The human louse, Pediculus humanus (Phthiraptera: Anoplura), has been of great concern to public health throughout human history. It is one of the most ancient haematophagous ectoparasites which are intimately related to its host [1]. Two ecotypes can infest Homo sapiens: Pediculus humanus corporis and Pediculus humanus capitis. P. h. corporis, known as the body louse, infests people living in poor hygienic conditions and is the principal vector of Rickettsia prowazekii (epidemic typhus agent), Borrelia recurrentis (relapsing fever agent), Bartonella quintana (trench fever agent) [2, 3] and probably Yersinia pestis (pandemic plague agent) [4]. P.h. capitis, known as the head louse, has a widespread infestation regardless of the hygiene conditions[ 5,6]. However, its capacity as a potential vector of disease remains misunderstood [7]. The genetic diversity of human lice has been extensively investigated using mitochondrial genes (cytochrome b [cytb], cytochrome oxidase subunit 1 [cox1] and 12S ribosomal RNA [12S]), allowing their classification into six divergent clades that may be grouped in three sister groups (A–D, B–F and C–E), each exhibiting a specific geographical distribution [8]. Clade A is the most prevalent with a worldwide distribution, while clade D is only found in central Africa. Clade B is reported in all continents, while genetically close clade F has a geographically restricted distribution and was recently reported in South America. In addition, clade C has been identified in lice from central Africa and Asia, whereas clade E is reported in central and west Africa [6–9].
Like all haematophagous lice, P. humanus directly depends on the presence of endosymbiotic bacteria to supplement their unbalanced diet and metabolic integration [9]. Symbiosis is a common widespread phenomenon, which strongly affects the biology of haematophagous arthropods. This interaction encompasses a whole range of possible symbiotic associations, from strict parasitism to obligate mutualism [10]. Body and head lice host the same primary endosymbiotic bacteria (Candidatus Riesia pediculicola) [9], which is a member of the Gamma-proteobacteria class, Enterobacteriaceae family within the Candidatus Riesia genus [11, 12]. The symbiont is housed in mycetome that is localized on the ventral side of the lice midgut. In adult females the developing ovum is also infected for transovarial transmission to progeny [9, 11]. Genomic study of the human body louse and its primary endosymbiont has provided new insights into Candidatus R. pediculicola [11]. This bacterium has a small genome (< 600 genes) containing a panel of genes encoding for the synthesis of essential B-group vitamins that are crucial for the host’s diet [13]. Indeed, the symbiont supplements the host with thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin and folate (B1, B2, B3, B5, B6, B7 and B9 vitamins) [13, 14]. In addition, removal of the mycetome in Pediculus females leads to their death a few days later, as well as to the production of deformed eggs [10, 13]. Moreover, Candidatus R. pediculicola can be targeted for possible lice control strategies, especially with the absence of antibiotic resistance genes [10, 13].
Human and chimpanzee lice (Pediculus schaeffi) diverged from a common ancestor, as did their human (Homo sapiens) and chimpanzees hosts (Pan troglodytes), somewhere between approximately five and seven million years ago. Interestingly, Candidatus R. pediculicola shared a common ancestor with the P. schaeffi endosymbiont (Candidatus R. pediculischaeffi), roughly 5.4 million years ago [14, 15]. The evaluation of this co-evolutionary association between louse and their endosymbionts might provide new insights into human evolution [15]. Furthermore, phylogenetic studies have shown a higher sequence similarity between clade A-head and body lice endosymbionts compared to a lower similarity in clade A and B-head lice endosymbionts. These results suggest that the endosymbionts co-evolved with their hosts’ clades [15].
In this study, we aimed to establish a co-evolutionary relationship between the endosymbiotic bacteria and their human lice hosts from different clades using molecular approaches. We investigated mitochondrial genes from the six divergent clades of human lice and the housekeeping genes of Candidatus R. pediculicola, in order to determine the lice clade using its endosymbiont bacteria.