Insects in general are an incredibly successful and diverse group. Part of their success is undoubtedly due to mutualistic primary endosymbiotic bacteria, which have enabled insects to radiate into niches that include nutrient-poor diets (5
), such as wood (e.g., termites), plant sap (e.g., aphids), and blood (e.g., sucking lice in the Anoplura
). Primary endosymbiotic bacteria (as opposed to secondary endosymbionts [S-endosymbionts]) are normally maintained inside specialized host cells called mycetomes (9
), usually exhibit nucleotide A+T bias greater than 50%, and show elevated sequence evolution with respect to their free-living counterparts (4
). These endosymbionts are required for host survival, and they provide nutrients that are not available in the insect's specialized diet (5
). Most primary endosymbionts (P-endosymbionts) leave their mycetomes to migrate to the ovaries so that they may be incorporated into developing eggs (transovarial transmission) and thus be passed onto the next host generation (9
), leading to long-term, shared coevolutionary histories between the insects and their symbionts.
It is estimated that there are 14,000 species of hematophagous insects (1
), but only a few P-endosymbionts have been described for these insects (e.g., Wigglesworthia
). Because blood is a nutrient-poor diet, all sucking lice likely have some form of endosymbiont and many are reported to have obligate primary endosymbionts based on microscopic observation. The P-endosymbiont in the human head and body louse (Pediculus humanus
) was first seen over 340 years ago (15
) with some of the very first microscopes. It has a complex migration (5
) that involves four different mycetomes and two extracellular migrations as it moves to the eggs in the adult female lice (28
). This migration was first observed by Ries (35
) and later shown by scanning and transmission electron microscopy by Eberle and McLean (11
is the only other bacterium that has been found among human lice (19
The complex migration associated with transovarial transmission stands as potential evidence of the importance of the relationship between the P-endosymbiont and the lice. If the mycetome is removed from a young female louse, she dies after only a few days and her eggs are deformed (3
). Furthermore, if the bacteria are removed from the eggs directly, the larvae survive only a few days (2
). Puchta (32
) demonstrated that lice without P-endosymbionts were able to survive if their diet was supplemented with nicotinic acid, pantothenic acid, and beta-biotin, suggesting a basis for a mutualistic long-term relationship.
Humans have three types of lice, head and body lice (Pediculus humanus
), which are currently classified as two distinct subspecies (Pediculus humanis capitis
and Pediculus humanis humanus
), and pubic lice (Pthirus pubis
). Body lice are known to transmit three diseases: louse-borne epidemic typhus (LBET), relapsing fever, and trench fever (6
). Although head lice can transmit LBET in a laboratory setting (14
), there has never been evidence of LBET transmission by head lice in nature. Currently, it is not known why one subspecies of P. humanus
can transmit three deadly bacterial agents while the other subspecies, epidemic in schoolchildren, effectively cannot. Although the secondary endosymbiont of tsetse flies (Sodalis glossinidius
) has been shown not to affect the ability of its host to transmit Trypanosoma congolense
), it is possible that there are differences in the endosymbiotic bacteria of head and body lice and that these differences may reinforce patterns of disease transmission in human lice.
Primate lice show a history of cospeciation with their hosts and have been used effectively to infer human evolutionary history (18
). Most of this work has relied upon mitochondrial DNA (mtDNA) from the lice and to a lesser extent on nuclear markers. If a single lineage of endosymbiont were found among primate lice, the endosymbiont could serve as still another marker of human and primate evolutionary history. In addition, this new three-tiered assemblage of primates, lice, and endosymbiotic bacteria would yield a new system in which to study relative and absolute rates of evolution in three disparate lineages (vertebrates, insects, and bacteria).
In this study, we describe the molecular characterization of the P-endosymbiont of primate lice, including head and body lice (Pediculus humanus
) characterized by Sasaki-Fukatsu et al. (38
), and present new data from chimpanzee lice (Pediculus schaeffi
) and human pubic lice (Pthirus pubis
). We used the full-cycle rRNA approach, including comparative 16S rRNA gene analysis and the detection of endosymbionts within the host cell by means of fluorescent in situ hybridization using specific 16S rRNA-targeted oligonucleotide probes. By sampling across a phylogenetically diverse assemblage of louse species, we expected to capture a greater percentage of the diversity in this lineage of P-endosymbiont.