Thread: pigments
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Old 10-26-02, 02:30 AM   #4 (permalink)
Bryan Self
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Join Date: Aug-2002
Location: So Cal
Age: 48
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Here is a artice of intrest. I will do it in two parts.

Toledo Herpetological Society Newsletter

Volume 10, Issue 12 December 1999

Color Anomalies in Snakes

by Myia Sindelar

Why are snakes called albino when they have obvious coloration? How does a pairing of two albino snakes produce normal offspring? What is the difference between leucistic and albino? The answers to these questions involve basic genetics and knowledge about skin pigment cells and the role they play in determining phenotype (outward appearance). While I in no way claim to know all about this subject, I have recently had the opportunity to learn some of the basics behind skin color aberrations in snakes (Actually Eileen forced (ha-ha) me to write a paper for her developmental biology class on the subject). The following paragraphs represent my feeble attempt to explain what I have learned.
Before I get into pigmentation, I thought it might be helpful to do a little background on the skin, where it comes from and what its function is. The simplest definition of skin is that it is a protective barrier between delicate body organs and the environment. The skin also prevents dehydration and protects the snake from toxic substances. Two other functions of the skin rely on the presence of melanin, they are protection from ultraviolet radiation and thermoregulation. The skin of a snake (or any vertebrate for that matter) is composed of two basic layers called the epidermis, the outermost layer, and the dermis, the underlying layer. The origins of these layers can be seen in embryonic development when cells differentiate into ectoderm, mesoderm and endoderm. The ectoderm will give rise to the epidermis along with skin glands and the nervous system. The dermis on the other hand will be formed from mesoderm along with muscles, bones, and the circulatory system. Lastly, endoderm gives rise to several organs and the lining of the respiratory and digestive systems.
Now for the interesting stuff. Skin color is determined by skin pigment cells called chromatophores. Chromatophores arise from the neural crest, a part of the embryonic ectoderm. Early in development these cells migrate to the skin where they may differentiate into one of three chromatophore types. These three types are melanophores, xanthophores and iridophores (this is where I consider reptiles lucky because warm-blooded creatures have only one type of chromatophore, the melanocyte).
Melanophores are cells responsible for synthesizing black and brown pigments. A process called melanosynthesis is essential to the production of melanin. Melanosynthesis involves the conversion of the amino acid tyrosine to melanin which is a somewhat complicated process that has two crucial steps. In step one tyrosine is converted to dihydroxyphenylalinine (dopa); step two converts dopa to dopaquinone. The key to these two steps is that it cannot occur without the enzyme tyrosinase. If tyrosinase is absent, tyrosine cannot be converted to melanin.
Xanthophores are pigment cells responsible for yellow, red, or a mixture of the two colors. The exact color of a xanthophore is determined by the number and combination of the yellow and red pigments in the cell.
Iridophores do not synthesize pigments, they do however produce color based on their physical properties. These cells house organelles called reflecting platelets that reflect and scatter light. The colors that can result from these cells are greens, blues, reds and browns.
These three cell types are situated in the epidermis and it seems that their location is predetermined by genes, although the exact mechanism is not fully understood. The density, distribution, quantity and quality of the pigment cells interact with each other to produce the colors and patterns we see. Pattern is more attributed to the melanophores and xanthophores and color quality is a result of the iridophores.
Variations from normal color and pattern occur for a number of reasons. The most common aberrancies are albinism, axanthism, leucism, piebaldism, and melanism. This is where the terminology can be confusing and misleading. To be perfectly honest, I am still trying to differentiate them in my mind as I write this. The best way to discuss these anomalies is to treat each one separately.
Albino as defined by Bechtel (1995) is a congenital (occurring from or before birth) decrease or absence of melanin in the skin, mucosa, and eyes. This is usually a result from an inherited defect in melanin formation. Mutations at various loci involving pigmentation can cause albinism or in simpler terms, more than one type of defect can result in an albino organism. The two most common forms of albinism are tyrosinase-negative albinism and tyrosinase-positive albinism. In tyrosinase-negative albinism, the organism is unable to synthesize tyrosinase, the enzyme necessary to convert tyrosine to melanin. This is the result when a homozygous recessive pairing of the gene controlling synthesis of tyrosinase occurs. Tyrosinase-positive individuals have the ability to synthesize tyrosinase but are unable to produce melanin for either or both of two reasons. The first reason is that although tyrosinase is present, tyrosine is not transmitted to the melanophore for conversion to melanin. The other reason is that tyrosinase inhibitors could prevent synthesis of melanin. Tyrosinase-negative and tyrosinase-positive albinos look the same, that is they have the same phenotype. Keep in mind that their genotypes are different. This is the reason why a cross between two albino specimens can result in all or some normal offspring. The following examples may be an easy way to understand this concept:
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