Study of tiny parasites points to gene loss from adaptation putting them on dead-end evolutionary course

Gliding through grease, and protected by our pores, tiny Demodex folliculorum mites lead a secretive life within our skin, only emerging at night to mate on our foreheads, noses and nipples. Successful as these sexual encounters are, their days as independent parasites may be numbered.

The first ever genome sequencing study of these mites appears to have caught them in the process of transitioning to internal symbionts, entirely dependent on us for their existence. Eventually, this process may even lead to their extinction.

Measuring just 0.3mm long, D folliculorum are carried by roughly 90% of people, and are most abundant in the wings of the nose, the forehead, the ear canal and the nipples. They lead a harmless life, feasting on the sebum naturally secreted by cells in the pores, and are likely to have been present since early life, having been transferred from our mothers during birth or breastfeeding.

“The long association with humans might suggest that they could have simple but important beneficial roles, for example, in keeping the pores in our face unplugged,” said Dr Henk Braig from Bangor University and the National University of San Juan in Argentina, who led the research.

To better understand this relationship, Braig and his colleagues sequenced the genomes of D folliculorum mites, collected from a person’s nose and forehead using a blackhead remover – with each collection yielding about 40 mites.

Their findings, published in Molecular Biology and Evolution, revealed that the mites survive with the minimum repertoire of proteins – the lowest seen in any insect, arachnid or crustacean so far.

This gene loss has resulted in an extreme reduction in the number of cells in adult mites – a likely first evolutionary step in their journey towards adopting an entirely symbiotic lifestyle within our tissues.

The more they adapt to us, the more genes they are likely to lose, until eventually they may become entirely dependent on us. And with no opportunity to gain additional genes from less closely related mites – they don’t appear to transfer between adult humans during close physical contact – their isolated existence and resulting inbreeding may ultimately have set the mites on course for an evolutionary dead end, and potential extinction.

If this ever happens, it could be bad news for us as well. “They are associated with healthy skin, so if we lose them you could face problems with your skin,” co-author Dr Alejandra Perotti from the University of Reading, told the BBC.