IYou have to admit, dust mites are not the stuff of dreams. When chiggers, ticks and other mites – responsible for scabies – begin to populate our nights, it’s a rather bad sign. However, this huge subclass of arthropods, which includes at least 50,000 species, hides a dizzying diversity. The land where we walk, the water where we wade, the plants, the animals that accompany us are all areas where they have learned to live.
Among them, Demodex follicular holds a special place. To the naked eye, the animal looks like… not much. It must be said that with its length of 0.3 mm, its worm silhouette plays with human eyes. Above all, hidden in the pores of our skin, preferably where it can cling to the hairs, it remains out of sight. An international team of researchers has nevertheless used great means to reveal the secrets of its biology. For the first time, its genome has been deciphered. The result, published in the journal Molecular Biology and Evolution, reveals an amazing world. And threatened.
At the risk of worrying some, we house almost all of these discreet eight-legged tenants: 3,600 on average on our face alone. During the day, they gorge themselves on the sebum produced in particular by the sebaceous glands. At night, they copulate. With a limited but original register of positions. The females (80% of the population) cling to a hair. The males then slip under them and insert the penis they carry into their lower back.
“Extreme adaptation”
The originality of the study led by the universities of Reading, England, and Bangor, Wales, is not limited to sexual gymnastics, however. Genomic analysis has found that while all mites (and all mites) have successfully parasitized mammals for nearly 200 million years, D. follicular has settled down an evolutionary path with no way out.
The price of success, indeed. Among humans, he found an ideal shelter. No need for UV protection, the pores take care of it. No need either for melatonin production to control the day/night cycle: our glands offer it to them. Over the course of evolution, the animal has gradually lost these functions and a few others. Researchers have discovered the smallest genome ever sequenced in an arthropod. Few genes, few proteins, few cells even.
The three segments of these eight legs are each animated by a single cell. “It is the sign of an extreme adaptation to their host”, emphasizes Henk Braig, of the University of Bangor, one of the coordinators of the work. The study also proved that the animal stopped transmitting between hosts of different species, and even horizontally between humans. Transmission is exclusively from mother to child, Demodex enjoying with delight the Montgomery glands located in the nipples.
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