Evolutionary relationships of "Candidatus Riesia spp.," endosymbiotic enterobacteriaceae living within hematophagous primate lice

Abstract

The primary endosymbiotic bacteria from three species of parasitic primate lice were characterized molecularly. We have confirmed the characterization of the primary endosymbiont (P-endosymbiont) of the human head/body louse Pediculus humanus and provide new characterizations of the P-endosymbionts from Pediculus schaeffi from chimpanzees and Pthirus pubis, the pubic louse of humans. The endosymbionts show an average percent sequence divergence of 11 to 15% from the most closely related known bacterium "Candidatus Arsenophonus insecticola." We propose that two additional species be added to the genus "Candidatus Riesia." The new species proposed within "Candidatus Riesia" have sequence divergences of 3.4% and 10 to 12% based on uncorrected pairwise differences. Our Bayesian analysis shows that the branching pattern for the primary endosymbionts was the same as that for their louse hosts, suggesting a long coevolutionary history between primate lice and their primary endosymbionts. We used a calibration of 5.6 million years to date the divergence between endosymbionts from human and chimpanzee lice and estimated an evolutionary rate of nucleotide substitution of 0.67% per million years, which is 15 to 30 times faster than previous estimates calculated for Buchnera, the primary endosymbiont in aphids. Given the evidence for cospeciation with primate lice and the evidence for fast evolutionary rates, this lineage of endosymbiotic bacteria can be evaluated as a fast-evolving marker of both louse and primate evolutionary histories.

FIG. 1.

Phylogenetic tree based on Bayesian phylogenetic analysis of 10 million generations, produced using MrBayes. Posterior probabilities above 0.50 are shown above nodes. Endosymbionts of primate lice are monophyletic, with a posterior probability of 1.00. GenBank accession numbers are given in parentheses.

FIG. 2.

Fluorescent in situ hybridization microphotograph of thorax and abdomen (ventral view) of a second-instar nymph of a head louse (Pediculus humanus capitis). This whole-insect mount was probed with the species-specific probe designed from the 16S rRNA gene endosymbiont sequence obtained from another individual of P. humanus capitis. Note the bacteria inside the mycetome, shown in yellow. Bar = 50 μm.