From parasite to symbiote
Hair follicle mites unite with humans
Von Hedvika Nyarsik
They are born on us, feed on us, mate on us and die on us: Tiny hair follicle mites are so dependent on humans that they gradually develop from a parasite to a symbiote, the researchers found. This is not necessarily a disadvantage.
They are about 0.3 millimeters long and live on us, with us, us: hair follicle mites of the species Demodex folliculorum. Most people host these mites, which spend most of their short lives hanging upside down in our hair follicles, especially on the face. In fact, humans are the only habitat for Demodex folliculorum. Their entire life cycle revolves around eating dead human skin cells.
New research now suggests that the survival of Demodex folliculorum is so dependent on humans that microscopic mites are in the process of evolving from an ectoparasite to an internal symbiote – one beneficial relationship with their host for both. In other words, these mites gradually merge with our bodies, so they live permanently in and on us.
For the first time, scientists have been able to create a complete DNA analysis of our small roommates. By sequencing the genomes, Alejandro Perotti’s team can draw conclusions about mating habits, body traits and the evolutionary future of Demodex folliculorum. “We found that these mites have a different arrangement of genes for body parts than other similar species because they have adapted to life in a protected environment in the pores,” explains a biologist at the University of Reading in the United Kingdom. “These changes in their DNA have led to some unusual bodily features and behaviors.”
Hair follicle mites reside in our hair follicles on the face, lashes and nipples. During its short two-week life, it feeds on sebum secreted by cells in the pores – their only source of nutrition. They are not exposed to food competitors, enemies or other threats.
Night sex with mites on our faces
This isolated existence, in which they do not compete with other mites and infest other hosts, has led to genetic reduction, according to a new study published in the journal Molecular Biology and Evolution. That is: they have become extremely simple organisms, their genome has been reduced to the most essential. His tiny legs propel only three single-celled muscles, and his body has the absolute minimum of protein – just enough to survive. Demodex folliculorum therefore has the lowest number of proteins ever observed in this or any related species.
Gene reduction is also the reason for their nocturnal behavior, the study said. Hair follicle mites only climb under the veil of darkness to crawl slowly and laboriously across the skin, find a partner and hopefully mate before returning to the safe darkness of the follicle.
Because mites lack UV protection. They also lost a gene that causes them to wake up in daylight. At the same time, however, they are unable to produce melatonin – a substance that makes small invertebrates active at night. The owner will take care of it for you. At night, people release melatonin through the skin, which is then ingested by Demodex folliculorum. Only in this way can they mate and reproduce.
The sex life of hair follicle mites is also somewhat unusual. Unlike other mites, the reproductive organs have moved to the front of the body due to a special arrangement of genes, with the male penis protruding forward and up from the back, the researchers write. This means that in order to mate, they must stand under the female and attach to human hair.
Challenging mating is important for the survival of small creatures. However, the potential gene pool is very small: there are very few opportunities to expand genetic diversity. This could mean that hair follicle mites are on their way to an evolutionary impasse, the study authors suggest. At worst, they could die out. Something like this has already been observed in bacteria that live in cells, but never in animals, according to studies.
What doesn’t seem like a big loss at first glance is not necessarily an advantage for people. Until now, the scientific world has assumed that Demodex folliculorum does not have an anus and instead that feces accumulate in their bodies, which are released after the mite’s death and can cause skin diseases. The research team refutes this in their study. Hair follicle mites have an anus that was probably overlooked in the past due to its small size.
“Mites have been accused of many things,” says Henk Braig, co-author of the study and zoologist at the University of Bangor in Wales. “However, their long connections with people could also indicate that they may have simple and useful tasks, such as keeping the pores open on our faces.”
And yet another misconception about this type of mite was refuted in the study. Mites have many more cells in the nymph stage, ie when they are young, than later in the adult stage. This contradicts the previous assumption that parasitic animals reduce their cell numbers at an early stage of development. All this, according to research, shows that mites are on their way from parasites to symbionts.