Human follicular mites: ectoparasites that become symbionts Summary Most humans carry mites in their skin hair follicles throughout their lives. Follicular mites are the only metazoans that live continuously on humans. We propose that Demodex folliculorum (Acari) represents a transitional stage from an obligate host-damaging parasite to an obligate symbiont. Here, we describe the profound impact of this transition on the mite genome and physiology. Genome sequencing revealed that permanent host association of D. folliculorum led to extensive genome reduction through relaxed selection and genetic drift, resulting in the smallest number of protein-coding genes yet identified. among panarthropods. Confocal microscopy revealed that this gene loss coincided with an extreme reduction in cell number. Individual uninucleated muscle cells are sufficient to operate each of the three segments that make up each walking leg. Although it has been assumed that the reduction in cell number in parasites begins early in development, we identified a greater total number of cells in the late stage of development (nymph) than in the terminal adult stage, suggesting that the reduction It begins in the adult or last stage of development. This is the first evolutionary step in an arthropod species that adopts a reductive, parasitic or endosymbiotic lifestyle. Somatic nuclei show underreplication in the diploid stage. New ocular structures or photoreceptors, as well as a unique human host melatonin-guided day/night rhythm, are proposed for the first time. The loss of DNA repair genes coupled with extreme inbreeding could have put this mite species on a dead-end evolutionary trajectory. |
The image shows the Demodex folliculorum mite on the skin under the Hirox microscope
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The first comprehensive DNA analysis of mites that live in the hair follicles of all humans reveals explanations for their strange mating habits, body characteristics and evolutionary future.
Microscopic mites that live in human pores and mate on our faces at night are becoming such simplified organisms because of their unusual lifestyles that they could soon become one with humans, new research has found.
Mites are transmitted during birth and are carried by almost all humans, with peak numbers in adults as the pores enlarge. They are about 0.3 mm long, found in the hair follicles of the face and nipples, including the eyelashes, and eat the sebum that the cells naturally release into the pores. They activate at night and move between the follicles looking to mate.
The first genome sequencing study of the D. folliculorum mite found that its isolated existence and resulting inbreeding are causing it to shed unnecessary genes and cells and move toward a transition from external parasites to internal symbionts .
Dr Alejandra Perotti, associate professor of invertebrate biology at the University of Reading, who co-led the research, said: "We found that these mites have a different arrangement of body part genes to other similar species because they "They adapt to a protected life inside the pores. These changes in their DNA have resulted in some unusual body characteristics and behaviors."
The in-depth study of Demodex folliculorum DNA revealed:
- 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 turned them into extremely simple organisms with tiny legs powered by only 3 single-celled muscles. They survive with the minimum repertoire of proteins, the lowest number ever seen in this and related species.
- This gene reduction is also the reason for their nocturnal behavior . The mites lack UV protection and have lost the gene that causes the animals to wake up in daylight. They also have not been able to produce melatonin, a compound that makes small invertebrates active at night; However, they can fuel their mating sessions throughout the night using melatonin secreted by human skin at dusk.
- Their unique genetic disposition also results in unusual mating habits of the mites. Their reproductive organs have previously moved, 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 while both clinging to human hair.
- One of their genes has been inverted, giving them a particular arrangement of mouth appendages that stick out more for collecting food. This helps their survival at a young age.
- Mites have many more cells at a young age compared to their adult stage. This contradicts the previous assumption that parasitic animals reduce their cell number early in development. The researchers argue that this is the first step for mites to become symbionts .
- Lack of exposure to potential mates who could add new genes to their offspring may have put the mites on the path to an evolutionary dead end and possible extinction . This has been observed before in bacteria living inside cells, but never in an animal.
- Some researchers had assumed that mites do not have an anus and therefore must accumulate all their feces throughout their lives before releasing it when they die, causing skin inflammation. However, the new study confirmed that they do have an anus and have therefore been unfairly blamed for many skin conditions .
The research was led by Bangor University and the University of Reading, in collaboration with the University of Valencia, the University of Vienna and the National University of San Juan. It is published in the journal Molecular Biology and Evolution .
Dr Henk Braig, co-lead author from Bangor University and St John’s National University, said: “Mites have been blamed for many things. "The long association with humans could suggest that they could also have simple but important beneficial roles , for example in keeping the pores on our faces unclogged."
