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Our body is home to a whole complex and diverse microecosystem, serving for example as a permanent host to thousands of tiny mites of the Demodex genus. Colonizing the pores of the skin (body and face), their way of life and their biology remain however still little known. Hypotheses suggested that these mites do not have an anus and die by evacuating all the waste accumulated during their short life. These theories have notably contributed to incriminating them for various skin diseases. Dust mite specialists are now changing the situation thanks to genomic sequencing: they have discovered that they finally have an anus. Moreover, these arachnids would be evolving towards an obligatory symbiotic lifestyle, and could bring us benefits for our health, like the intestinal microbiota. Another important discovery: the decline of their genome would lead to their extinction as a separate species.
Both the inside and the outside of our body are teeming with life. First discovered in the human ear canal in the 1800s, two species of mites inhabit our skin, including Demodex folliculorumfound mostly on the face and Demodex brevis. Follicular mites are also the only metazoans (multicellular animals) to live their entire lives on humans. Being arachnids (including ticks, spiders, etc.), however, they are so tiny that it is impossible to feel them move or see them with the naked eye.
Demodexes are shaped like small, stocky eight-legged worms and like to cling to our sebum-rich hair follicles, on which they feed. They thus spend the majority of their short life (no more than three weeks) upside down in the pores, and only emerge to mate. Furthermore, the composition of the cutaneous mite population is inherited from the parents from the first moments of life, and is thus specific to each individual.
Biologists assumed that the Demodex did not have an anus. This hypothesis thus suggests that fecal waste therefore accumulates inside their small body throughout their life, and is only released at the time of their death.
Dermatologists speculated that the abundance of these mites (and the bacteria they are believed to excrete when dying) were implicated in skin diseases such as rosacea. ” It’s easier and faster to blame the mites says Alejandra Perotti, co-lead author of the new study and researcher at the University of Reading in the UK.
New research co-led by the Universities of Bangor and Vienna appears to refute these deeply held assumptions. The results of genomic sequencing, presented in the journal Molecular Biology and Evolutionshow that Demodex folliculorum would indeed have an anus, and can excrete its waste throughout its life. They also reveal that these mites evolve from an obligate parasite harming the host to an obligate symbiont (providing benefits to the host while depending and feeding on it).
A genome in decline
According to the authors of the new study, the excessive number of mites detected in people suffering from rosacea or other skin diseases could be a consequence of these diseases and not the cause, as assumed by old theories. In addition, the sequencing of the genome of the mites has shown that by being provided with anuses, these animals do not ultimately discharge the supposedly large quantities of excrement filled with bacteria, supposed to infect the skin, all at once.
The researchers of the new study made another important discovery, according to which dust mites evolved by reducing their entire genetic code. In particular, they are endowed with a very simple genome (the smallest number of coding genes among the panarthropods), probably allowing them to live with the minimum necessary cells. Their legs, for example, are only controlled by a single muscle cell.
Moreover, the reduction in the number of cells in parasites generally begins early in their developmental stage. However, the largest total number of cells in the Demodex folliculorum is observed only in the pupal state (at the terminal adult stage), suggesting that cell reduction does not begin until adulthood. This would include the first evolutionary step of the species adopting an endosymbiotic lifestyle (a form of symbiosis where one of the two organisms lives inside the other).
The demodex would also have evolved to adopt a unique day/night rhythm of life and conditioned by the melatonin of the human host. It would thus have lost the ability to survive ultraviolet light. In addition to extreme endogamy (the choice of a mate within a small group), the loss of DNA repair genes could lead to the extinction of the species as a distinct individual. Already observed in bacteria, this may be the first time that the phenomenon has been observed in an animal.