Humanity is experiencing an unexpected evolutionary process. But it’s not our bodies that are evolving. Instead, new research explains, the diminutive mites that live in the skin on our faces are evolving to become one with us.
Here’s a piece of unpleasant news: there are mites on your face, right now. They live there. In fact, they meet other mites and (maybe) fall in love and have tiny mite babies on your face. Some of them also live on your nipples. They are passed on during birth from our mothers and are present on virtually every human being alive today, growing in numbers as our pores become larger in adulthood. For breakfast, lunch, and dinner, they munch on the sebum — a fatty substance — released by our pores, and are most active at night.
The first-ever genome sequencing study of these mites, known as Demodex folliculorum, shows that they are slowly making the transition from external parasites to internal symbionts. This evolutionary direction is fueled by inbreeding and the resulting shedding of genes this causes.
Evolving towards integration
“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,” says Dr. Alejandra Perotti, Associate Professor in Invertebrate Biology at the University of Reading, who co-led the research. “These changes to their DNA have resulted in some unusual body features and behaviors.”
The team explains that due to their isolated, sheltered existence — there are no known external threats for these mites, nor direct competitors — as well as having virtually no exposure to other mites with different genes, D. folliculorum has been experiencing a process of genetic reduction.
Essentially, they have been losing parts of their genome, becoming extremely simple organisms: currently, they move on legs powered by only 3, single-cell muscles. They also function using the narrowest range of proteins ever seen in any of its related species, according to the team. Even their nocturnal behavior is a product of this genetic reduction: they have lost any protection from ultraviolet light. Furthermore, these mites have lost the genes that encode the production of melatonin, a compound that makes small invertebrates active at night, and instead rely on the melatonin our skin secretes at dusk.
Other shifts make the mites highly adapted to life on human skin, but helpless anywhere else. Their reproductive organs have moved anteriorly, and the tiny invertebrates need to cling to human hair to be able to copulate. A particular arrangement of their mouth appendages let them more easily access sebum in our pores.
Unlike other parasites, the mites contain more cells when young compared to their adult stage. This, the team explains, shows that the species has already started becoming symbionts.
In the long run, however, the lack of exposure to mates that could add new genes to their genomes means that D. folliculorum is at an evolutionary dead end and may even become extinct. While such a process has been observed in unicellular organisms before, this is the first time it’s been noted in multicellular animals.
All in all, it seems that the mites are well on their way to becoming completely dependent on human beings to survive. Their genetic isolation and hyperspecialization is very likely to result in the species entering a dependent relationship with us. Although it may be icky to think that our faces are inhabited by these mites, they can also benefit us to an extent. Normal populations of D. folliculorum help maintain skin health as they consume dead cells, and unclog our pores of excess sebum.
“Mites have been blamed for a lot of things. 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,” explains Dr. Henk Braig, co-lead author from Bangor University and the National University of San Juan.
The paper “Human follicular mites: Ectoparasites becoming symbionts” has been published in the journal Molecular Biology and Evolution.
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