If you’re reading this, you’re probably not alone.
Most people on Earth are habitats for mites that spend most of their short lives buried, upside down, in our hair follicles, mostly on the face. In fact, humans are the only habitat of Demodex folliculorum. They are born of us, they feed on us, they mate with us and they die on us.
Its entire life cycle revolves around biting dead skin cells before kicking the small bucket.
D. folliculorum depends so much on humans for survival, new research suggests, that microscopic mites are evolving from an ectoparasite to an internal symbiont, and one that shares a mutually beneficial relationship with their hosts. (it’s us).
In other words, these mites are gradually merging with our bodies so that they now live permanently within us.
Scientists have now sequenced the genomes of these ubiquitous little beasts, and the results show that their human-centered existence could be causing changes not seen in other mite species.
“We found that these mites have a different arrangement of genes in body parts than other similar species because they adapt to a protected life within the pores,” said invertebrate biologist Alejandra Perotti of the University of Reading in the United Kingdom. United.
“These changes in your DNA have led to some unusual body characteristics and behaviors.”
D. folliculorum seen in a preparation of potassium hydroxide from human skin. (KV Santosh / Flickr, CC BY 2.0)
D. folliculorum is actually a fascinating little creature. Human skin detritus is its only food source, and it spends most of its two-week shelf life in search of it.
Individuals emerge only at night, on the cover of darkness, to slowly crawl through their skin to find a mate and hopefully copulate before returning to the safe darkness of a follicle.
Its tiny bodies are only a third of a millimeter long, with a clump of tiny legs and a mouth at one end of a long sausage-shaped body, just to remove the hairs of human hair to get to the tasty names out there.
The work on the mite genome, led jointly by Marin and geneticist Gilbert Smith of Bangor University in the United Kingdom, revealed some of the fascinating genetic traits that drive this lifestyle.
Because their lives are so crisp: they have no natural predators, no competition or exposure to other mites, their genome has been reduced to just the essentials.
Their legs are propelled by three unicellular muscles, and their bodies have the absolute minimum number of proteins, only those necessary for survival. It is the smallest number ever seen in its largest group of related species.
This reduced genome is also the cause of some of D. folliculorum’s other strange peccadillos. For example, the reason why he only goes out at night. Among the lost genes are those responsible for protecting against UV radiation and those that wake animals up in daylight.
They are also unable to produce the hormone melatonin, which is found in most living organisms, with different functions; in humans, melatonin is important in regulating the sleep cycle, but in small invertebrates, it induces mobility and reproduction.
This does not appear to have prevented D. folliculorum, however; you can catch melatonin secreted by your guest’s skin at dusk.
This is not convenient. (Smith et al., Mol. Biol. Evol., 2022)
Unlike other mites, their reproductive organs of D. folliculorum have moved to the front of their bodies, with the penises of the male mites pointing forward and upward from the back. This means that they have to be tucked under the nut while they are precariously placed on a hair for mating, which they do all night, in the AC / DC style (presumably).
But while mating is quite important, the set of potential genes is very small: there are very few opportunities to expand genetic diversity. This could mean that mites are on their way to an evolutionary dead end.
Interestingly, the team also found that in the stage of nymph development, between larva and adult, is when the mites have the largest number of cells in their body. When they reach adulthood, they lose cells, the first evolutionary step, the researchers said, in the march of an arthropod species toward a symbiotic lifestyle.
One might wonder what possible benefits humans can derive from these peculiar animals; something else the researchers found could partially hint at the answer. For years, scientists have thought that D. folliculorum does not have an anus, but accumulates waste in its body to explode when the mite dies and therefore causing skin conditions.
The arrow points to the mite’s anus, and now you’re probably on a sort of watch list. (University of Reading)
The team found that this is not the case. Mites do have small holes; your face is probably not full of posthumously expelled mite poop.
“The mites have been to blame for many things,” said zoologist Henk Braig of Bangor University and the National University of San Juan in Argentina. “The long association with humans could suggest that they could also have simple but important beneficial functions, for example, to keep the pores of our face disconnected.”
The research has been published in Molecular Biology and Evolution.