A new contribution to the taxonomy and phylogeny of the ciliate genus Spirostomum (Alveolata, Ciliophora, Heterotrichea), with comprehensive descriptions of two species from wetlands in China

Species of the ciliate genus Spirostomum Ehrenberg, 1834 are distributed worldwide and have a research history spanning more than two centuries. However, species delimitation and phylogenetic relationships within this genus are still uncertain due to the paucity of stable morphologic characters for species separation and the unavailability of accompanying morphological data for most molecular sequences in public databases. In the present study, S. yagiui Shigenaka, 1959 (three populations) and S. caudatum (Muller, 1786) Delphy, 1939 (one population) were investigated using morphological and molecular methods for the first time in China. Detailed morphological data for the two species were documented, and improved diagnoses were supplied based on a combination of previous studies and the current work. It should be highlighted that there were three different atypical morphotypes identified in a Ningbo population of S. yagiui which may represent various stages in conjugative reproduction. Molecular phylogenies based on 18S, ITS1-5.8S-ITS2, and 28S rRNA gene sequences show that the genus Spirostomum is monophyletic, however, the internal relationships inferred from different genes were poorly resolved but suggest that the species with a moniliform macronucleus comprise an early-diverging clade within this genus. Finally, the global distribution of Spirostomum is summarized based on previous and present studies.