Leucism

Leucism is a condition characterized by reduced pigmentation in animals.

Leucism is a general term for the phenotype resulting from defects in pigment cell differentiation and/or migration from the neural crest to skin, hair or feathers during development. This results in either the entire surface (if all pigment cells fail to develop) or patches of body surface (if only a subset are defective) having a lack of cells capable of making pigment.

Since all pigment cell-types differentiate from the same multipotent precursor cell-type, leucism can cause the reduction in all types of pigment. This is in contrast to albinism, for which leucism is often mistaken for. Albinism results in the reduction of melanin production only, though the melanocyte (or melanophore) is still present. Thus in species that have other pigment cell-types, for example xanthophores, albinos will not be entirely white instead displaying a pale yellow colour.

More common that a complete absence of pigment cells is localized or incomplete hypopigmentism, resulting in irregular patches of white on an animal that otherwise has normal colouring and patterning. This partial leucism is known as a "pied" or "piebald" effect; and the ratio of white to normal-coloured skin can vary considerably not only between generations, but between different offspring from the same parents, and even between members of the same litter. This is notable in horses, the urban crow^[1] and the ball python^[2] but is also found in many other species. In contrast, albinism always affects the entire animal.

A further difference between albinism and leucism is in eye colour. Due to the lack of melanin production in the retinal pigmented epithelium (RPE), albinos typically have red eyes due to the underlying blood vessels showing through. In contrast, leucistic animals have normally coloured eyes. This is because the melanocytes of the RPE are not derived from the neural crest, instead an outpouching of the neural tube generates the optic cup which, in turn, forms the retina. As these cells are from an independent developmental origin, they are typically unaffected by the genetic cause of leucism.

Genes that, when mutated, can cause leucism include, c-kit [3], mitf [4] and ednrb [5]

Gallery

• Leucistic Alligator (Alligator mississippiensis) at Audubon Zoo
  Leucistic Alligator (Alligator mississippiensis) at Audubon Zoo
• Leucistic Alligator (Alligator mississippiensis) at Audubon Zoo
  Leucistic Alligator (Alligator mississippiensis) at Audubon Zoo
• Leucistic Leopard Gecko (Eublepharis macularius)
  Leucistic Leopard Gecko (Eublepharis macularius)
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  Leucistic Texas Rat Snake (Elaphe obsoleta)
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  Leucistic Common Grackle (Quiscalus quiscula)