What Human Follicle Mites and the Spanish Hapsburgs Have in Common

Mites that Mate on Your Face Headed for Likely Extinction

Creative Commons Acarologiste

NO AI TRAINING: Without in any way limiting the author’s [and publisher’s] exclusive rights under copyright, any use of this publication to “train” generative artificial intelligence (AI) technologies to generate text is expressly prohibited. The author reserves all rights to license uses of this work for generative AI training and development of machine learning language models.

_______

Each night, the mites that live in your face — arachnids whose kin include ticks and spiders — pick their way out of your pores on eight stumpy legs with lovin’ on their minds.

Their comically microscopic, knob-like limbs are operated by three lonely muscle cells. Not three muscles, three cells — one per segment. In spite of the legs’ obvious shortcomings, they somehow still manage to power (if such a word can be used) mites toward connubial bliss at a rate of around 12 millimeters per hour, which, in automotive terms, is 0.00000746 miles per hour.

Yet that is more than sufficient to the rice-shaped arachnids’ purpose: finding a mate, acrobatically clinging to one of your hairs while in flagrante delicto all … night … long (I mean, Rome wasn’t built in a day), then retiring to your pores to lay their eggs and sleep it off as the sun rises. Even if your love life has hit a dry spell, rest assured your follicle mites never lack for partners or libido, and reproduce themselves on your face — and potentially many other exciting body parts — with vigorous abandon roughly every two weeks. Although, it should be added, because you have many mites, this is probably happening somewhere, and possibly multiple special somewheres, every night.

There’s only one small issue: due to a hardly noticeable loss of a mite developmental gene, the male’s genitalia are no longer located in the usual spot. Instead, they have migrated to his back, in the position of a shark’s dorsal fin. In order to complete his mission, the male has to position himself in a very un-missionary position under the female.

Oh, and in addition to that tiny mix-up, and the fact their legs are running on absolute minimum thrust, there’s also the small matters of the complete loss of genes that protect them from UV light, the ones that tell them to wake up when it’s light outside, and something like, oh, 27 DNA repair genes. They’ve also lost the gene that makes melatonin, but no problem — they’ve just outsourced it by freeloading on ours. Laissez les bons temps roulez!

As it turns out, the clearly decaying human follicle mite has the smallest number of functional genes of any known joint-legged animal (and its genome is just over half the size of that of a tiny animal called Trichoplax that is nothing more than a millimeter-wide gliding sheet of cells). For reasons we will come to shortly, the genome of the human follicle mite is falling apart, setting them on an evolutionary trajectory in which they may be doomed to extinction. But it is an extinction in extreme slow motion, one whose contours are illuminating parts of the process we have never seen before. It also begs the question: where does a symbiont end and a human begin?

If the realities of your arachnid housemates creep you out, take comfort in the fact that we’re far from alone. The mites — called Demodex, and ranging in size from about one-tenth to three-tenths of a millimeter long — likely infect, infest, or co-habit (depending on your perspective) more than nine in 10 adult humans. Such mites are not unique to us either; they are common in mammals and marsupials. They live in our pores, feeding on … well, we’re not 100% sure, but probably body oil called sebum and maybe some of the cells lining your hair follicles (trust me, you do not miss them).

The story of probable follicle mite doom is tied up in the story of how mites move between humans — or rather, how they don’t. In a study published last summer in Molecular Biology and Evolution, a team of scientists working in the United Kingdom, Austria, and Spain found that follicle mites are primarily passed from mother to child and remain with us our entire lives, the only animal known to do so. They compared the DNA of mites on 31 people in 10 families, including children, grandchildren, and spouses. They found six examples of maternal transmission — in two cases passed down two generations to grandchildren — but only one example of transmission between spouses.

Follicle mites are thus, like sub-cellular machines called mitochondria, living heirlooms inherited from your maternal ancestors. You likely have the same mites your mother’s mother’s mother’s mother did, to the point where your mites’ DNA can be used to determine the geographical region from which your maternal ancestors hailed.

How exactly they reach you from your mother is another set of harsh realities you may regret knowing by the time you finish this piece. For yet another true fact about follicle mites is that they rather have a thing for human areolas and nipples, dwelling there happily and abundantly. It is suspected that the “increased temperature and moisture levels” at the nipple during breastfeeding facilitate their moving house.

The less likely human follicle mite-transmission scenario does not improve on matters. Among the other possible dwelling-places of human follicle mites are follicles in vulvas (and, it should be added, in penises; I will let you form your own emotional reaction to both pieces of news). So it is also possible your mites acquired new homes during the well-known messy and — thanks to humans’ possibly unfortunate move to begin walking upright — excruciatingly slow process of childbirth. Even a follicle mite, apparently, can catch a birthing baby under full throttle.

It may be that babies are the primary means of mite transmission because they present choice real estate that also happens to be unoccupied. Because a mother’s mites necessarily get there first, it may be nearly impossible for later explorers to oust them, unless the baby remains unoccupied into adulthood. Once happily ensconced on their new sweet-smelling (if noisy) homes, most mites take up residence on the wings of the nose, the forehead, eyelash follicles, ear canals, and in the nipples, but, depending on the human, other locations may provide happy, oily homes.

So are follicle mites parasites? Scientists expect that parasites should spread from person to person in the community, not from mother to child. This team hypothesizes that the evolution of efficient mother-to-child transmission eliminated the need for them to be “freely contagious” and simplified their lives considerably. But it has also led to isolation and inbreeding; to a transition from free-agent, host-injuring parasites to couch-potato, possibly beneficial (they may help clean our pores) but doomed symbionts. If our mites never mingle, they are also mating eternally with their brothers, sisters, and cousins.

The team of scientists also examined the follicle mite genome in detail, and based on their analysis, the consequence of this turn of events is genome decay that may be irreversible and ultimately fatal. Because they live in stable, cozy homes with abundant food (us) and without competitors, a situation called “relaxed selection” results, with effects not unlike the acquisition of relaxed blue jeans (See also: Dad jeans). The relentless inbreeding on its own wouldn’t be a problem if the population was allowed to expand or mix with other populations with non-broken genes. But when inbreeding is combined with relaxed selection, the relentless bottlenecking that occurs when a few founders crawl from boob or vulva to new baby, and a phenomenon called “genetic drift” — the takeover by pure chance of versions of genes in small populations — the results are catastrophic. Mutations arise, they take over by chance or because relaxed selection allows it, inbreeding prevents corrective or beneficial versions of genes from re-entering the population, and the bottleneck between generations seals the deal.

The resulting genetic and morphological decay is stunning. Follicle mites, as mentioned, have both the smallest number of protein-coding genes and the second-smallest genome (the shortest belongs to a plant parasitic mite). In Demodex, mutation has run so wild that even the mite’s normally sacrosanct Hox developmental genes have been tampered with (probably resulting in the unorthodox penis geography).

They’ve lost a set of 21 non-Hox body sculpting genes whose disappearance appears to have left them with legs just 15 micrometers long, shorter than many fungal spores and only as long as a few average-sized bacteria. The cycle becomes vicious and perhaps even exponential when the failing genes start to include those that repair DNA, and as mentioned, something like 27 have now broken.

The decay extends to the very number of cells in a mite. Demodex has more than 500-times fewer cells than a fruit fly — around 600-900 cells versus more than 600,000 in the fly — which they discovered because someone (presumably ill-starred grad students) counted. The mites have dispensed with eyes and have only very basic photosensors. The entire function of the midgut — a hollow, multicellular tube, as you will recall, in most animals — has been replaced by individual giant cells through which digesting food physically passes. (Yet Demodex does, in fact, still have a tiny true hindgut from which it poops during the course of its short three-week life. Reports that they expire with all that poop saved up inside, leading to a massive post-mortem poosplosion that inflames human skin, are scandalous falsehoods according to the authors.)

The adult Demodex actually appears to have fewer cells even than its own most senior juvenile stage: around 900 in females and 600 in males compared to 1,400 in the eldest juvenile. This was a surprise. Scientists had assumed that a reduction in cells would start early in development, and not, seemingly inefficiently, later. It is the first example of its kind, and, the authors hypothesize, perhaps the first step in becoming an animal symbiont.

Muller’s Ratchet is the idea that sex is favored over cloning because its signature shuffling of gene variants defeats deleterious mutations. But when follicle mites mate relentlessly with their brothers and sisters and cousins for thousands of generations, through repeated regular bottlenecks, and in cozy, easy conditions that lead to sloppy genomes, they can shag all they want on your sleeping face but they’re not going to escape Muller’s Ratchet. They are functionally — and ironically — asexual. Extinction by decrepitude has actually happened before to symbiotic bacteria and fungi (they get replaced by more functional species), but has never been before observed in an animal; the human follicle mite may be the first that we know of because we are watching it happen.

You know what this should remind you of, of course. That would be the House of Hapsburg, which through successive and relentless generations of uncles marrying nieces and double cousins marrying double cousins resulted in the Spanish King Charles II (El Hechizado, “The Hexed”). Based on the Hapsburgs’ well-documented pedigrees, Charles’s genome was slightly more inbred (Inbreeding Coefficient (F) = 0.254) than if his parents had been brother and sister or parent and child (F = 0.250). Charles was so sickly and disabled (he may have had not just one but two recessive genetic diseases) that it was impossible for him to father children. With his death, the Hapsburg dynasty died in Spain.

(The inbreeding record holder among the Hapsburgs was actually a woman: Maria Antonia of Austria (F = 0.3053), daughter of Austrian Emperor Leopold I and his niece Margaret of Spain. The insatiable Hapsburgs actually had plans to marry her to Charles II, which would no doubt have been a genetic catastrophe on par with the sinking of the Titanic or the Hindenberg disaster, but fortunately for all concerned these plans came to nothing.)

The Hapsburg political philosophy was expressed in the late Middle Ages as Bella gerant alii, tu felix Austria nube – 'Let others wage war: thou, happy Austria, marry’. The Hapsburgs, who thought this both a prudent and clever way of acquiring and maintaining territories, were quite unaware of its biological consequences and no doubt happy with their strategy right up until the end, but it nonetheless doomed their Spanish branch to extinction. Human follicle mites seem to have taken the Hapsburg strategy to 11; the death warrant of their species may likewise have been signed when they discovered the most expeditious way to reach the next generation was to just hop on the nearest baby. Felicitous in the short term, maybe; in the mites’ best long term interests, no. Both groups fell into a biological trap. At this point, the authors of the new paper write, follicle mite extinction may be a mathematical and statistical certainty.

Should we be sad about that prospect? Are follicle mites a part of us? They’re a lot more part of us than even our gut microbes, long considered human symbionts. Although gut bugs are well-known to be transmitted from mother to baby at birth and through breast milk (incredibly, immune cells may ferry maternal gut bacteria to the boobs, which dispense up to one million bacteria a day), we don’t keep those early bugs for long and our microbiome is subject to change throughout our lives. Our gut microbiome is also made of a whole ecosystem of species, can be transmitted from person to person, and many human species may even be able to survive in other mammals’ guts.

On the other hand, the sub-cellular machines called mitochondria that, perhaps a few billion years ago, were free-living microbes that fused symbiotically with other microbes to give rise to most large life forms, are utterly inseparable from us. Even though they still have their own small genome, mitochondria cannot survive outside cells, and cells cannot survive without mitochondria. They are us.

So where does that leave follicle mites? They can, and obviously do, walk right off us onto new hosts. So they are separable from a human. But they are not separable from humans. They can live nowhere else, associate with no other species, and are totally dependent on us for almost everything (except, obviously, scoring). So, probably not human — but almost as close as it’s possible to get. Which is pretty darn astounding, when you think about it, considering that they are, after all, still arachnids.