Endosymbionts of ticks and their relationship to Wolbachia spp. and tick-borne pathogens of humans and animals

TLDR

The study examined the presence, distribution, and phylogenetic placement of endosymbiotic bacteria in four specific‑pathogen‑free tick species. PCR assays targeting 16S/23S rRNA and specific Wolbachia, Rickettsia, and Francisella genes were performed on the ovaries and Malpighian tubules of Ixodes scapularis, Rhipicephalus sanguineus, Haemaphysalis longicornis, and Ornithodoros moubata. Endosymbionts were found in all four tick species, with Ixodes scapularis harboring Rickettsia‑related alpha‑proteobacteria in its ovaries, while the other species carried gamma‑proteobacteria related to Coxiella burnetii and Francisella tularensis in ovaries and Malpighian tubules; these symbionts are distinct from insect Wolbachia and appear to be ancient and widespread.

Abstract

The presence, internal distribution, and phylogenetic position of endosymbiotic bacteria from four species of specific-pathogen-free ticks were studied. These included the hard ticks Ixodes scapularis (the black-legged tick), Rhipicephalus sanguineus (the brown dog tick), and Haemaphysalis longicornis and the African soft tick Ornithodoros moubata. PCR assays for bacteria, using two sets of general primers for eubacterial 16S and 23S rRNA genes (rDNAs) and seven sets of specific primers for wolbachial, rickettsial, or Francisella genes, indicated that I. scapularis possessed symbiotic rickettsiae in the ovaries and that the other species harbored eubacteria in both the ovaries and Malpighian tubules. Phylogenetic analysis based on the sequence of 16S rDNA indicated that the symbiont of I. scapularis belonged to the alpha subgroup of proteobacteria and was closely related to the members of the genus Rickettsia. The other species had similar microorganisms in the ovaries and Malpighian tubules, which belonged to the gamma subgroup of proteobacteria, and formed a monophyletic group with the Q-fever pathogen, Coxiella burnetii. O. moubata harbored another symbiont, which formed a monophyletic group with Francisella tularensis and Wolbachia persica, the latter a symbiont previously isolated from Malpighian tubules of the soft tick Argas (Persicargas) arboreus. Thus, the symbionts of these four tick species were not related to the Wolbachia species found in insects. The two symbionts that live in the Malpighian tubules, one closely related to C. burnetii and the other closely related to F. tularensis, appear to be of ancient origin and be widely distributed in ticks.