Ultrastructure of Apocolpodidium etoschense (Ciliophora) and its Systematics Enlightenment for the Class Nassophorea

The Class Nassophorea is not monophyletic with unsolved relationship of four orders, which calls for discussion to combine morphological features and molecular phylogeny. In the present study, the ultrastructure of Apocolpodidium etoschense in the order Colpodidiida is first studied. Comparisons between orders of Nassophorea were conducted and a discussion of systematics was performed based on a SSU rRNA gene-based phylogeny. The order Colpodidiida and Nassulida shared the following features: Two pairs of alveolocysts in the cortex, the presence of a ‘‘B-cartwheel’’ in the distal region of the kinetosome, the presence of cytostomal lamellae and subcytostomal lamellae in the cytopharyngeal basket, and spindle trichocysts with a simple tip. These similarities shape a core group of Nassophorea, which are morphologically and genetically different from the order Microthoracida. Consequently, Microthoracida should be regarded as an independent taxon rather than a member of Nassophorea. Within the core group of Nassophorea, Colpodidiida as an independent order is further validated by its delicate cytopharyngeal basket which lacks nematodesmal lamellae; while the non-monophyly of the order Nassulida might be explained by differentiation of the cartwheels in kinetosomes and the arrangement of kinetosomes with postciliary microtubules in the nassulid organelle 3 within its members.