There are two species of “face mites,” Demodex, that live on humans. One, D. folliculorum, lives mostly on the hair follicles, and mostly on the face. It’s small (0.1-0.4 mm), has a transparent body with four pair of anterior legs (remember, mites are arachnids), and nearly all of us harbor them. They’re harmless, although in large numbers they can cause a form of mange. Here’s D. folliculorum; remember, they’re all over you!!! (Read Ed Yong’s popular piece about them here.)
The other species. D. brevis, lives in the sebaceous glands at the base of the hairs, so its niche is slightly different. Because those are buried in our pores, they’re less likely to be transmitted between people than are individuals of D. folliculorum.
Before we get to the new paper on D. folliculorum, I was interested in seeing if the two hair-inhabiting species on our bodies are sister species: each other’s closest relatives. That would imply that, since these species are limited to humans, they speciated on our bodies, i.e., a form of sympatric speciation (speciation in the same small area without the need for geographic barriers, so that species form while they’re still exchanging genes). Now it’s possible that they are sister species but still speciated allopatrically (in geographic isolation), with a common ancestor evolving into D. folliculorum in one part of the world, into D brevis in another part, and then later came to cohabit single humans when these isolated human populations came back together and swapped mite species.
But that’s not the way it happened. Our hair mites are not sister species, as the phylogeny below. The closest relatives of D. folliculorum are two species in the genus that inhabit white-tailed deer (WTD) and mice (D. musculi) respectively. A less-related species inhabits dogs, and D. brevis is even less closely related:
These two species, then, did not separate on the human body, but are the result of two separate invasions of distinct species from other animals. This is a common finding for parasites—relatd species on one host are almost never sister species—supporting the idea that speciation in parasites usually requires some kind of geographic separation (i.e., speciation on other hosts, which in effect are isolated “islands”.) See chapter 4 of Speciation, by Coyne and Orr, for more detailed discussion of this issue.
But that’s my own take; now on to a new paper on D. follicularum, published by Michael Palopoli and colleagues in the Proceedings of the National Academy 0f Sciences (USA). Reference is at bottom; download is free.
Since the “parasite” (it could be seen as a commensal since it rarely harms us) is an obligate resident on human bodies, Palopoli et al. wanted to see how its evolution mirrored that of its human hosts. So they sampled D. folliculorum from 70 humans distributed over the globe (except in North America), and looked at the evolutionary relationship between the mites by sequencing a segment of their mitochondrial DNA. Here’s how they cluster based on geographic origin and evolutionary relationship; the colors of the segments show where the hosts are from (key at top), and their evolutionary relationships are in the “tree graph” inside the circle:
What does this show? First, that there are four “clades” or evolutionarily-related groups, labeled A-D in the diagram. Second, these clades in general cluster according to geographic origin (As and Bs, for instance, come from Africa and Asia respectively, Cs are mostly African, and Ds are largely of European origin); but note that there is variation within each geographic area, as displayed in the map below:
This shows that, in general, these mites have evolved in geographic isolation from each other, as expected if they had a fairly ancient origin and co-evolved with humans as they migrated over the globe.
It also shows that the mites probably jumped onto humans in Africa, for that is where they are most genetically diverse, and thus probably older. As humans moved out over the planet from Africa, the genetic variation drops. That makes sense because only a limited number of people left Africa about 60,000 years ago; ergo a limited sample of their hair mites went with them.
There’s one exception to this, though: Latin Americans have a genetically diverse sample of mites—nearly as diverse as Africans. But that makes sense if you think about it, for Latin America harbors a human mixture of diverse origin: Europeans, Africans (most brought as slaves), and Native Americans. If they had sampled the U.S., I suspect they’d also find a diverse mixture, though not as diverse as Latin America (we have fewer Native Americans).
There are a few other results of interest. Mites are stable on individual humans for a long time, as found by sampling the mites on single people over several years. That makes sense too, since they reproduce on our bodies. Also, when an individual moves between continents, say someone born in Africa who moves to the US, the ancestral mite genotypes remain stable: such individuals harbor mites having DNA that’s found only in Africa. (An alternative hypothesis is that different human ethnic groups impose different selection pressures on their hair mites, so the close and persistent match between mite and human genotypes could reflects natural selection as well as evolutionary history.)
The notion that there’s not much movement between individuals, though, is supported by looking at mite DNA from human families. The authors found that mite genotypes were often shared between members of families, but rarely between ethnically similar individuals from different families. That also makes sense because humans in the same family have a lot more skin-to-skin or hair-to-skin contact than do humans of different families.
How old is the divergence between genetically different D. folliculorum mites? Using a “molecular clock” (i.e., the amount of DNA divergence coupled with the shaky assumption that, in these mites, DNA diverges at about the same linear rate it does in other arthropods), the authors calculate that the major mite clades diverged between 3 and 4 million years ago. That is, the mite groups above come from a common ancestor about as old as the genus Homo itself—or older. And that, in turn, implies that these little guys have been with us since we were small, hairy apelike denizens of the savanna. But even if the clock isn’t calibrated correctly, they’re still old, antedating our movement out of Africa.
After reading the paper and writing all this, I’m strongly tempted to go to the lab, pull out a few eyelashes, and examine them under the microscope. Like many biologists, I’m not squeamish about what I’ll find. After all, I did harbor a 1.5 inch botfly maggot in my head for almost two months!
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Palopoli, M. F., D. J. Fergus, S. Minot, D. T. Pei, W. B. Simison, I. Fernandez-Silva, M. S. Thoemmes, R. R. Dunn, and M. Trautwein. 2015. Global divergence of the human follicle mite Demodex folliculorum: Persistent associations between host ancestry and mite lineages. Proceedings of the National Academy of Sciences (USA). Dec. 14, 2015, online; doi: 10.1073/pnas.1512609112
Oh! Squeeee! Do check your lashes under the microscope, and be sure to take plenty of fresh film. We want pictures!
Now I know that arachnids crawl all over my body, I will probably die within a week due to heart failure caused by a lack of sleep. Seriously, the gene pool needs a lifeguard.
I was pleased to note that according to the diagrams in this post, NZers do NOT have hair mutes! 🙂
I think I just found a reason to do a Trump on ALL immigrants. 🙂
Surely that means all the New Zealanders of European ancestry leave and only the Maori are allowed to stay? 😛
How do the mites of Maori compare with other (Poly-| Micro-) nesians?
Weeeeell, there is a small movement of people who say that, so if they’re reading this, perhaps they’ll include it in their propaganda. 🙂
No you have them; they just didn’t sample New Zealanders. I can guarantee that these things are swarming in your eyelashes at this very moment.
You know I was kidding mate, yet still you add to my distress! 🙂
Although the truth is one of the few things I’ve managed to learn from science is I’m covered in creepy crawlies, and it doesn’t worry me, and it was fascinating to learn some more about some of them. So a genuine “thanks” for that Jerry. 🙂
Horton Hears A Who was my favorite children’s book. Makes the thought of being colonized by microscopic critters not all that objectionable.
What!?! Ugh, creepy thought, but I guess if I’ve had them all this time & survived, I’ll make it awhile longer…
Bleech … now I get the whole shaved-head thing. First time I’ve ever wanted to be-like-Mike in that respect.
I’ll be scratching like a third-base coach the rest of the day now.
It doesn’t matter if you shave your head: they’re in your eyelashes, too!
There is a common SF trope that “Hoomins who invade other planets will be denuded of their microflora before shipping out, ‘to control the new ecosystem.'”
On a scale from ‘humility’ to ‘hubris’, that strikes ma as pretty close to ‘hubris.’ Also, “unlikely to end well.”
“Baby in a bubble” stories come around from time to time. But to genuinely achieve a microfauna stripping, you’d need to crew a “generation ship” with bubble babies. Or maybe, give up on the idea, already.
Has a “bubble baby” ever made it to reproductive age, however defined, let alone reproduced?
Including the fauna in the human digestive tract?
They best stock up on the Dannon yogurt and sweet acidophilus milk for the ride back.
AIUI, bubble babies are immuno-compromised; that doesn’t mean they don’t have microflora. I don’t think humans could survive without it, plus one imagines that our co-evolved inhabitants don’t set off the immune system in the first place.
There’s a lot we know that we don’t know about the interactions between gut, gut flora and immune system. It’s a very active research area.
My bet is that the SF trope is “not even wrong”, though we may be decades or centuries from being able to show why, with confidence.
Still rather a cool premise for a story, though.
I’m trying to remember /why/ the trope was developed. For Asimov’s ‘Lije Bailey stories, it seemed to be just to increase the difficulty of the detective (‘Lije) getting to the scene of the crime(s). In general SF it has been a reason for space hoomins to not have to worry about disease. Off the top of my head, the only time I can think of “deflora-ing” people having been a significant plot element is in “RingWorld”, where Niven actualy attacks the trope by proposing a mutated gut bacterium producing a bug that eats a mythical room-temperature superconductor, and brings about the fall of the first Hoominoid civilisation on the Ring. But he later recants and makes the RT-SC eating bug the product of biological warfare by the Puppeteers.
Looking back, it may be just the sort of thinking that was going around in the 1950s and 60s, which had astronauts sitting in quarantine, and meteorites being examined for microbes of non-terrestrial origin. And of course, Hoyle and Wickrandomsinghe seeing space invaders in every nasal drip.
And after fictional bacteriology, I suppose I’d better sterilise and help Dad with the overburdening of our digestive tracts.
Aw, your Dad cooked dinner? 🙂
What is the check on the population of these critters? I imagine they have an unlimited food supply, and no predators — why is it that although we all harbor them, the numbers are so low that we don’t even notice?
The check is probably INTRA-specific competition. The organism MOST likely to need the same resoources as you is your sibling, not a 3-headed dood from Betelgeuse. Despite Zaphod’s rishithra inclinations to Trillian. Or Kirk’s beastiality with [names too numberous to mention, but there is probably a Wikipedia list]
Ha! — Kirk’s “bestiality”. I had never thought of it quite that way.
But my point was: with a pretty much unlimited amount of hair available (well, formerly, anyway) — an all-you-can-eat buffet for the little buggers — how come my head isn’t seething with them?
That’s my question too. There are predator mites, but they are usually bigger than their prey. If there are mites that prey on these things, I am surprised we don’t notice them.
Interesting question, and I have no answer. But maybe for the ones that live on hair follicles they are limited by the # of hair follicles.
What’s wrong with the idea of them reaching carrying capacity? Only so many sebaceous glands to inhabit, say, or so many dry skin cells to eat. (Though judging by my dry skin cells, that would mean I’m short quite a few million mites…)
How much of us is actually “human” once you’ve separated out the fauna and the flora that have permanent residence and / or squatter rights?
By mass, around 90% ; by population count, about 10%
That assumes a 1:1000 volume (and mass) ratio for prokaryotes vs eukaryotes, which is a good starting point.
[SIGH] Sis to me “will the delivery wait? We’re away for a few days,”
Me to Sis : “It’s good for one, but not two, geosynclinal cycles. Call it 400 million years.”
I mean, she’s had decades to get used to me! Whole decades!
It’s kind of fun to explain about these mites to a biology class. People start rubbing their eyes and generally squirming.
My partner and I had to use an anti-Demodex wash for a couple months as she was treated for possible causes of eye irritation. The doctor showed us photos of the mites and eggs at the bottom or her lashes, but I couldn’t see them as thoroughly as I’d have liked because she was utterly grossed out.
After all that, it turned out that although the Demodex may have contributed to the irritation, the basic cause was allergy to our cat! We had to find him a new home. His new place is very good for him, but we both miss him.
The honourable gentleperson needs a microscope in his home. Like any good home, really (thinking about a reflected light microscope ..) .
And a reasonable stock of materiel and knowledge for preparing adequate slides.
Sorry, a thought pattern that is currently active.
Stupid question – who of the 37510 followers of this blog has a favourite rock which they’d like to examine further? Again, a current conversation elsewhere.
Yes to your “stupid question”. Can you provide a link to where this is “a current conversation elsewhere”?
Around the table at the local (for Dad) Field Society’s Microscopy section. I’m not aware if either the society or the Microscopy Section has a website.
When I came down to visit the family, Dad had this monthly meeting arranged, and so I decided to set up his petrological microscope for a change from the normal biological materials (Dad is FLS, naturally ; I’m FGS). After dealing with unexpected issues (the small range of wavelengths emitted by LED lamps makes interference colours and figures much different to under incandescent lights), I eventually got the scope set up for the first time in a decade or more … and then had to come up with suitable mineral specimens.
Inspiration struck, and I applied the “Professor Levitating Frog” trick to a specimen of mica and of selenite. Which worked sufficiently well.
But then the question of how to do thin sections arose.
ebaY has since done it’s stuff, but with the average petrological slide going there for around £10 … that’s painful. And getting a commercial maker to section your favourite rock is going to be more like £40. So I’ve been trying to figure out ways to bring this back towards a sane, amateur-friendly price.
Sorry – so the conversation is in real life. Me, Dad, Uncle Roger, and a couple of others.
Totally agree. And there are hundreds of microscopes needing foreverhomes on ebay.
Aw, that’s a sad ending, sedgequeen!
Is the itching feeling one gets when discussing mites cultural or genetic?
Anybody ever done a study to see if the general population believes that *they* couldn’t possibly have these living on them?
You are asking a HIGHLY biased population.
To de-bias (somewhat), you’d need to ask who here knows someone # who is unfamiliar with the biological sciences # that would belive that they could (etc).
Doing a vox pop may be easier, and less inaccurate.
Sorry, should have specified the population I was thinking of was not WEIT reader, but more along the line of people who see the virgin mary on wall, but don’t believe we landed on the moon, etc. That sort of population.
Maybe this is addressed in the paper. Since this is a human-specific species, but what with the spectrum of different hair characteristics in humans, what is it about human hair vs. deer and mice that keeps them human-specific? Or is it something else about the habitat?
Maybe it has more to do with the skin or secretions than the hair itself? Just a guess, assuming they don’t actually eat the hair.
“keeps them human-specific”?
Wrong question, I suspect.
Do the organisms in question come into sufficiently frequent contact with the other (host) species that ONE alternative-possible-host can carry enough of the invading species that two of opposite genders can meet and bred.
I did a related thought experiment a while ago, looking at humans arriving on Australia on debris-rafts from an Indonesian volcano/ earthquake/ tsumami. Giving it 14 years from birth to (male) fertility (small over-estimate), and 45 years individual survival time, I ball-park that one transport event per CENTURY would result in colonisation once the first (newly) pregnant female arrived. Any higher rate of individual arrival would reduce the initial hiatus, but not be terribly important otherwise, as long as the geneflow continued. After the population reached a coouple of thousand, the immigrants woould barely matter.
I went through this mental exercise aftr reading about the success (measure : pups surviving per litter) change in a “coloniser” population of wolves in Southern Sweden. A single wolf crossed the divide between populations and spread his genes everywhere. The research (based on shit DNA, to track individuals and ther parentage) was published about 8 years ago.
Given the millennia over which humans & canines have been sharing beds…
(Interesting thought experiment & wolf story!)
This mite story is reminiscent of the lice story. As I recall, body lice are of two kinds – head and pubic. They would have started as a single species on our early hairy bodies but when the sea of hair shrunk leaving two islands, voila, you have isolation and speciation.
The data now show that they aren’t different species, as they exchange genes prolifically and body lice are not monophyletic with respect to head lice. We can think of them as “ecotypes”, and one can turn into the other in just two generations when forcibly put in the opposite niche.
That’s an interesting development. As I recall, originally the case had been used to date when humans lost their hair. Maybe that doesn’t apply any longer.
“when humans lost their hair” – sounds like Kipling.
So Pediculosis capitis and Phthirius pubis are one and the same?
I learned those off the label on Kwell shampoo when I was an orderly at Alexandria Hospital, summers between college terms, and would be designated to take newly-admitted “House” patients to the shower to go thru de-lousing. They did the lathering, I just made sure they did, but I was still a tad worried that one might somehow find its way onto me.
I have a microscope and looked at my hair and eyelashes and didn’t see any mites (this was a decade or so ago). I obviously couldn’t discern them. Now that I know what they look like, I’ll have to give it another go. I have looked at my semen and sperm are fun to watch. 😉 For the most part, I enjoy looking at the water from my turtle pond- those little drops of water are teaming with different life forms- there’s a world down there!
I would like to get another microscope with better lighting (mine is reflected light) and an eyepiece that can accommodate a digital camera of some sort. I don’t know if there is an eyepiece mount for a digital SLR. I have a pretty good camera on my smartphone; maybe that would work. I’ll have to do some research.
I got my 11-year-old son a microscope a few years back. A good reflected light micro. Decent ones are remarkably cheap now (well, I don’t balk at a (really, really cool) $200 gift for my kid.)
We spent a lot of time looking at the pond water and, wow, is it full of life! Very fun.
As many a student knows, you can hold your cell phone camera up to the eyepiece and take a reasonable picture. The photomacrography forum web site (http://www.photomacrography.net/) has lots of people who rig up cameras to microscopes. They can give you lots of suggestions.
Equal parts fascinating and terrifying. Thanks (I think).
We searched for demodex in my undergrad parasitology class. My eyelashes yielded no results, but using a note card folded in half, and scraping the folded trough across my forehead yielded one! Fascinating to look at under a scope. Approximately half of the class found one more.
Reblogged this on My Selfish Gene and commented:
I have previously written about the human microbiota. Here is an excellent post from Jerry Coyne’s Why Evolution is True about the evolution of human hair mites. And just like bacteria covering pretty much every accessible exterior and interior space on our body these tiny guys love to hang out anywhere there is hair.
Enjoy!
I suppose the old adage, “it was a problem you never knew you had,” sits nicely here.
We all have a bacterial layer too, hell the mites do too.
It’s stuff all the way down. 🙂
Mike
It looks to me like they did; that’s what the pale blue dots in the inner ring represent: people born in North America of European, Asian, or African ancestry.
What they apparently didn’t sample is people born in North America of Native American ancestry (nor Aussies of Aboriginal ancestry).
Oooh, creepy-crawlies.
I’ll just have to think of them as arachnids. I’m OK with spiders 😉
cr
“So they sampled D. folliculorum from 70 humans distributed over the globe (except in North America),…”
There is no “except”, actually the majority of the samples appear to be from North America (based on the colourful chart you included in the post).
I can understand our mites being closest related to those on mice, but why deer? Why not dogs or cats, something with which we also cohabitate? In many places, our ancestors slept with cows, pigs, and other farm animals, piling them into the house in winter for heat, but our mites are most closely related to deer mites. Can it be solely from wearing them as skins?
I wonder if mites can identify a sinking ship (i.e., the deer skin they’re on is no longer alive) which then causes them to actively seek other hosts; unlike those that are still on their original living organism?
As the man said, “we are each a multitude”.