What the Mites on Your Face Say About Where You Came From

David Shultz, Science Mag, December 14, 2015

Right now, deep in your hair follicles and sweat glands, tiny mites are feeding on dead skin cells, mating, and laying eggs. The microscopic arthropods, known as Demodex folliculorum, live on virtually all mammals–especially their faces–and cause no harm under most circumstances. Now, a new study shows that people of different ancestry carry different subgroups of the bugs, and that the mites’ distribution throughout the global population may even reflect how our species has migrated and evolved over the course of history.

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In order to get a handle on the diversity within the mite population, scientists collected samples from 70 human hosts. The participants were from diverse backgrounds, with European, Asian, African, and Latin American ancestries, and the team analyzed DNA from the mites’ mitochondria (the energy-generating parts of cells) and looked for differences in the sequence. Overall they found four distinct groups, or “clades,” of mite mitochondrial DNA. The results suggest that your ancestry seems to determine which mite clades live on your body, the team reports online today in the Proceedings of the National Academy of Sciences. People of African descent, for example, had a mixture of all the different types, whereas people of European descent tended only to have mites from one group.

{snip} Though it was the least sampled geographic area, people of African descent had the most diverse mites, possessing all four clades. From there, the authors theorize that people (carrying their mites) spread out to the other geographic regions and that, along the way, certain groups of the mites didn’t make it.

“As they diverged into Asia and Europe, some individual lineages were lost,” explains the study’s first author Michael Palopoli, an evolutionary biologist at Bowdoin College in Brunswick, Maine. Thus, all European mites are mitochondrial type D. Human genes show a pattern similar, in some respects, to Africans possessing the most diverse genomes. Exactly when these die-offs occurred is impossible to know from the experiment, but he suggests that future studies could give us a clearer idea of the timeline.

The researchers also found that each individual’s mite population was stable over time periods as long as 3 years–even in people who had moved to new regions of the world with different dominant clades. Furthermore, the mite profiles appear to be passed across generations–a second generation person of African descent living in Europe will most likely retain the mites of her ancestors, rather than acquire a European profile.

The stability of the mite populations, especially between generations, suggests to the authors that differences between the hosts’ skin may explain why people from different regions have different subsets of the bugs. Human populations differ in skin hydration, hair follicle density, and lipid production. These differences have likely arisen over evolutionary time, and the authors theorize that the changes may have given certain mite clades a competitive advantage in certain skin types. Ancient Europeans, for example, may have acquired mutations in their skin that heavily favored the D clade over A, B, or C.

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