NEW YORK (GenomeWeb) – Genetic lineages of microscopic mites living in human hair follicles largely match the divergence patterns in human populations, according to a study appearing online this week in the Proceedings of the National Academy of Sciences.
"They aren't just bugs on our faces, they are storytellers," senior author Michelle Trautwein, a comparative genomics researcher with the California Academy of Sciences, said in a statement. "Mites tell us about our own ancient history — it's a complex story, and we've only just scratched the surface."
Trautwein led a team of researchers from the US, Spain, and Denmark that did mitochondrial genome sequencing on more than 200 Demodex folliculorum mites isolated from 70 human hosts living in different parts of the world. Placing these mite mitochondrial sequences in a phylogeny, they identified deeply divergent mite lineages. Among them: lineages that are mainly found in individuals with African, Asian, Latin American, or European ancestry.
"The continent where a person's ancestry originated tended to predict the types of mites on their faces," Trautwein said.
Moreover, she explained, the genetic findings suggest mites don't move easily from one individual to another. Instead, mite lineages are mainly shared by individuals in close contact with one another, with the same lineages sometimes persisting across generations.
Past studies of organisms living in, on, or near humans — for instance, bacteria, lice, or rodents — have offered clues to past human population histories, Trautwein and her co-authors explained. As such, they reasoned that they might glean similar clues from studying the genetics of Demodex mites found in human hair follicles and sebaceous glands.
"Demodex mites are more tightly associated with human bodies than are lice, while their generation times are slower than those of bacteria and viruses but are faster than those of rodents, making them a complementary system by which to understand the evolution of both humans and human associates," they wrote.
For their analysis, the researchers focused on the Demodex species that's found relatively close to the skin's surface in hair follicles. They amplified and sequenced some 930 bases of mitochondrial DNA sequence from mites isolated from 37 human hosts with European ancestry, 18 hosts of Asian descent, seven hosts with African ancestry, and eight Latin American hosts.
With the resulting 241 mtDNA sequences — including sequences for three-dozen mites collected from one European individual over three years — the team found that mite mtDNA sequences fell into clusters that coincided with human host ancestry.
In particular, the researchers identified four main mite clades known as A, B, C, and D. Whereas most individuals with European ancestry carried clade D mites in their hair follicles, clade A and B mites were absent in the Europeans. On the other hand, hosts of African, Asian, or Latin American descent were more likely to have D. folliculorum from the A and B clades.
Despite the follicle mites' proximity to the skin surface, their results suggest that mites from the same lineage tend to persist in individuals from a given population, even when they relocate to a region where mites from another lineage are typically more common.
"We found that mite lineages can persist in hosts for generations. Even if you move to a faraway region, your mites stick with you," Trautwein noted.
More broadly, the mite lineages observed seemed to show genetic divergence patterns that fit with known human migration histories, including human migration out of Africa. As with humans, the mites showed declining genetic diversity with increasing distance from Africa.
"Overall," the authors concluded, "D. folliculorum evolution reflects ancient human population divergences, is consistent with an out-of-Africa dispersal hypothesis, and presents an excellent model system for further understanding the history of human movement."