ABSTRACT. We isolated a new ciliate, Bistichella variabilis n. sp., from slightly saline soil of the Yellow River Delta in China and studied it using live observation, protargol staining, and small subunit (SSU) rRNA gene sequencing. Bistichella variabilis is elongated ellipsoidal and about 220 × 70 μm in vivo, with a variable number and arrangement of cirri— ventral row VI mostly very short, but a long row observed also in one specimen, and an extra row occasionally exists besides the left or the right marginal row. Bistichella variabilis differs from all congeners except Bistichella buitkampi (Foissner, 1982) in the macronuclear pattern (four vs. two or many nodules); and from B. buitkampi by the usual oblique arrangement of transverse cirri (vs. perpendicular); and the ventral row VI mostly restricted in the posterior quarter of the cell (vs. long and stretching to the anterior quarter). Phylogenetic analyses of SSU rDNA sequences indicated that B. variabilis grouped with the clade of Orthoamphisiella breviseries and Uroleptoides magnigranulosus with high support values. Despite their distinct differences in morphology, these three species have sequence similarities of 99.5–99.7%. Nonetheless, it is still premature to assign Bistichella together with Uroleptoides to the family Orthoamphisiellidae because of the taxonomic uncertainties and undersampling.
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