Phylogeny of trichostome ciliates (Ciliophora, Litostomatea) endosymbiotic in the Yakut horse (Equus caballus).

Ciliates of the subclass Trichostomatia inhabit the fermentative regions of the digestive tract of herbivores. Most available small subunit ribosomal RNA (SSrRNA) gene sequences of trichostomes are from species isolated from the rumen of cattle or sheep and from marsupials. No ciliate species endosymbiotic in horses has yet been analyzed. We have sequenced the SSrRNA genes of five ciliate species, isolated from the cecum and colon of four Yakut horses: Cycloposthium edentatum, Cycloposthium ishikawai, Tripalmaria dogieli, Cochliatoxum periachtum, and Paraisotricha colpoidea. Based on their morphology, Cycloposthium, Tripalmaria, and Cochliatoxum are classified as Entodiniomorphida, while Paraisotricha is considered a member of the Vestibuliferida. Phylogenetic analyses using Bayesian inference, distance, and parsimony methods confirm these placements. The two Cycloposthium species cluster together with the published Cycloposthium species isolated from a wallaby in Australia. Tripalmaria and Cochliatoxum branch as a sister group to or basal within the Entodiniomorphida. The Vestibuliferida remain paraphyletic with Paraisotricha and Balantidium branching basal to all other trichostome species, but not closely related to Isotricha and Dasytricha.