“Albinism”的意思、由来-开放百科全书

Albinism is the "congenital absence of any pigmentation or coloration in a person, animal or plant, resulting in white hair, feathers, scales and skin and pink eyes in mammals, birds, reptiles, amphibians and fish and other small invertebrates as well."^[1] 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 and their lack of camouflage to avoid predators.^[2] Albino animals have characteristic pink or red eyes because the lack of pigment in the iris allows the blood vessels of the retina to be visible. Familiar albino animals include in-bred strains of laboratory animals (rats, mice, guinea-pigs and rabbits), but populations of naturally occurring albino animals exist in the wild, e.g. Mexican cave tetra. Albinism is a well-recognized phenomenon in molluscs, both in the shell and in the soft parts. It has been claimed by some, e.g.^[3] that "albinism" can occur for a number of reasons aside from inheritance, including genetic mutations, diet, living conditions, age, disease, or injury. However, this is contrary to definitions where the condition is inherited.

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.^[4] 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".^[5]

Biological colouration

Biological pigments are substances produced by living organisms that have a colour resulting from selective colour absorption. What we perceive as "colour" are the wavelengths of light that are not absorbed by the pigment, but instead are reflected. Biological pigments include plant pigments and flower pigments.

Animal colouration

Animals can appear colored due to two mechanisms, pigments and structural colors. Animals may have both biological pigments and structural colors, for example, some butterflies with white wings.

Pigments

Many animal body-parts, such as skin, eyes, feathers, fur, hair, scales and cuticles, contain pigments in specialized cells called chromatophores. These cells are found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, have a class of cells called melanocytes for colouration. The term chromatophore can also refer to coloured, membrane-associated vesicles found in some forms of photosynthetic bacteria. Chromatophores are largely responsible for generating skin and eye colour in poikilothermic animals and are generated in the neural crest during embryonic development. Mature chromatophores are grouped into subclasses based on their colour under white light:

Structural colours

Animals can also appear coloured due to structural colour, the result of coherent scattering^[6] perceived as iridescence. The structures themselves are colourless. Light typically passes through multiple layers and is reflected more than once. The multiple reflections compound one another and intensify the colours. Structural colour differs according to the observer's position whereas pigments appear the same regardless of the angle-of-view. Animals that show iridescence include mother of pearl seashells, fish, and peacocks. These are just a few examples of animals with this quality, but it is most pronounced in the butterfly family.

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. Whilst 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"^[8] and "a person or animal with white skin and hair and pink eyes"^[9] 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"^[10]

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."^[11] 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 leucistic and albino individuals – the colour of the eye."^[12]

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"^[12] 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’ ".^[14]

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",^[13] 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, and the American albino horse is a chestnut with a dilution gene.

A clear definition appears to be – "Congenital absence of any pigmentation or coloration in a person, animal, or plant, resulting in white hair and pink eyes in mammals."^[1] 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 coloration..." 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, eumelanin (producing dark browns and blacks) and pheomelanin (producing light reddish tans and blondes). The dark and light melanins have their influence either alone or in conjunction, making either plain or multi-coloured coats. Sometimes, in a condition called agouti, they make multi-coloured individual hairs. The production of melanin occurs in melanocytes in a complex process involving the enzyme tyrosinase. Mammals have a gene that codes for the presence of tyrosinase in cells – called the TYR gene. If this gene is altered or damaged, melanin cannot be reliably produced and the mammal becomes an albino. Besides the TYR gene, several other genes can cause albinism. This is because other hormones and proteins are involved in melanin production, the presence of which is genetically determined. In mice, a total of 100 genes are known to affect albinism.^[14]

All the genetic traits for albinism are recessive traits. This means that their influence is hidden when paired with stronger traits. For the recessive albino trait to be expressed in a mammal, the offspring must inherit a recessive gene from both parents.^[14]

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.^[19] In mammals, albinism occurs once in every 10,000 births, but in birds, the rate is once in every 1,764 births.^[20]

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.^[15]

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 red blood cells in the retina can be seen through the iris, which has no pigment to obscure this. Some albino animals may have pale-blue eyes due to other colour generating processes. Albino vertebrates exposed to intense light typically lose photoreceptors due to apoptosis.^[16]

In all albino mammals studied, the centre of the retina is under-developed and there is a deficit of rod cells; the central ganglion cell density is approximately 25% below normal (except for the gray squirrel). In nearly all mammals, the overwhelming majority of photoreceptors are rods rather than cones. Albinism specifically affects the rod cells, but the number and distribution of the cones is unaffected. In contrast, the retina of birds is cone rich meaning that the vision of albino birds is affected less than albino mammals.^[23]

Reduced protection from sunlight

Melanin protects the skin from ultra-violet radiation in sunlight. Melanosomes block harmful electromagnetic radiation from the sun while allowing beneficial frequencies to enter the body.

Survival disadvantages

Many animals with albinism lack their protective camouflage and are therefore less able to conceal themselves from their predators or prey: The survival rate of animals with albinism in the wild can be low,^[17]^[18] 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.^[14] Furthermore, albino animals may be excluded from families or other groups,^[19] or rejected as mates.^[14]

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 and behaviour.^[20]

Reduced viability

Studies on medaka fish in the laboratory, i.e. with no predators, sufficient food supply, controlled temperatures, etc., found that albinos had considerably reduced viability; from 800 albino embryos, only 29 survived to full adulthood.^[30] Early studies on fish led some researchers to describe albinism as a "semi-lethal mutation".^[30]

Hearing disorders

Pigmentation disorders such as albinism are occasionally associated with hearing impairments in mice, rats, guinea pigs and cats.^[21]

In mammals

Artificially selected

Intentionally bred albinistic strains of some animal species are commonly used as model organisms in biomedical research and also as pets. Examples include the BALB/c mouse and Wistar and Sprague Dawley rat strains, laboratory rabbits, guinea-pigs and ferrets. Albino axolotl (an amphibian) are also used widely in the laboratory as their transparent skin allows observation of the underlying tissues during limb regeneration.^[33] Some researchers have argued that albino animals are not always the best choice for scientific studies due to the consequences of albinism (e.g. hearing and visual impairments).^[22]

Many individual albino mammals are in captivity and were caught as youngsters. It is doubtful whether these individuals would have survived to become adults without the protection and care they receive in captivity.

Naturally occurring

It has been claimed that "Squirrels are the only known albino mammal to survive successfully in the wild".^[23] In all albino mammals studied, the eye color is green, blue or red due to absence of eumelanin in the iris, and the centre of the retina is under-developed with a deficit of rod cells; the central ganglion cell density is approximately 25% below normal. In nearly all mammals, the overwhelming majority of photoreceptors are rods rather than cones. Albinism specifically affects the rod cells, but the number and distribution of the cones is unaffected. The retina of the squirrel (Sciurus carolinensis) is unusual for mammals as it is rich in cones. Central cell densities are less than 5% lower in albino squirrels than in pigmented individuals. This relatively minor disruption to vision is thought to assist in the survivability of albino squirrels in the wild. This is supported by observations that the behaviour of albinos in the wild, e.g. leaping from branch to branch, is similar to pigmented squirrels.^[23]

A 2012 survey of the literature reported that in India, there were several records of albino mammals including the tiger, lesser rat-tailed bat, chital, common palm civet, Northern palm squirrel five-striped palm squirrel and wild boar.^[24]

Albino macaques have been reported in several occasions including a toque macaque (M. sinica), rhesus macaque (M. mulatta), and bonnet macaque (M. radiata).^[24]

Albinism was observed in jungle cats (Felis chaus) and jackals (Canis aureus) along the coastline of the southern Western Ghats (Kerala and Kanyakumari coast, India). Albinism was observed in jungle cats from the Amaravila area of Trivandrum district in the Kerala State. Albinism in jackals was observed from the Polooni area in Malappuram district and Chaliyam area of Calicut district (Kerala). As albinism is observed in those areas where the density of these mammals is comparatively low, it is concluded that continuous inbreeding could be the reason for expression of albinism.^[25]

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.^[26]

Albinism can also occur in marsupials and monotremes as well such as echidnas, kangaroos, koalas, possums, wallabies and wombats as well.

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.^[41]

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 color 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 7 pinniped species but there were no known reports of anomalously white sea otters (Enhydra lutris) or sirenians.^[27]

Whales and dolphins also may appear white if extensively scarred, or covered with a fungus, such as Lobo's disease (also known as "lobomycosis").^[27]

Famous albino mammals

Famous albino mammals include "Migaloo", a humpback whale living off the coast of Australia; Pinky, a bottlenose dolphin living in and around in Calcasieu Lake, Louisiana; "Carolina Snowball", a popular albino bottlenose dolphin displayed at the Miami Seaquarium in the early 1960s; "Snowflake", a Barcelona Zoo gorilla, and "Mahpiya Ska", (Sioux for "White Cloud"), a buffalo in Jamestown, North Dakota.^[28]^[29] The inspiration for Herman Melville's novel Moby-Dick was a sperm whale known as Mocha Dick.

In birds

The most important pigments that determine plumage coloration 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 flamingos, which owe their distinct pink colour to the presence of red carotenoids in their natural food. When these carotenoids are in short supply, these birds appear white after the next moult. Mutations causing changes in carotenoid-based colour pigments are rare; melanine mutations occur much more frequently. Two types of melanin, eumelanin and phaeomelanin, are present in birds. In the skin and eyes, only eumelanin is present. In some bird species, the colour is completely caused by eumelanin, however, both types of melanin are found in most species. In birds, albinism has been defined as "a total lack of both melanins in feathers, eyes and skin as a result of an inherited absence of tyrosinase",^[30] however, this ignores the effects of other pigments and structural colours.

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 common house martin and the American robin.^[19] Famous albino birds include "Snowdrop", a Bristol Zoo penguin.^[31]

In one study, albinism in birds has been categorised according to the extent of pigment absence.^[32] –

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’.^[30]

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.^[33]

Artificially selected

Several types of aquarium fish are have "albino" in their names, e.g. albino driftwood catfish, albino festivum, albino red tailed shark, and albino tiger shark, however, photographs of these four fish indicate that the latter three have dark eyes and/or pigmented fins.^[34]

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 (Silurus glanis) and South American bronze catfish (Corydoras aeneus). Similarly, Mexican blind cave fish (Astyanax mexicanus) do not differ in hearing sensitivity from the normally pigmented and eyed surface-dwelling populations. Fish lack melanin in the inner ear, meaning that hearing in fishes is less likely to be affected by albinism than in mammals.^[21]

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.^[36]

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).^[37]

In the wild, albinism is reasonably common in the teleosts, especially the Pleuronectiformes (flatfish), however, it is rarely reported in the elasmobranchs.^[38] Albinism has been reported in hagfish, lampreys, sharks, rays and numerous teleost fishes, e.g. catfishes, grunts or cyprinids.^[21]

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.^[39]

Some wild cave fish have populations that are albinistic. The Mexican cave tetra is a species that has evolved specialized characteristics in a series of independent caves. One of these is albinism linked to the Oca2 gene, a known pigmentation gene, This trait has evolved independently in at least two caves.^[40]

In Chondrichthyes

In the class chondrichthyes, several species of naturally occurring albino rays and sharks have been recorded. Furthermore, an albino individual spotted ratfish (Hydrolagus colliei) from the order Chimaeriformes has been reported.^[41]

Albinistic individuals of the following shark species have been reported:^[42]^[43]

A study published in 2006 reported albinistic individuals of the following ray species:^[43]

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.^[33]

In Sarcopterygii

Albinism in the African lungfish has been reported on at least two occasions.^[44]

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 etherin (red). An amelanistic reptile therefore, may still have pale yellow, orange, or red pigmentation.

The California Academy of Science, in the Steinhart Aquarium, as of 2015, has on display an albino American alligator named "Claude". The alligator is partially blind because of lack of pigment in its eyes.^[45] The albino alligator hatched from the egg in 1995 in Florida, and was brought to the Academy in 2008. This alligator would not have survived in the wild because its whiteness would have made it too easy a prey object. The only known albino alligators are in captivity.^[46] While extremely rare, white-colored crocodiles and alligators do exist in other places. However, most of these animals are leucistic given that they have a general loss of pigmentation with some color tinges remaining on their bodies although looking at first like other albino creatures, thus creating the misconception that the reptiles are albino themselves when they are not. Four such alligators are kept at the Gatorland theme park in the U.S. state of Florida.^[47] In Australia, a crocodile believed to be "part-albino" and nicknamed by people in the area as "Michael Jackson", attacked and killed a man.^[48]

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.^[49]

Albino tortoises and turtles are uncommon; Sulcata tortoises are the most likely type of turtle to express albinism. The shells have an almost yellow colouration and they have pink eyes. For turtles, a pure white colour is nearly impossible, even with albinism. Albino turtles can have a longer lifespan than many other albino animals; their hard shells help to prevent predation and other environmental challenges. Vision and sensory organs are slightly affected.^[49]

An albino anole has been reported and photographed.^[50] In 2007, it was reported that an albino stumpy-tail lizard (possibly a shingleback lizard), approximately 12 cm long and about one-year-old, had been found in Victoria, Australia.^[51] (Also see [https://web.archive.org/web/20150301025919/http://www.snakeranch.com.au/snake-profiles/bluetongued-lizard/])

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".^[52]

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, xantophores, erythrophores, leucophores, cyanophores and iridophores.^[76] 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 hind legs 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.^[53]

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).^[54]

In invertebrates

Albinism in molluscs has been recognized to be a hereditary phenomenon at least since 1900.^[55] Albinism in molluscs can exist to a variable degree. Sometimes an individual snail has a normally pigmented body, but the shell is completely without the normal pigmentation because of a defect in the cells of the mantle. Shells of certain mollusc species can be translucent when they lack the normal pigmentation.^[56]

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 ia that they are gregarious locusts. However, the albino strain shows more solitarious behaviour.^[57]

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.^[58]

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 upper side 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.^[59]

In arachnids

"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.^[61]

Related pigment disorders

In some animals, albinism-like conditions may affect other pigments or pigment-production mechanisms:

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.^[4] 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".^[5]

Albino redwoods are rare examples of an albino tree with white needles; despite its lack of chlorophyll it may grow to substantial size as a parasite, usually on the base of the (normal) redwood tree from which it first grew.^[65]^[93]^[66] Only about sixty examples of albino redwoods are known.^[67] Additionally, an even smaller number of "chimeric" redwood trees have both normal and white needles.

Albinism has frequently occurred in progeny of Black Tartarian, Bing and Hedelfingen varieties of sweet cherry.^[68]

In human culture

The Albino Squirrel Preservation Society was founded at the University of Texas at Austin in 2001. Members of the society at the University of North Texas petitioned for an election to name their albino squirrel as the university's secondary mascot. The University of Louisville in Kentucky also has a documented population of albino squirrels.^[69]

Albino animals are often kept as pets, for example, African clawed frog, guinea pigs and peacocks.^[69]

See also

References

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