If you’re reading this, you’re probably not alone.
Most people on Earth are habitats for dust mites, which spend the majority of their brief lives buried, headfirst, in our hair follicles, mostly on the face. In fact, humans are the only habitat for Demodex folliculorum. They are born on us, they feed on us, they mate on us and they die on us.
Their entire life cycle revolves around chewing up dead skin cells before kicking in the tiny bucket.
If dependent is D.folliculorum 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 its hosts (that’s us).
In other words, these mites gradually merge with our body so that they now live permanently in us.
Scientists have now sequenced the genomes of these ubiquitous little beasts, and the results show that their human-centric existence could cause changes not seen in other mite species.
“We found that these mites have a different arrangement of body part genes compared to other similar species, as they adapt to sheltered life inside the pores,” explained invertebrate biologist Alejandra. Perotti from the University of Reading in the UK.
“These changes in their DNA resulted in unusual body characteristics and behaviors.”
D.folliculorum is actually a fascinating little creature. Human skin detritus is its only food source, and it spends the majority of its two-week lifespan foraging for it.
Individuals only emerge at night, under the cover of darkness, to crawl laboriously slowly over the skin to find a mate and hopefully mate before returning to the safe darkness of a follicle.
Their tiny bodies are only a third of a millimeter long, with a cluster of tiny legs and a mouth at one end of a long, sausage-like body – just enough to descend human hair follicles for get the tasty names therein.
Work on the mite’s genome, co-led by Marin and geneticist Gilbert Smith of Bangor University in the UK, has revealed some of the fascinating genetic characteristics that drive this way of life.
Because their lives are so difficult – they have no natural predators, no competition and no exposure to other mites – their genome has shrunk to the bare essentials.
Their legs are powered by three single-celled muscles, and their bodies contain the absolute minimum number of proteins, only what is necessary for survival. This is the lowest number ever seen in its larger group of related species.
This refined genome is at the origin of certain D.folliculorumother weird peccadilloes too. For example, why he only comes out at night. Among the genes lost are those responsible for protection against UV rays, and those that wake animals up in daylight.
They are also unable to produce the hormone melatonin, present in most living organisms, with varying functions; in humans, melatonin is important in regulating the sleep cycle, but in small invertebrates it induces mobility and reproduction.
It didn’t seem to bother me D.folliculorum, Nevertheless; it can harvest melatonin secreted by its host’s skin at dusk.
Unlike other mites, their reproductive organs of D.folliculorum moved forward in their body, with male mites’ penises pointing forward and upward from their back. This means he has to settle under the female as she precariously perches on a hair to mate, which she does all night, AC/DC style (presumably).
But although mating is quite large, the potential gene pool is very small: there is very little scope for expanding genetic diversity. This could mean that the mites are on the way to an evolutionary dead end.
Interestingly, the team also found that the pupal stage of development, between larva and adult, is when the mites have the highest number of cells in their bodies. As they mature into adults, they shed cells – the first evolutionary step, researchers say, in an arthropod species’ march toward a symbiotic lifestyle.
One wonders what benefits humans can derive from these particular animals; something else the researchers found might partially suggest the answer. For years, scientists thought that D.folliculorum lacks an anus, instead accumulating waste in its body to explode when the mite dies, and thus causing skin ailments.
The team found that this is simply not the case. Mites do indeed have tiny little assholes; your face probably isn’t full of posthumously expelled mite poo.
“Mites have been blamed for many things,” said zoologist Henk Braig of Bangor University and the National University of San Juan in Argentina. “The long association with humans might suggest that they might also have simple but important beneficial roles, for example, keeping our facial pores unplugged.”
The research has been published in Molecular biology and evolution.