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Louse Genetic Diversity Shows Pattern of Lice, Human Migrations

NEW YORK (GenomeWeb News) – Louse populations from around the world fall into different genetic clusters based on geography, researchers from the Florida Museum of Natural History reported in PLOS One yesterday.

The louse is thought to be an ancient human parasite — lice have been found within Incan and Egyptian mummies, with the oldest known louse remains dating back about 10,000 years, as Marina Ascunce, a postdoctoral researcher and the first author on the PLOS One study, noted in an email to GenomeWeb Daily News.

Because the louse, or Pediculus humanus, has co-existed with people for so long, its evolution could be used to fill in some blanks about human migrations. In this most recent paper, based on microsatellite analysis, the researchers reported that lice from Central America and Asia are more closely related than Central American and North American louse populations are. This led the researchers to speculate that Central American lice were brought by people migrating from Asia while the North American lice likely arrived with European settlers.

"This study shows that the study of the genetic diversity of human lice can help us better understand patterns of human migration worldwide at different temporal scales," Ascunce said.

Louse infestations, the researchers added, are on the rise as lice are becoming resistant to the insecticides used to control them. In addition, there are two kinds of lice that affect humans, the head louse, Pediculus humanus capitis de Geer, that mainly affects children and the clothing louse, Pediculus humanus humanus Linnaeus, that mostly affect homeless populations or people living in refugee camps. Clothing lice, and increasingly head lice, can serve as vectors for disease, like epidemic typhus and relapsing fever.

To examine the genetic structure of louse populations, Ascunce and her colleagues first dug through the P. humanus genome to look for DNA stretches that could serve as microsatellite markers. They uncovered nearly 300,000 tandem repeat motifs, including dinucleotides, trinucleotides, and tetranucleotides, and tested about 150 primer pairs, eventually focusing on a set of 15 loci.

The researchers then validated those markers on 93 lice samples that they and their colleagues collected from 11 sites across the world.

Through a Bayesian clustering approach, the researchers found that the lice populations they sampled belonged to four genetic clusters: one cluster contained clothing lice from Canada; another cluster was made up of North American and European head lice; head lice from Honduras comprised another cluster; and Asian head and clothing lice made up the final cluster. While they reported little evidence of gene flow among the populations, the researchers noted that there was some gene flow between Central American and Asian lice populations.

"Although preliminary, our study suggests that the Central America-Asian cluster is mirroring the (human host) colonization of the New World if Central American lice were of Native American origin and Asia was the source population for the first people of the Americas as has been suggested," Ascunce and her colleagues wrote. "The USA head louse population might be of European decent, explaining its clustering with lice from Europe. Within the New World, the major difference between USA and Honduras may reflect the history of the two major human settlements of the New World: the first peopling of America and the European colonization after Columbus."

Additionally, the clothing lice populations from Canada and Nepal appeared to be more closely related to the Central America-Asian cluster, a finding the researchers said is consistent with the theory that clothing lice arose from one mitochondrial haplogroup of head lice about 83,000 years ago.

They added, though, that they were "puzzled" by the distant genetic relationship between the Canadian clothing lice population and the New York head lice population.

Both head and clothing lice showed heterozygosity deficits and other patterns indicating inbreeding, which, the researchers noted, is not surprising for a parasite that spends its life on a single host.

That, plus the low levels of gene flow observed between populations, has implications for controlling lice as different insecticide resistance alleles may have arisen in different populations.

"In human lice, because of the strong genetic structure with low gene flow among populations, it seems more likely that human lice will follow a model of parallel adaptive evolution, where new resistance alleles evolve in different populations," the researchers wrote. They added that regional epidemiological studies many be needed to find ways to control lice.

Such findings, the researchers argued, show that their microsatellite panel is useful for investigation both lice and human evolution. Studies of lice could be used, Ascunce added, "as a proxy to understand certain aspects of human evolution that were unclear from studies of direct human evidence, such as fossil or molecular data."