If you are reading this, you are probably not alone.
Most people on Earth are mite habitats that spend most of their short lives buried, head first, in hair follicles, mostly on the face. In fact, humans are the only habitat for Demodex folliculorum. They are born of us, feed on us, mate with us and die from us.
Their entire life cycle revolves around eating your dead skin cells before kicking the little bucket.
D. folliculorum is so dependent on humans for their survival, new research shows that microscopic mites are evolving from an ectoparasite into an internal symbiont – and one that shares a mutually beneficial relationship with its hosts (that’s us).
In other words, these mites are gradually merging with our bodies, so they are already living permanently in us.
Scientists have now sequenced the genomes of these ubiquitous small beasts, and the results show that their human-centered existence could cause changes not seen in other mite species.
“We found that these mites have a different arrangement of genes on body parts than other similar species due to their adaptation to protective life in the pores,” said invertebrate biologist Alejandra Perotti of the University of Reading in the United Kingdom.
“These changes in their DNA have led to some unusual characteristics and behaviors in the body.”
D. folliculorum, observed in a preparation with potassium hydroxide on human skin. (KV Santosh / Flickr, CC BY 2.0)
D. folliculorum is actually a charming little creature. Human skin detritus is the only source of food and he spends most of his two weeks living in pursuit of it.
Individuals appear only at night, in the cover of darkness, to crawl carefully slowly on the skin to find a mate and hopefully mate before returning to the safe darkness of the follicle.
Their small bodies are only a third of a millimeter long, with a bunch of small legs and a mouth at one end of a long sausage-shaped body – just to scoop up human hair follicles to get to the delicious nomines in them.
Work on the mite genome, led by Marin and geneticist Gilbert Smith of Bangor University in the United Kingdom, has revealed some of the fascinating genetic traits that drive this lifestyle.
Because their lives are so turbulent – they have no natural predators, no competition and no exposure to other mites – their genome is reduced to the bare essentials.
Their legs are powered by three unicellular muscles, and their bodies have the absolute minimum number of proteins, only what is needed for survival. This is the smallest number ever seen in its wider group of related species.
This reduced genome is also the cause of some of the other strange D. folliculorum pecadillas. For example, the reason to go out only at night. Among the lost genes are those responsible for protection from 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 various functions; in humans, melatonin is important in regulating the sleep cycle, but in small invertebrates it causes mobility and reproduction.
However, this does not appear to have prevented D. folliculorum; can collect melatonin secreted by the skin of its host in the evening.
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 male mites’ penises pointing forward and upward from their backs. This means that it should be arranged under the female while landing precariously on hair for mating, which they do all night, in AC / DC style (probably).
But although mating is quite important, the potential gene pool is very small: there is very little room for expanding genetic diversity. This may mean that the mites are on their way to an evolutionary dead end.
Interestingly, the team also found that at the stage of nymph development, between a larva and an adult, mites have the highest number of cells in their bodies. As they move into adulthood, they lose cells, the first evolutionary step, the researchers said, in an arthropod-like march toward a symbiotic lifestyle.
One may wonder what possible benefits people can get from these special animals; something else the researchers found may partially hint at the answer. For years, scientists believed that D. folliculorum did not have an anus, but instead accumulated debris in its body to explode when the mite died, thus causing skin diseases.
The arrow points to the mite’s anus and you are probably on a watch list now. (University of Reading)
The team found that this was simply not the case. Mites do have small holes; your face is probably not full of mite feces expelled posthumously.
“Mites have been blamed for many things,” said zoologist Hank Braig of the University of Bangor and the National University of San Juan in Argentina. “Long communication with people may suggest that they could also have simple but important useful roles, such as keeping our facial pores unclogged.
The study is published in Molecular Biology and Evolution.
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