Systematic position and phylogenetic relationships of the genera Bursaridium, Paracondylostoma, Thylakidium, Bryometopus, and Bursaria (Ciliophora: Colpodea)

The morphology and in fraciliature of Paracondylostoma cavistoma oligostriatum ssp. n. (differs from P. cavistoma cavistoma by non-overlapping morphometric characteristics), P. setigerum chlorelligerum ssp. n. (differs from P. setigerum setigerum by having symbiotic green algae), and Bursaridium pseudobursaria were studied in live and silver-impregnated specimens. Paracondylostoma and Bursaridium are sister groups due to a unique synapomorphy, namely, a circumoral ciliary ribbon produced by narrowly spaced somatic kinetids at the anterior end of the somatic kinetids. Bursaridium differs from Paracondylostoma by the euplanktonic mode of life and the paroral membrane, the middle portion of which has very loosely spaced kinetids. Based on the morphological details discovered, a Hennigian phylogeny of the genera Bursaridium, Paracondylostoma, Thylakidium, Bryometopus, and Bursia in is proposed. These genera are linked by four synapomorphies, namely: ( I) an apical oral opening secondarily lost in Bryometopus, which ventralized the oral apparatus: (2) a ventral vestibular cleft occupied by the ventralized oral structures in Bryometopus; (3) a conspicuous adoral zone of adoral organelles; and (4) a simple paroral membrane composed of a row of dikinetids secondarily amplified to a conspicuous field of short, dikinetidal kineties in Bursaria, which is thus derived.