Many questions have been asked of me as a herpetologist and veterinarian. One of these is the nature of leucism. First of all, it is NOT pronounced "loo-si-zm" saying that immediately identifies a person as poorly educated in scientific and medical terminology. In Classical Latin the C is always pronounced like K - the so called hard C sound. You do not call a neoplasm of blood cells "loo-see-mee-ah" it is pronounced "loo-kee-mee-ah." The rule is the same for the prefix leuc- or leuk- across the board. White blood cells are pronounced "loo- ko- site" not "loose- o- site" (which incidentally, is spelled leukocyte or leucocyte with the k form being more common, but both correct). It is "loo- ko-" in the words leucoencephalomalacia and all other words with the prefix. Arrogant as it sounds, in many medical circles the mispronunciation of basic words like that makes people think of you as poorly educated and without a firm grasp of scientific or medical language. In fact, one colleague of mine once heard another doctor say "loo-sis-tic" and said "did you hear that? Where did he get his doctorate? From an online college staffed by trailer trash?" Ok, I agree that is harsh, but similar (though more tactfully expressed sentiments ) are frequently found in the halls of academia. So mispronouncing words can make people dismiss you as a rube, so make an effort not to do it.
Where does this come from? Leuc- is the Latin form of the Greek Leukos. Thus, technically any work that is spelled with the leuc- prefix can be spelled with the Greek prefix instead and spelled leuk-. An example is the word leukocyte. The Greek is used, but it is acceptable (though more rare) to spell it with the Latin to form leucocyte. In the case of leucism the opposite has become true. The Latin form has become more widespread, but the Greek is equally valid. Thus, leukism is correct. In fact, I have increasingly begun to spell it with the Greek spelling because of the pronunciation issue.
Unfortunately, most people that pronounce it "loo-si-zm" are hobbyists that are poorly trained in medical terminology, if they are trained at all. Most know nothing about science beyond their high school biology and chemistry classes. It is very difficult to correct people that have formed an entire community which is equally badly educated. You fall into a form of peer pressure to be wrong. If you pronounce a word correctly when everybody else is pronouncing it wrong you are looked at as a jerk or a wierdo. Veterinarians and some herpetologists then adopt the incorrect pronunciation so they will not offend their clients. This is what scientists and medical professionals have to combat. Peer ignorance pressure is difficult to overcome. I can remember speaking to a group of hobbyists not long ago and someone asked me a question about leukism. I corrected their pronounciation very politely, but you should have seen the looks from the whole room. I said, "I'm sorry, do you mean leukism?" The person looked a little puzzled. I continued by saying "the condition is called leukism, it comes from the Greek leukos meaning white." The whole room smiled and looked rather odd. I asked several people afterward why they looked odd. They laughed and said "everybody says 'leusism'." When I pointed out that was not correct, they replied "maybe, but if you say it like you say it, people will think you are wierd."
For that reason (as I mentioned before) I still tend to write using the more common spelling with a C, but I have increasingly begun to spell it with a k when dealing with hobbyists.
Concepts to Consider
There is a great deal of bad information out there (hence the new term "wikipedian information"). If you have not run across that term, you will eventually. One of the greatest sources of misinformation is wikipedia. I have read the article on leucism there and there is a great deal of misunderstanding. One of the things that is not understood is the fact that the words leucism (or leukism) and albinism are essentially the same in their roots. They both mean white. The medical field is what delineated a difference between them. Those not in the medical field rarely use the distinction correctly. Just because something looks white does not mean it is either leukistic or albino. There are other genetic mutations out there that cause feathers or hair to be white which have to do with deposition of melanin or other pigments (carotenoids in many avian species for example) that have nothing to do with leukism or albinism. Many PhD's (and I am one as well as a DVM) have a very poor concept of what constitutes these conditions. So I will break them down in the most basic forms.
The first thing we must remember is the definitions are artificial. The term albino and leucistic actually literally mean the same thing - white. The artificial division between them began in the veterinary and herpetological communities, and rather recently too. In fact the word leucism has not made it (as of this writing) into most dictionaries. Among those that study chromatophore biology and pigment mutations there are a set of definitions for these words that are accepted as the standard.
1) ALBINISM- genetic mutations that alter the pigment cells of the skin and other tissues in such a way that the pigments themselves are not formed in their final, normal biological form. NOTE I said skin and other tissues. If the skin and rest of the body is not devoid of pigment, but the hair or feathers are white, that does not equate to albino. Also albinism is a derangement of pigment formation, not deposition. There are numerous forms of albinism. In humans, there are two pigments. Eumelanin is brown to black and pheomelanin is rusty or even orange or red. They travel along a similar cascade when being formed but differ in the amount of sulfur in the final melanin compound. Any disruption along the cascade can cause a form of albinism. Some albinos have red hair because they have a gene that is faulty for the formation of eumelanin, so they are really only eumelanistic albinos. Other pigments found in reptiles can also have faulty genes. The pteridines and drosopterins in the other cells (xanthophors and a subset of xanthophores called erythrophores) can cause other forms of albinism. Currently the iridophores (which use crystals and refraction to cause color instead of pigment) are not really known to be faulty in the same way since there are not pigments, so iridophoric albinism is something that simply does not occur.
2)LEUKISM (LEUCISM)- medically defined this is a defect in the skin, not the pigment cells. There are other derangements of pigment that can cause a whitening effect, but they are not classical leukism. Classical leukism is caused by a faulty gene, or set of genes, that causes the skin to be unable to support pigment cells. Experiments have been done that illustrate this. In one set of experiments normal pigment cells from a normal animal were placed in albino skin and the cells were normal and produced pigment. This demonstrated that the albino defect was in the pigment cells of the albino but not in the skin itself. The same experiment done in leukistic skin caused the normal pigment cells to die. Some have claimed that the reason eyes are pigmented in leukistic animals is because the pigment in the eye comes from another origin (the non-neural crest theory). This is really not the case. In fact some (unfortunately as yet unpublished research that really needs to get published) experiments were done transplanting RPE eye pigment cells into the skin and they died. Conclusion? Well nothing. The eye pigment cells can't survive out of the eye is all that proved. So melanophores from the iris were transplanted and they died in leukistic skin but survived in albino skin. Conclusion? The defect has to do with the skin, not the origin of the pigment cells. Further evidence of this can be found in numerous species that have melanin or other pigments present in other tissues such as the peritoneum but are typical of leukistic animals on the outside when alive.
However, some leukistic animals are also leukistic internally. What does this mean? At present it is unknown. It might reflect a subtype of leukism where there is agenesis, dysgenesis or complete necrosis embryologically of the chromatophores. This could represent another branch on the leukism scheme and might indicate a disorder we might call Complete Leukism. Where forms just limited to the skin might be termed Cutaneous Leukism. One thing is clear, the definition of leukism is only semi set. There is room for other forms, but it should be understood that there must be a standard definition defined in pathological terms.
So are there other forms of leukism? Possibly, but one must not confuse leukism with dysregulation of dysfunction of chromatophores. For example, if the chormatophore cannot produce pigments, but is otherwise functional, that is albinism. However, what about a mutation in a receptor that causes the pigment cell to be unable to receive signals (a MSH receptor for example) to produce pigment? That situation is more closely related to albinism since the pigment cells are present but not functioning, though they are dysfunctional from a different cause. Thus it is probably better call the condition something else in order to eliminate confusion. I personally refer to these potential disorders as receptor mediated chromatophoropathies (or chromatopathy) or RMC's. I first coined that term back in 2003, but have had no real case where this could be proven. Since many of the immunohistochemical markers for mammal receptors do not work in reptiles and leucistic or RMC mammals are much harder to come by, I have not been able to publish the term in the mainstream literature. But published or not, it is useful for this discussion. We can separate some of the confusion like this:
- Classic leukism is due to chromatophore necrosis, apoptosis, dysgenesis or agenesis - and is the the absence of recognizable chromatophore cells on histopathology.
- Receptor Mediated Chromatophoropathy (RMC) is a white state due to chromatophores not receiving signals or are receiving only low level signals to produce pigment due to a mutation in some receptor, signaling pathway or a defect in the production of melanophore stimulating hormone (MSH), but chromatophores are present in the skin on histopathology.
- Albinism is a defect of pigment production within the chromatophores without loss of chromatophores. Chromatophores are present in the skin, but are not able to produce pigment or fully formed pigment.
3)Any white animals with pigmented eyes are leukistic? NO. Particularly in those animals where their color or percieved color comes from keratin structures like hair or feathers. There are other mutations out there where the pigment cells are working but the (in the case of mammals for example) melanocytes are prevented from injecting their melanosomes into hair shafts. This causes white coats, but pigmented skin. Some white haired horses are an example of this. They may have black skin, but white hair. They are not leukistic. Other animals have this kind of situation too and it can arise as a mutation in a population. Birds may also have a condition like this where they are really normal as far a pigment production, but not in terms of deposition in the feathers.
4)Animals with patterns are leukistic, right? NO. Leukistic animals should be all white. There was a picture of a giraffe circulating about the internet a few years ago that was black and white. No brown. People started calling it leukistic. NO. It had black, so it was not leucistic. It may have had a pheomelanin defect or other mutation, but it was definitely not leukistic. On the wikipedia website under leucism there is (as of this writing) even a picture of several avians with black feathers but white feathers also. That is NOT leukism. Pattern mutations are something separate as is piebaldism. Piebaldism was believed to be a related condition to leukism, but it is often a progressive condition over time in animals, though may be static. Animals that are born with a pattern that is maintained over the course of their life may not be piebald, that is often something else, like mosaicism or a pattern mutation it depends on the nature of the color pattern. Too often I have had someone show me an animal with a clear case of pattern mutation where the normally white bands (kingsnakes are a good example) are wider than usual and less neat, and they call it piebald. It is not so.
Typical progressive piebald animals start out normal then loose patches of pigment over time until they reach or get near maturity when it often stops progressing, though some can progress to complete loss of pigment. However, some species have static piebaldism too, but whatever the type, piebaldism is random, not patterned. Thus animals with white patches that form a symmetrical pattern are not piebald, but are suffering from a pattern mutation. One must also be careful not to throw the word around carelessly. A spotted horse is not piebald just because it is spotted. Horses are not piebald just because they have white. If the skin under the white spots is also devoid of pigment, then that may be considered piebald. But caution must be used with piebald too.
What exactly consititues a piebald? Piebald is where normal pigmented skin and structures (hair, feathers) are randomly distributed around the body with non-pigmented skin and structures. If the skin under the coat is normal, it is not piebald. Some argue that there is also another condition to piebald and that it must be an abnormal condition.
Under this definition of abnormal, paint horses are not piebald since their color is normal for the breed. Calico cats are not piebald, though they have a random arrangement of color. I personally do not accept this definition. I do, however, accept the prerequisite that the distribution of piebaldness is random.
A bi-colored crow or grackle that is symmetrical and has a distinct pattern is not piebald or leukistic, but has a pattern mutation. White pigeons with black speckles are not piebald or leukistic, they have a speckled pattern mutation. The list goes on. If you want to see misidentified pictures just look at any number of websites - they will often have a spotted pigeon and call it leukistic or piebald.
I must also bring a point of standardization here. A sparrow with a white patch around its head is more in keeping with piebaldness than leukism. Leukism is complete lack of pigment over the body. Not patches. Not blotches. Not stripes. Random patches of depigmentation are due to a pathologic disease resulting in depigmentation or due to something like piebaldism. The purity of the term leukism must be preserved for the sake of standardization. The way the current use is (especially in birding circles) if you say something is leukistic, you do not know if it is piebald, hypomelanistic, has autoimmune depigmentation, been burnt and has a depigmented white area, has a birth mark causing the hair or feathers to be white, or is completely white with pigmented eyes. UNACCEPTABLE, ASININE AND INTOLERABLE. Especially the scientific community should know better that allow this kind of confusion.
5) For lack of a better term... PSEUDOLEUKISM - I choose to coin the term pseudoleukism to identify the condition of false leukism seen in those animals (birds particularly) where pigment is dietary. Picture a flamingo which is stark white and has pigmented eyes. It is leukistic! Wrong! In point of fact this condition is common in avians. Many avian species do not deposit the classical pigments in their feathers. Many dispose of excess pigment from their diet by excreting it into the feathers as they are forming. Carotenoids are a common one. You would be surprised how many people have told me they have seen leukistic flamingos at the zoo. If flamingos, cocks of the rock or other species are not provided carotenoids in their diet, they go white. They are not leukistic or albino.
I added the following because of some questions generated by some of you. You e-mailed me some places to visit that were birding sites where they claim that washed out birds with low levels of pigment are leukistic. I hope this explanation helps.
6)HYPOMELANISM - another source of confusion out there is the really bad tendency of the literature particularly the non-peer reviewed literature of mammal and bird color morphs to call those with lower than normal levels of pigment (not absence) leukistic. The problem with that is that there is a wholly different pathogenesis going on there. I have had the opportunity to examine some of these birds at necropsy and I examined their skin and feathers. The ones I have examined were very similar to hypomelanistic reptiles. The melanophores are present but have a reduced level of melanin production. The exact pathogenesis of hypomelanism is not worked out but it is known that in some species it is a Mendelian recessive gene. It is not classical leukism, nor should it be referred to as leukism (leucism).
So there is a brief run down on leukism (leucism). Be careful what you read out there. Some explanation on leukism can be found in the book "Reptile and Amphibian Variants" by H. Bernard Bechtel. He talks about the skin micro-environment defect briefly and the transplant experiments. Keep your questions coming and I will try to answer. I hope my students have found my new blog. I will be e-mailing you all again to update you since it is now up. I will try to post my old posts eventually, but necropsy has been heavy lately and I am swamped with cases.