From parasite to symbiont
Hair follicle mites become one with humans
by Hedviga Nyarsik
06/22/2022, 8:28 PM
They’re born on us, they feed on us, they mate on us, and they die on us: Researchers have found that tiny hair follicle mites are so dependent on humans that they gradually evolve from a parasite to a symbiont. This is not necessarily a defect.
It is about 0.3 mm long and lives with us, with us, by us: Demodex folliculorum hair follicle mite. Most people host these mites, which spend most of their short lives hanging upside down in our hair follicles, primarily on the face. In fact, humans are the only habitat of Demodex folliculorum. Their entire life cycle revolves around eating dead human skin cells.
New research now suggests that Demodex folliculorum is so dependent on humans for its survival that the microscopic mite is in the process of evolving from an external parasite to an endosymbiont – a beneficial relationship with its host. In other words, these mites gradually merge with our bodies so that they live permanently in and on us.
For the first time, scientists have succeeded in creating a complete DNA analysis of our young roommates. By sequencing the genome, Alejandra Perotti’s team can draw conclusions about mating habits, body traits, and the evolutionary future of Demodex follicularis. “We found that these mites have a different gene arrangement for body parts than other similar species because they have adapted to life protected in the pores,” explains the biologist from the University of Reading in the UK. “These changes in their DNA led to some unusual body traits and behaviors.”
Hair follicle mites are found in hair follicles on the face, eyelashes, and nipples. During its short two-week lifespan, it feeds on the fats secreted by cells in the follicles – their only source of nourishment. They are not exposed to food competitors, enemies, or any other threats.
Night time sex is a mite on our faces
This isolated presence, in which other mites neither compete nor infect other hosts, led to epigenetic reduction, according to the new study, published in the journal Molecular Biology and Evolution. This means that they have become very simple organisms, and their genomes have come down to basics. Its tiny legs are powered by only three single-celled muscles, and its body contains minimal protein – just enough to survive. Therefore Demodex folliculorum contains the fewest number of proteins ever observed in this species or related species.
The study says gene reduction is also the reason for their nocturnal behaviour. Hair follicle mites just emerge under a blanket of darkness to crawl slowly and laboriously through the skin, find a mate and hopefully mate before returning to the safe darkness of the follicle.
Because mites lack protection from UV rays. They’ve also lost the gene that makes them wake up in broad daylight. However, at the same time, they cannot produce melatonin – a substance that makes small invertebrates active at night. The owner takes care of this for you. Humans secrete melatonin through their skin at night, which is then eaten by Demodex folliculorum. This is the only way for them to mate and reproduce.
The sex life of hair follicle mites is also somewhat unusual. Unlike other moths, the researchers wrote, the genitals have shifted to the front of the body due to a special genetic arrangement, with the male’s penis protruding forward and upward from the back. This means that in order to mate, he must position himself under the female and stick to human hair.
Evolutionary dead end
Hard mating is important to the survival of the young creatures. But the pool of potential genes is very small: there are very few opportunities to expand genetic diversity. The study authors believe this may mean that hair follicle mites are on their way to an evolutionary dead end. At worst, they could die. The study says that something like this has already been observed in bacteria that live in cells, but has never been observed in an animal.
What doesn’t seem like a big loss at first isn’t necessarily an advantage for people. Until now, the scientific world assumed that Demodex folliculorum does not have an anus and instead accumulates feces in its body, which is released after the death of the mite and can cause skin diseases. The research team refutes this in their study. The hair follicle mite has an anus, which may have been overlooked in the past due to its small size.
“Moths have been blamed for a lot,” says Henk Braig, co-author of the study and a zoologist at Bangor University in Wales. “However, their long association with humans may also indicate that they may have had simple, useful tasks, such as keeping the pores on our face open.”
However another misconception about this type of mite was refuted in the study. Moths have more cells in the nymph stage, that is, when they are young, than in the adult stage. This contradicts the previous assumption that parasitic animals reduce their cell number early in development. According to the research, all this shows that mites are on the way from parasites to symbionts.