It's well known that redheads are particularly sensitive to the sun. Their skin pigmentation offers very little protection against UV rays. This is due to the type of melanin they produce.
The pigment is associated with an increased risk of skin cancer
The pigment responsible for hair and skin color, the melaninIt exists in two forms: eumelanin, which is dark brown in color, and pheomelanin, which ranges from yellow to red.
Eumelanin, the predominant form in people with brown or black hair, absorbs up to 751 TP3T of ultraviolet rays. Once absorbed, the energy is dissipated as heat, protecting cells from the sun. Conversely, pheomelanin, the most prevalent form in people with blond or red hair, is less resistant to sunlight. Under the influence of UV radiation, it produces substances that are toxic to cells.
In addition to this toxicity, the red hair pigment has been associated with an increased risk of skin cancer – regardless of UV exposure. So why has pheomelanin been conserved during evolution? Especially since some mammals and birds with orange feathers also carry this pigment.
Researchers in evolutionary biology at the National Museum of Natural Sciences in Madrid, Spain, have been investigating this question. The adaptive physiological advantages associated with pheomelanin synthesis remain poorly defined. However, their identification could help in designing personalized strategies against melanoma. (skin cancer, editor's note) and to understand the evolution of pigmentation phenotypes in nature.”, the authors write in their paper published in the journal PNAS Nexus.
"Pheomelanin synthesis prevents cell damage."
In the absence of human experiments, the researchers used mandarin diamonds as a model (Taeniopygia guttata), small birds native to Indonesia. In this species, the orange feathers sported by the male are colored by pheomelanin. The researchers then fed the zebra finches a high dose of cysteine. Excess of this amino acid causes oxidative damage in cells, but it is used as a reagent for the synthesis of pheomelanin.
Male (left) and female (right) Zebra Finch. The male's orange plumage is colored by pheomelanin, while the female's is not. Credits : Ismael Galván.
Their results showed that females, which do not produce this orange pigment, exhibited more cellular damage. Conversely, males, which naturally produce the pigment from cysteine, showed no toxic effects and grew feathers rich in orange pigment. This initial finding suggested that the synthesis of pheomelanin allowed the body to eliminate excess toxic cysteine.
To test this hypothesis, the researchers then administered a drug that blocks pheomelanin synthesis, ML349. This time, the males fed cysteine and treated with ML349 showed an increase in oxidative damagecompared to males that received only cysteine. The females, however, were not affected by this medication.
"These results demonstrate that pheomelanin synthesis prevents cellular damage by excreting excess cysteine into inert keratinized structures such as feathers," the authors explain in their article. This capacity constitutes the only known physiological role of this pigment at present. This could explain the persistence of genetic variants favoring pheomelanin, despite their association with an increased risk of skin cancer.
The next work of the Spanish biologists should explore the role of environmental factors in the evolution and maintenance of the diversity of colors in animals.
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