Morphology and phylogeny of three trachelocercids (Protozoa, Ciliophora, Karyorelictea), with description of two new species and insight into the evolution of the family Trachelocercidae

Abstract

Although trachelocercid ciliates are common in marine sandy intertidal zones, methodological difficulties mean that their biodiversity and evolutionary relationships have not been well documented. This paper investigates the morphology and infraciliature of two novel Trachelolophos and one rarely known form, Tracheloraphis similisRaikov and Kovaleva, 1968, collected from the coastal waters of southern and eastern China. The small subunit (SSU) rRNA gene sequences of two of the species are presented, allowing the phylogenetic position of the genus Trachelolophos to be revealed for the first time. Phylogenetic analyses based on SSU rRNA gene sequences indicate that Trachelolophos branches with Kovalevaia and forms a sister clade with the group including Prototrachelocerca, Trachelocerca and Tracheloraphis. The monophyly of Trachelocerca is not rejected by the approximately unbiased (AU) test (P=0.209,>0.05) but that of Tracheloraphis is rejected (P=3e-033,<0.05). The newly sequenced genus Trachelolophos, and recent studies on the morphology and phylogeny of the family Trachelocercidae, suggest two new hypotheses about the evolution of the seven genera within Trachelocercidae, based on either infraciliature or molecular evidence. Both hypotheses suppose the compound circumoral kineties in the oral apparatus is a plesiomorphic feature while the single circumoral kinety is synapomorphic. More evidence is still needed, however, as to whether the closed circumoral kinety with no brosse feature in Trachelocerca is ancestral or secondarily reduced.