Abstract
Within the ciliate class Oligohymenophorea, many evolutionary relationships among taxa remain unresolved. This study incorporates 97 new sequences from 30 oligohymenophorean populations, including nuclear small subunit ribosomal (SSU-rRNA) genes, nuclear ITS1-5.8S-ITS2 rRNA regions, nuclear large subunit ribosomal (LSU-rRNA) genes, mitochondrial cytochrome oxidase c subunit I (cox 1) genes, and mitochondrial small subunit ribosomal RNA (mtSSU-rRNA) genes. With the addition of these new sequences, we performed comprehensive multigene phylogenetic analyses of Oligohymenophorea. The main findings are: (1) Utilizing multiple genes is instrumental in improving phylogenetic relationships within class Oligohymenophorea; (2) class Oligohymenophorea is divided into two distinct groups: (i) encompassing the subclass Hymenostomatia, Scuticociliatia, Apostomatia, Astomatia, and Urocentria; and (ii) comprising the subclasses Peniculia and Peritrichia; (3) Revising the phylogenetic placement of the subclass Urocentrida reveals its transitional role as a taxon between Group I and Group II. It emerges as a sister clade to Hymenostomatia; (4) the phylogenetic positions of Astomatia and Apostomatia within the clade 'SAA' become clearer; and (5) the order Loxocephalida represents an early member of Scuticociliatia and serves as a potential prototype for this taxonomic group. This study provides more information for understanding the evolutionary relationships within Oligohymenophorea.