Genetic diversity of human head lice (Pediculus humanus capitis) across the Americas

The human louse (Pediculus humanus) is a highly specialized blood-sucking insect that lives in the human body (head or clothes). Over millions of years, this close association has led to coevolution in which louse diversification parallels that of humans. Therefore, we can use louse molecular data to infer the evolutionary history of lice and also of their human hosts. Despite its ubiquitousness, the genetic diversity of the human louse worldwide is poorly understood. In this study, we analyzed mitochondrial DNA sequences and microsatellite variability from 450 head lice in the Americas. Haplotypes clustered into two well-supported haplogroups, known as Clade A and B, respectively. We found 12 mitochondrial haplotypes belonging to Clade A and 25 in Clade B. Haplogroup frequencies differ geographically. Haplogroups A and B were both relatively common in North America, with 58% haplogroup A and 42% haplogroup B. In Central America, haplogroup A was much less common than haplogroup B (18% and 82%, respectively). In contrast, 95% of the head lice from South America belonged to Clade A with only 5% of the haplotypes from Clade B. We suggest that multiple colonization events throughout the Americas could account for the differing haplogroup frequencies seen today. These events could include both the first peopling of the New World and more recent European colonizations. Microsatellite analysis revealed high genetic structure in the nuclear genome even at micro-spatial scales (e.g., among classmates in different grades at a single school).