Abstract
Background Hypotrichia are a group with the most complex morphology and morphogenesis within the ciliated protists. The classification ofGastrostyla-like species, a taxonomically difficult group of hypotrichs with a common ventral cirral pattern but various dorsal and ontogenetic patterns, is poorly understood. Hence, systematic relationships within this group and with other taxa in the subclass Hypotrichia remain unresolved. Results 18S rRNA gene sequence of a newGastrostyla-like taxon was obtained. Phylogenetic analyses based on the 18S rRNA gene sequences indicate that this ciliate represents a new genus that is closely related toHeterourosomoidaandKleinstylawithin the oxytrichid clade of the Hypotrichia. However, the position of this cluster remains unresolved. All three genera deviate from the typical oxytrichids by their incomplete (or lack of) dorsal kinety fragmentation during morphogenesis. Morphology and morphogenesis of this newly discovered form,Heterogastrostyla salinanov. gen., nov. spec., are described.Heterogastrostylanov. gen., is characterised as follows: more than 18 fronto-ventral-transverse cirri, cirral anlagen V and VI develop pretransverse cirri, and dorsal ciliature inUrosomoida-like pattern. Conclusions Similar to the CEUU-hypothesis about convergent evolution of urostylids and uroleptids, we speculate that the shared ventral cirral patterns ofGastrostyla-like taxa might have resulted from convergent evolution.