New Finds on the Systematics of Cycloposthiid Ciliates (Ciliophora: Entodiniomorphida: Cycloposthiidae) Based on New 18S-rDNA Sequences from a Brazilian Capybara.

The family Cycloposthiidae (Entodiniomorphida) comprises ciliated protists that are symbionts of the gastrointestinal tract of several herbivore mammals, such as rodents, elephants, equids, primates, hippopotamus, marsupials, rhinoceros and tapir, where they contribute to the digestion of their host's plant-based diet. Despite the significance of these ciliates to the evolution of their hosts, many characters used in the taxonomy of the group are homoplastic and most of the valid species do not have molecular data available. For these reasons the systematics of this family is poorly understood. Here, we sequenced the 18S-rDNA of ten cycloposthiids, including nine Cycloposthium spp. and Monoposthium cynodontum, all of them isolated from the cecum of a Brazilian capybara. Our phylogenetic analyses indicate that the family Cycloposthiidae might be polyphyletic, while M. cynodontum and Cycloposthium spp. constitute a single monophyletic group. Given the great morphological and molecular similarities between members of M. cynodontum and Cycloposthium ciliates, it is possible that this species, although it has been described in the genus Monoposthium, is actually a Cycloposthium ciliate.