If you are reading this, you are probably not alone.
Most people on Earth are habitats for mites that spend the majority of their
brief lives burrowed, head-first, in our hair follicles, primarily of 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 munching your dead skin cells before
kicking the teeny tiny bucket.
So reliant is D. folliculorum on humans for their survival, new research
suggests, that the microscopic mites are in the process of 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 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 wreaking
changes not seen in other mite species.
"We found these mites have a different arrangement of body part genes to
other similar species due to them adapting to a sheltered life inside
pores," explained invertebrate biologist Alejandra Perotti of the University
of Reading in the UK.
"These changes to their DNA have resulted in some unusual body features and
behaviors."
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| D. folliculorum seen in a potassium hydroxide preparation of human skin. (K.V Santosh/Flickr, CC BY 2.0) |
D. folliculorum is actually a fascinating little creature. Human skin
detritus is its sole food source, and it spends the majority of its two-week
lifespan in pursuit thereof.
The individuals emerge only at night, in the cover of darkness, to crawl
painstakingly slowly across the skin to find a mate, and hopefully copulate
before returning to the safe darkness of a follicle.
Their tiny bodies are just a third of a millimeter in length, with a cluster
of tiny legs and a mouth at one end of a long, sausage-shaped body – just
right for scooching down human hair follicles to get at the tasty noms
therein.
The work on the genome of the mite, co-led by Marin and geneticist Gilbert
Smith of Bangor University in the UK, revealed some of the fascinating
genetic characteristics that drives this lifestyle.
Because their lives are so cruisy – they have no natural predators, no
competition, and no exposure to other mites – their genome has reduced down
to just the bare essentials.
Their legs are powered by three, single-cell muscles, and their bodies have
the absolute minimum number of proteins, only what is needed for survival.
It's the smallest number ever seen in its wider group of related species.
This pared-down genome is the reason for some of D. folliculorum's other
strange peccadilloes, too. For instance, the reason it only comes out at
night. Among the genes lost are those responsible for protection against UV
radiation, and those that wake animals up at daylight.
They are also unable to produce the hormone melatonin, found in most living
organisms, with varying functions; in humans, melatonin is important for
regulating the sleep cycle, but in small invertebrates, it induces mobility
and reproduction.
This hasn't seemed to have hindered D. folliculorum, however; it can harvest
melatonin secreted by the skin of its host at dusk.
Unlike other mites, their reproductive organs of D. folliculorum have moved
towards the front of their bodies, with male mites' penises pointing
forwards and upwards from their backs. This means he has to arrange himself
underneath the female as they perch precariously on a hair for mating, which
they do all night, AC/DC-style (presumably).
But although mating is pretty important, the potential gene pool is very
small: there is very little opportunity for expanding genetic diversity.
This could mean that the mites are on track for an evolutionary dead end.
Interestingly, the team also found that, at the nymph stage of development,
between larva and adult, is when the mites have the greatest number of cells
in their bodies. When they move on to the adult stage, they lose cells – the
first evolutionary step, the researchers said, in the march of an arthropod
species to a symbiotic lifestyle.
One might wonder what possible benefits humans can gain from these peculiar
animals; something else the researchers found might partially hint at the
answer. For years, scientists have thought that D. folliculorum doesn't have
an anus, instead accumulating waste in its body to explode out when the mite
dies, and thus causing skin conditions.
The team found that this is simply not the case. The mites do indeed have
tiny little buttholes; your face probably isn't full of mite poop expelled
posthumously.
"Mites have been blamed for a lot of things," said zoologist Henk Braig of
the University of Bangor and the National University of San Juan in
Argentina. "The long association with humans might suggest that they also
could have simple but important beneficial roles, for example, in keeping
the pores in our face unplugged."
The research has been published in Molecular Biology and Evolution.
Reference:
Gilbert Smith, Alejandro Manzano Marín, Mariana Reyes-Prieto, Cátia Sofia
Ribeiro Antunes, Victoria Ashworth, Obed Nanjul Goselle, Abdulhalem Abdulsamad
A. Jan, Andrés Moya, Amparo Latorre, M. Alejandra Perotti, Henk R Braig, Human
follicular mites: Ectoparasites becoming symbionts, Molecular Biology and
Evolution, 2022;, msac125,
DOI: 10.1093/molbev/msac125