Phylogenetic analyses of cyclidiids (Protista, Ciliophora, Scuticociliatia) based on multiple genes suggest their close relationship with thigmotrichids.

Cyclidiids and thigmotrichids are two diverse groups of scuticociliates, a diverse clade of ciliates that is often difficult to investigate due to the small size and conserved morphology among its members. Compared to other groups (e.g. hypotrichs and oligotrichs), the scuticociliates have received relatively little attention and their phylogenetic relationships are largely unresolved. To contribute to our understanding of their evolutionary history, we characterized 26 sequences for three linked genes (SSU-rDNA, 5.8S and LSU-rDNA) from 14 isolates of cyclidiids and thigmotrichids. Phylogenetic analyses reveal the following: (1) traditional cyclidiids are associated with thigmotrichs rather than pleuronematids as expected; (2) the validity of the newly-reported genus Falcicyclidium is confirmed by the molecular data and we suggest to transfer this genus to the family Ctedoctematidae; (3) both the genera Cyclidium and Protocyclidium are not monophyletic and the separation of Protocyclidium from Cyclidium is not supported; (4) the genus Cristigera is a well supported monophyletic group and may stand for a new family; (5) according to both morphological and molecular information, Cyclidium plouneouriDragesco, 1963 should be assigned in the genus Falcicyclidium and thus a new combination is suggested: Falcicyclidium plouneouri (Dragesco, 1963) n. comb.; and (6) based on the data available, a new genus is suggested: Acucyclidium gen. nov. with the type species, Acucyclidium atractodes (Fan et al., 2011a) n. comb.