We examined the vector competence of the tick, Ornithodoros moubata, using laboratory-reared gerbils as hosts. Transmission of the relapsing fever agent Borrelia duttonii occurred efficiently from infected ticks to uninfected gerbils and from infected gerbils to uninfected ticks. Spirochetes were maintained stably in the ticks for at least 3 months, but they disappeared from the bloodstream of infected gerbils after three episodes of spirochetemia. We also examined transovarial transmission of B. duttonii during the gonotrophic cycle and filial generation. No spirochetes could be detected from the offspring generation of the ticks by culture and polymerase chain reaction (PCR) methods, although spirochetes were still found in the female ticks. The results indicate that, because of the rarity of transovarial infection, the role of transovarial passage of B. duttonii to eggs and larval O. moubata ticks is limited in maintaining B. duttonii. Our findings strongly suggest that B. duttonii is maintained through the O. moubata tick-human transmission cycle in tick-borne relapsing fever endemic areas.

The gene content of a mitochondrial (mt) genome, i.e., 37 genes and a large noncoding region ( LNR ), is usually conserved in Metazoa. The arrangement of these genes and the LNR is generally conserved at low taxonomic levels but varies substantially at high levels. We report here a variation in mt gene content and gene arrangement among chigger mites of the genus Leptotrombidium . We found previously that the mt genome of Leptotrombidium pallidum has an extra gene for large-subunit rRNA ( rrnL ), a pseudo-gene for small-subunit rRNA ( PrrnS ), and three extra LNR s, additional to the 37 genes and an LNR typical of Metazoa. Further, the arrangement of mt genes of L. pallidum differs drastically from that of the hypothetical ancestor of the arthropods. To find to what extent the novel gene content and gene arrangement occurred in Leptotrombidium , we sequenced the entire or partial mt genomes of three other species, L. akamushi , L. deliense , and L. fletcheri . These three species share the arrangement of all genes with L. pallidum , except trnQ (for tRNA-glutamine). Unlike L. pallidum , however, these three species do not have extra rrnL or PrrnS and have only one extra LNR . By comparison between Leptotrombidium species and the ancestor of the arthropods, we propose that (1) the type of mt genome present in L. pallidum evolved from the type present in the other three Leptotrombidium species, and (2) three molecular mechanisms were involved in the evolution of mt gene content and gene arrangement in Leptotrombidium species.

To better understand the evolution of mitochondrial (mt) genomes in the Acari (mites and ticks), we sequenced the mt genome of the chigger mite, Leptotrombidium pallidum (Arthropoda: Acari: Acariformes). This genome is highly rearranged relative to that of the hypothetical ancestor of the arthropods and the other species of Acari studied. The mt genome of L. pallidum has two genes for large subunit rRNA, a pseudogene for small subunit rRNA, and four nearly identical large noncoding regions. Nineteen of the 22 tRNAs encoded by this genome apparently lack either a T-arm or a D-arm. Further, the mt genome of L. pallidum has two distantly separated sections with identical sequences but opposite orientations of transcription. This arrangement cannot be accounted for by homologous recombination or by previously known mechanisms of mt gene rearrangement. The most plausible explanation for the origin of this arrangement is illegitimate inter-mtDNA recombination, which has not been reported previously in animals. In light of the evidence from previous experiments on recombination in nuclear and mt genomes of animals, we propose a model of illegitimate inter-mtDNA recombination to account for the novel gene content and gene arrangement in the mt genome of L. pallidum.