BY LUCA GIOVAGNOLI DVM
The Taiji Whale Museum announced a couple weeks ago that two white/piebald Risso’s dolphins captured during the 2014-15 drive hunt season, are now on public display. Both dolphins had been kept away from public view. The museum in celebrating its ‘White Day,’ also released previously unknown details about the animals.
The two Risso’s dolphins are both males with black eyes. One is clearly piebald and the other — originally thought to be an albino, has tiny splashes of color. This indicates that unlike the bottlenose dolphin ‘Angel’ — captured last season and a true albino with pink eyes, both Risso’s dolphin are leucistic.
Genetically, let’s clarify two very important concepts of inheritance as often there is confusion with the terms leucism and albinism. What are the differences between the two?
Leucism (from the Greek, leukos, white) is a genetic peculiarity due to a gene, recessive in most cases, which gives a white color to the fur or feathers of animals which normally would have a different color.
Albinism (from the Latin albus, white) is a congenital anomaly consisting of a total or partial deficiency of melanin pigmentation in the skin, iris and choroid, and also in the fur or plumage. It is caused by an absence or defect of the enzyme tyrosinase, which is involved in the synthesis of melanin — the pigment that gives skin, hair, and eyes their color.
DNA contains all the genetic information essential to living beings, and can be divided into small sequences called genes. A gene is an instruction written in the language of the DNA code. One “expression” of these genes is the process in which the information encoded within a gene is translated into a biological functional protein.
It is written into our DNA that whenever the body ingests something, enzymes are produced to tackle it, be it proteins or pathogens. For example, when we are attacked by pathogens, our bodies respond with antibodies.
Genes also control our physical appearance and metabolic processes. These traits make us different from one another. An astonishing 99.9% of human DNA is identical. Only 0.1% makes us unique compared to the rest of humanity.
Certain genes in mammals control the pigmentation of the hair skin and eyes. Let’s focus on the genes that identify the color of the hair. The pigmentation is given by melanocytes, specialized cells that produce and contain melanin.
Melanocytes are formed during embryonic development in a part of the embryo defined as the neural crest. In addition to these cells, the neural crest give birth to various organs.
During development, melanocytes migrate to the epidermis, hair follicles, but to the eye, inner ear and other organs.
A stable and heritable (able to be inherited) modification of the sequence of nucleotides in the genetic code is known as mutation. Nucleotides are organic molecules that serve as the subunits of nucleic acids like DNA and RNA. They are composed of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group.
In a mutation, not all melanocytes grow and migrate properly. They will also not produce melanin and white spots are observed. These can be more or less extensive but are nothing more than hairs devoid of pigment.
An example is the coloring of the hooded rat, or the rat with the colored cap on the head. This is a perfect example of a delayed migration of melanocytes from the neural crest. Areas more distant from the median dorsal (feet, belly, chest) are depigmented.
Melanin is present in two forms: Eumelanin is a brown-black pigment responsible for dark color, and phaeomelanin is the red pigment responsible for yellow-light coloring. The final color of the hair comes from a combination of these two.
Different mutations in this control mechanism means the hair can be lighter, darker, or completely depigmented — as in the case of albinism. In albinism, the enzymes that would normally produce melanin are completely inactive. This is why the hair looks completely white, the skin is pink and and eyes are either red or blue because the iris is unable to produce pigment. The albino mammal then, is the result of a random mutation.
So what happens in nature?
This mutation usually skips between generations, often because the white coat makes it easy to spot the animal in its habitat and therefore it is more easily attacked. For this reason an albino puppy is rejected by his mother and not breastfed, so the genetic mutation is not passed on.
What else may cause animal albinism or a partial lack of pigment?
Albinism being a mutation in genes that control the process of melanin synthesis (second group of genes), should not interfere with other organs. The defects are mainly related to the iris. There is some discomfort when exposed to direct light (photophobia) and difficulty in perceiving movement.
In animals depigmented (leucistic animals), the situation is more complex. Leucism is a process affected in the early stages of embryonic development (the first group of genes) that influence the development of other parts of the body.
Melanocytes are derived of the same cells in the embryo that give rise to the central nervous system, and therefore mutations of this type are often linked to neurological defects. It can start with the alteration of sight and hearing, and gradually, the damage may be even more serious, resulting in the the death of the prenatal litter.
One effect responsible for alterations in genetic structures is pollution, in particular substances with mercury and PCBs. “Fragments” of these pollutant molecules in the nuclei penetrate, creating genetic alterations, including albinism.
Another element seen as harmful in the alteration of genes is prolonged exposure to radioactive substances. In this case the action of certain pollutants such as cesium, causes genetic alterations that are immediate and permanent.
Damage from exposure to radioactive material occurs in a very short time. The nuclear charge is stronger, so the greater the damage. This is clearly seen in the dolphins being held in the Taiji Whale Museum in Japan. These animals have a number of features not perfectly defined. Classic albinism and the associated leucistic phenomena, is most likely due to the action of radioactive substances.