Image showing Demodex folliculorum mite on skin under Hirox microscope. Credit: University of Reading
The mites that live in our faces at night are becoming simpler due to their unusual lifestyles, which may lead to them becoming one with humans.
The mites are passed on during birth and are carried by almost every human as they get older. They are small and are found in the hair on the face and nipples, as well as in the eyelashes. They are active at night, moving between their follicles.
The first ever study of the D. folliculorum mite found that their isolated existence is causing them to shed unneeded genes and cells and move towards a transition from external parasites to internal symbionts.
Dr. Alejandra Perotti, Associate Professor in Invertebrate Biology at the University of Reading, who co-led the research, said, "We found these mites have a different arrangement of body part genes to other similar species." Some strange body features and behaviors have been caused by the changes to their genes.
Demodex folliculorum mite under a microscope walking. Credit: University of Reading
There was an in-depth study of the sample.
Due to their isolated existence, with no exposure to external threats, no competition to infest hosts and no encounters with other mites with different genes, genetic reduction has caused them to become extremely simple organisms with tiny legs powered by just 3 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 to be awakened by daylight. 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 inverted, giving them a particular arrangement of mouth-appendages that are especially protruding for gathering food. This aids their survival at young age.
The mites have many more cells at a young age compared to their adult stage. This counters 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 symbionts.
The lack of exposure to potential mates that could add new genes to their offspring may have set the mites on course for an evolutionary dead end, and potential extinction. This has been observed in bacteria living 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.
Microscope image of the posterior end of the anus of a Demodex folliculorum mite. The presence of an anus on this mite had been wrongly overlooked by some previously, but this study confirmed its presence. Credit: University of Reading
Image shows unusually-positioned penis of a Demodex folliculorum mite. Credit: University of Reading
Microscope image of the posterior end of the anus of a Demodex folliculorum mite. The presence of an anus on this mite had been wrongly overlooked by some previously, but this study confirmed its presence. Credit: University of Reading
Image shows unusually-positioned penis of a Demodex folliculorum mite. Credit: University of Reading
Bangor University, the University of Reading, the University of Valencia, the University of Vienna, and the National University of San Juan collaborated on the project. The journal is about biology and evolution.
"Mites have been blamed for a lot of things," said Dr. Henk Braig, co-lead author from Bangor University. The long association with humans might suggest that they can have simple but important beneficial roles, such as keeping the pores in our face unplugged.
More information: Gilbert Smith et al, Human follicular mites: Ectoparasites becoming symbionts, Molecular Biology and Evolution (2022). DOI: 10.1093/molbev/msac125 Journal information: Molecular Biology and Evolution
