Parasitic mites that live on our faces have simplified genetically

They say that no man is an island, but to the tiny mites that inhabit our skin, we are the island of their dreams. These microscopic mites (Demodex folliculorum) live in the pores and hair follicles on the face, including the eyelashes, and the nipples of almost every human being. They are only around 0.3mm long and are passed on to us at birth. From then on, they live in our pores, feed on the sebum that is naturally produced in the cells of the pores, and reproduce on our skin in the darkness of night. 

A new study, led by researchers from the University of Reading has found that life for the mites living on a person’s face is not all roses, however. Their isolated existence results in inbreeding during the lifetime of the host person, which leads to the loss of genes and cells in the parasites. In the study, D. folliculorum genomes were studied for the first time ever, and the results showed that the mites are becoming so simple, genetically and anatomically, that they can be considered in transition from external parasites to internal symbionts.  

“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. These changes to their DNA have resulted in some unusual body features and behaviors,” explained Dr. Alejandra Perotti, who co-led the research.

The study of the D. folliculorum DNA, published in the journal Molecular Biology and Evolution, makes the following fascinating (and bizarre) findings concerning the mites:

• Due to their isolated existence on a single person’s face and nipples, with no exposure to external threats, no competition to infest hosts and no encounters with other mites with different genes, the mites have experienced genetic reduction. This has caused them to become extremely simple organisms, with tiny legs, each powered by just three single-cell muscles. They survive with the minimum repertoire of proteins – the lowest number ever seen in this and related species.
• This gene reduction is the reason for their nocturnal behavior too. The mites lack UV protection and have lost the gene that causes animals wake up when it is daytime. They have also been left unable to produce melatonin – a compound that makes small invertebrates active at night. However, they are able to fuel their all-night mating sessions using the melatonin secreted by human skin at dusk.
• Their unique gene arrangement also results in the mites’ unusual mating habits. Their reproductive organs have moved anteriorly, and males have a penis that protrudes upwards from the front of their body meaning they have to position themselves underneath the female when mating, and copulate as they both cling onto the human hair.
• One of their genes has been inverted, giving them a particular arrangement of mouth-appendages that stick out and enable them to feed. This aids their survival when they are young.
• The mites have many more cells when they are young than they do as adults. This contradicts the previous assumption that parasitic animals reduce their cell numbers early in development. The researchers argue this is the first step towards the mites becoming symbiotic inhabitants within the cells of the skin.
• The lack of exposure to potential mates that could add variety to their gene pool means that the mites may well be on course for an evolutionary dead end, and potential extinction. This has been observed in bacteria that live inside cells before, but never in an animal.
• Some researchers had assumed the mites do not have an anus and therefore must accumulate all their feces through their lifetimes before releasing it when they die, causing skin inflammation. The new study, however, confirmed they do have anuses and so have been unfairly blamed for many skin conditions.

Research collaborators from the University of Valencia, University of Vienna and National University of San Juan also contributed to the study.

Dr Henk Braig, co-lead author from Bangor University and the National University of San Juan, said: “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.”

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By Alison Bosman, Earth.com Staff Writer