The Family Borreliaceae (Spirochaetales), a Diverse Group in Two Genera of Tick-Borne Spirochetes of Mammals, Birds, and Reptiles

Spirochetes of the family Borreliaceae are, with one exception, tick-borne pathogens of a variety of vertebrates. The family at present comprises two genera: Borrelia (Swellengrebel), which includes the agents of relapsing fever, avian spirochetosis, and bovine borreliosis, and Borreliella (Gupta et al.), which includes the agents of Lyme disease and was formerly known as 'Borrelia burgdorferi sensulato complex'. The two genera are distinguished not only by their disease associations but also biological features in the tick vector, including tissue location in unfed ticks and transovarial transmission. Borrelia species transmitted by argasid (soft) ticks tend to have more exclusive relationships with their tick vectors than do other Borrelia species and all Borreliella species that have ixodid (hard) ticks as vectors. The division of genera is supported by phylogenomic evidence from whole genomes and by several specific molecular markers. These distinguishing phylogenetic criteria also applied to three new species or isolates of Borrelia that were discovered in ixodid ticks of reptiles, a monotreme, and birds. Although the deep branching of the family from other spirochetes has been a challenge for inferences about evolution of the family, the discovery of related microorganisms in the gut microbiota of other arachnids suggests an ancestral origin for the family as symbionts of ticks and other arachnids.