Ontogeny and phylogeny of the anaerobic genus Urostomides with supplementing morphological characterization of the type and one little-known species (Alveolata: Ciliophora: Armophorea)

Urostomides is an obligate anaerobic ciliate genus in the family Apometopidae (Metopida). Although most congeners have been described using modern criteria, detailed ontogenetic data for both the family and genus have not been reported yet. In this study, we investigate the morphogenesis of two Urostomides species, viz., U. minimus (Kahl, 1927) comb. nov. and U. striatus (type species) in detail. Our findings reveal that the process closely resembles that of several described Metopidae species, except for a notable difference in the origin of the new adoral membranelles either from all perizonal rows or from other somatic kineties. Additionally, the data of U. minimus on the ciliature and SSU rRNA gene sequence are provided and those of U. striatus are supplemented based on Chinese isolates. We show the diplostichomonad morphology of the paroral membrane, a feature not previously observed in Urostomides species. Furthermore, our phylogenetic analyses confirm the monophyly of Urostomides.