a single albino adult could be found.[39]
In European Salamandridae, albinism has been recorded in the fire salamander (Salamandra salamandra), gold-striped salamander (Chioglossa lusitanica), Italian crested newt (Triturus carnifex), marbled newt (Triturus marmoratus), Iberian ribbed newt (Pleurodeles waltl), alpine newt (Ichthyosaura alpestris) and two sub-species of the smooth newt (Lissotriton vulgaris vulgaris and Lissotriton vulgaris meridionalis).[68]
Genetics
Genetic studies of albinism in amphibians have focused on mutations in the tyrosinase gene. The albino phenotype of the leopard frog (Rana pipiens) has been attributed to a failure in post-translational control in a single recessive tyrosinase gene which still has some tyrosinase and DOPA oxidase activity. This is in contrast with mammals, some of which have mutations that show no tyrosinase or DOPA oxidase activity in albinos.[69] The albino phenotype of the pond frog (Pelophylax nigromaculatus) has been attributed to one of three mutations that created a dysfunctional tyrosinase. Two of those mutations involve an insertion of a thymine (T), a frameshift mutation, resulting in a truncated isoform of the TYR protein that is defective. The other mutation involves the deletion of a codon, three nucleotides that code for a lysine (Lys). In the wrinkled frog (Glandirana rugosa) and in the rice frog (Fejervarya kawamurai), a substitution from a guanine (G) to an adenine (A) creates a missense mutation, in which a glycine (Gly) changes to an aspartic acid (Asp) and an arginine (Arg), respectively. These changes in the polypeptide chain causes a dysfunctional tyrosinase.[70]
Albino axolotl (an amphibian) are also used widely in the laboratory as their transparent skin allows observation of the underlying tissues during limb regeneration.
In invertebrates
Albinism in
-
An exhibit showing albino and normal specimens of nine local species of marine molluscs, both gastropods and bivalves (The Bailey-Matthews National Shell Museum in Sanibel, Florida).
-
Normal (left) and albinistic (right) forms of thePseudofusulus varians,. Note that in the albino both the body and the shell are lacking the normal pigmentation.
-
Albino freshwater snailhaemoglobin. Without its normal pigment, the shell of this species is translucent.
In insects
The neurohormone [His7]-corazonin induces darkening of the cuticle of Locusta migratoria. The Okinawa strain of this species is deficient in [His7]-corazonin and is albino. One of the typical features of Locusta migratoria is that they are gregarious locusts. However, the albino strain shows more solitarious behaviour.[73]
The yellow mutation in fruit flies is a mutation causing a congenital lack of normal pigment; it is a similar phenomenon to albinism in other organisms.[74]
In echinoderms
The Japanese sea cucumber (Apostichopus japonicus) is an echinoderm that is caught in the wild or cultivated for food. Normal Japanese sea cucumbers start to develop pigmentation when they are about 1 cm long. The upperside becomes a dull, yellowish -brown to maroon and the underside a light brown. The body walls of adult, albino Japanese sea cucumbers contain only 0.24% melanin compared to 3.12% in normal adults. The difference in melanin content becomes visually apparent at 60 days of age. The epidermis is thinner in the albinos and contains fewer melanocytes. Albino individuals are similar to normal individuals in growth rate, digestion rate and fertility.[75]
Astaxanthin is the main carotenoid in marine crustaceans (and fish). It has been shown that adding astaxanthin to the feed can improve the skin and muscle colour of marine organisms and thereby increase their commercial and ornamental value.[75]
In arachnids
"Depigmented" arthropods have been found, usually in cave populations.[76] "Albino" individuals of normally red citrus red mites (Panonychus citri) occasionally appear in laboratory colonies, however, these still contain green and yellow pigments. This albinism does not affect mortality.[77]
Related pigment disorders
In some animals, albinism-like conditions may affect other pigments or pigment-production mechanisms:
- "Whiteface", a condition that affects some parrot species, is caused by a lack of psittacins.Audubon Aquarium of the Americas, New Orleans, Louisiana
- xanthophore metabolism is affected rather than synthesis of melanin, resulting in reduction or absence of red and yellow pteridine pigments.[79]
- Leucism differs from albinism in that the melanin is, at least, partially absent but the eyes retain their usual colour. Some leucistic animals are white or pale because of chromatophore (pigment cell) defects, and do not lack melanin.
- Melanism is the direct opposite of albinism. An unusually high level of melanin pigmentation (and sometimes absence of other types of pigment in species that have more than one) results in an appearance darker than non-melanistic specimens from the same gene pool.[80]
In plants
In plants, albinism is characterised by partial or complete loss of chlorophyll pigments and incomplete differentiation of chloroplast membranes. Albinism in plants interferes with photosynthesis, which can reduce survivability.[9] Some plant variations may have white flowers or other parts. However, these plants are not totally devoid of chlorophyll. Terms associated with this phenomenon are "hypochromia" and "albiflora".[10]
Plants that are pale simply from being in the dark are termed etiolated.
Albinism has frequently occurred in progeny of
Some herbicides (e.g. glyphosate and triazines) can cause partial chlorosis in plants, even several seasons or years after applicating.[85]
In human culture
The Albino Squirrel Preservation Society was founded at the
Albino animals are often kept as pets, for example,
See also
- Albinism in humans
- Albinism in chickens
- Albino and white squirrels
- Dyschromia
- Erythrism, unusually red pigmentation
- Isabellinism
- Leukoderma
- Mongolian spot
- Xanthochromism, unusually yellow pigmentation and lack of yellow pigment
- Leucism
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Some dominant chromogenic mutations are known and in fact there are more than predicted by the Haldane-Fisher theory of evolution of wild type dominance (Searle, 1968, p. 250)"..."Of 32 dominant mutations known in the mouse as many as nine are chromogenic (Hadorn, 1961, p. 132). This is a very high percentage of the 40 coal-colour genes known among the mouse and implies that dominant mutations are rather characteristic of colour genes. The dilation gene of the palomino horse is dominant and there is a dominant white in many mammals so that dominance/recessivity is not necessarily positively correlated with excess/deficiency. the dominant white is not so deficient in pigment as the true albino and in particular as the normal eye-pigmentation, so that in Atz' lobster the pigmentation of this organ can be controlled independently of that integument. It would seem that the retention of essential pigment by the dominant white has been naturally selected and that presumably its whole phenotype has been selected. Arctic white races and species no doubt have this type of allele. True recessive albinos lack tyrosinase (Onslow, 1917; Foster, 1952; Barnicott, 1957; Fitzpatrick et al., 1958) or have the enzyme but no free tyrosine (Verne, 1926, p. 535; Fox and Vevers, 1960, p. 37) but in the dominant white the enzyme is present in the skin, under inhibition.
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