Trends in Parasitology
Volume 28, Issue 12, December 2012, Pages 563-571
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Review
Biology and genetics of human head and body lice

https://doi.org/10.1016/j.pt.2012.09.003Get rights and content

Head lice and body lice have distinct ecologies and differ slightly in morphology and biology, questioning their taxonomic status. Over the past 10 years many genetic studies have been undertaken. Controversial data suggest that not only body lice but also head lice can serve as vectors of Bartonella quintana, and a better understanding of louse epidemiology is crucial. Here, we review taxonomic studies based on biology and genetics, including genomic data on lice, lice endosymbionts, and louse-transmitted bacteria. We recommend that studies of human lice employ morphological and biological characteristics in conjunction with transcriptomic date because lice seem to differ mainly in gene expression (and not in gene content), leading to different phenotypes.

Highlights

► Head and body lice taxonomy has long been studied based on morphology and biology ► These past 10 years, lice taxonomy has been studied based on genetics ► We review differences between human head and body lice ► We include genomic data on lice, lice endosymbiont, and lice infesting bacteria ► Lice differ mainly in gene expression leading to different phenotypes.

Section snippets

Human lice

The order of Phthiraptera (lice) is divided into two main groups: the sucking lice that comprise the Anoplura suborder and the chewing lice that comprise three other suborders: Amblycera, Ishnocera, and Rhynchophthirina (Figure 1) [1]. Lice are obligate ectoparasites, and each host species has its own type of louse [2]. Indeed, parasite speciation often occurs at approximately the same time as speciation of the host (cospeciation). The two genera of sucking lice that parasitize humans are

Human lice taxonomy before molecular biology

The morphology and biology of head and body lice, as reported over several decades, were used to assess their taxonomic status (Table 1).

Chromosome structure

Genome sequencing of the human body louse [13] confirmed that body lice and head lice have the smallest genomes of any insect reported to date (108 Mb for females and 109 Mb for males), as previously estimated by flow cytometry in 2007 [31]. Lice are diploid organisms that have six chromosomes (five metacentric chromosomes and one telocentric chromosome) [32]. The average guanine-cytosine (GC) content of the P. humanus genome is 28%, making this genome unusually AT rich. Transposable elements

Generalities about the endosymbiont

The human louse endosymbiont is a bacterium belonging to the family Enterobacteriaceae in the γ-Proteobacteria class. Its closest relatives are species in the genus Arsenophonus, and it was termed Candidatus Riesia pediculicola [12]. Many studies were undertaken during the past 5 years on louse endosymbionts 38, 39, 40. The microorganism is primarily located in a disc-shaped organ located on the ventral side of the midgut (the mycetome) and is transmitted from the female louse to its progeny

Genomic data on louse-infesting bacteria

As mentioned above, three main intracellular bacteria are transmitted by lice: R. prowazekii, B. quintana, and B. recurrentis [4]. Interestingly, in addition to all being highly pathogenic, these bacteria share another common characteristic: an unusually reduced genome size compared to close relatives. Hence, B. recurrentis appears to be a degraded subset of the tick-borne relapsing fever-causing agent Borrelia duttonii [45]. In addition, B. quintana is described as a genomic derivative of the

Genetic studies of human head and body lice

Genetic tools questioned the division of human lice into head lice and body lice (Table 2). The first study was based on the 18S rRNA gene [52], and subsequent studies focused on mt genes 53, 54, 55, 56 and intergenic spacers 24, 57. These studies revealed that there are three clades of head lice, one of which may also be body lice (Clade A) 53, 54. Recently, a transcriptome study of human head and body lice revealed that there is only one gene that is present in body lice but not in head lice.

Concluding remarks

Body lice are only found in one lineage (Clade A). The theory that body lice evolved from head lice when humans began to wear clothes [56] is incompatible with genetic studies. The data suggest that evolution of body lice from head lice, and vice versa, takes place constantly among Clade A lice, and that this evolution is facilitated by mass infestations (Figure 3, Figure 4). This finding is strengthened by the identification of body louse nits in the cap of a homeless person that may have

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