It must be admitted, dust mites are not a dream. 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 folliculorum 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 a stunning 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.
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. folliculorum has settled into a dead-end evolutionary path.
The price of success, indeed. Among humans, he found an ideal shelter. No need for UV protection, the pores take care of it. Nor do you need melatonin production to control the day/night cycle: our glands provide it. 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’s a sign of extreme adaptation to their host,” said Henk Braig of Bangor University, 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 takes place exclusively from mother to child, Demodex taking advantage with delight of the Montgomery glands located in the nipples.
This change is not without consequences. The more a host is different, the more it imposes on the parasite to evolve. For the authors, D. folliculorum has ceased its parasitic life. Now commensal, it would be in the process of adopting a symbiotic diet, like certain bacteria in the microbiota. Provided, however, that it does not simply disappear.
Because, by dint of simplifying, losing functions and diversity, our mites have probably settled in what Henk Braig and his colleagues call “an evolutionary cul-de-sac”. Good news for obsessives of skin protection. Aren’t they held responsible for certain skin diseases? “They are the black sheep of dermatology, reacts Henk Braig. Admittedly, they sometimes take advantage of diseased skin. But, if not, they probably allow the contrary to keep the pores open. Will we soon miss the mites on our face?