Toward a clarified taxonomy of Parentocirrus hortualis (Ciliophora, Hypotrichia): an integrative analysis of ontogenesis, ultrastructure, morphology, and molecular phylogeny.

There is a paucity of ultrastructural data on hypotrichous ciliates. In the present work, a comprehensive investigation of the ultrastructure, morphology, morphogenesis, and molecular phylogeny of a hypotrichous species, Parentocirrus hortualis, was performed. Historically, the taxonomy of Parentocirrus has been complicated by morphological variability. While P. brasiliensis is nearly identical to the type species P. hortualis in most respects, it differs in the presence of additional dorsal kinetids, a key distinction that now appears unstable. Consequently, we propose that P. brasiliensis should be recognized as a synonym of P. hortualis. In terms of the ultrastructure of P. hortualis, we found that: (1) the semi-rigid or rigid cortex of stylonychine species might be related to the presence of a single or double layer(s) of subpellicular microtubules, respectively; (2) the highly degenerated kinetosome of the dorsal bristle unit, i.e., the parakinetosomal body, has only been observed in two distantly related species of the subfamily Stylonychinae, suggesting this feature might have evolved independently; and (3) the ultrastructure of other organelles, such as the perilemma, buccal lip, buccal seal, and pharyngeal disks, was also documented. This is the first report on the ultrastructural features of P. hortualis and of the genus Parentocirrus, which should help improve the diagnosis and definition of this widely distributed species.