Refining the “Melting Pot” Genus Holosticha s. l. (Protozoa, Ciliophora, Hypotrichia) Based on Multigene Datasets with Establishment of a New Species Caudikeronopsis monilata sp. nov.

The genus Holosticha s. l. is a typical “melting pot” group with an intricate history, and so far, it has been divided into eleven genera. Both newly obtained taxonomic and molecular data provide the opportunity to gain more insights to outline the taxa in it and to understand their systematic and evolutionary relationship. Here, we describe Caudikeronopsis monilata sp. nov. from intertidal sediment on the China coast of the Yellow Sea and analyze the phylogenetic relationships of Holosticha s. l. by obtaining a total of 16 new sequences of seven isolates. The results demonstrate that (1) the morphological features of Holosticha s. str. are outlined very well, but its systematic relationship with Uncinata is still puzzling; (2) based on both morphological and molecular databases, the genera Adumbratosticha, Arcuseries, Caudikeronopsis, Extraholosticha, and “Holosticha + Uncinata” complex are separated clearly from each other in the phylogenetic analyses; and (3) the Anteholosticha isolates are dispersed among the urostylids in the phylogenetic analyses, even though its generic diagnostic features are described very clearly. In the present work, however, the secondary structure predictions do not provide better resolutions for understanding the systematic and evolutionary relationships among the holostichids. And the genus Anteholosticha becomes a new “melting pot” taxon.