Morphology, morphogenesis, and molecular phylogeny of Sterkiella tetracirrata n. sp. (Ciliophora, Oxytrichidae), from the Silent Valley National Park, India

The morphology and morphogenesis during cell division of Sterkiella tetracirrata n. sp., isolated from a soil sample collected from the Silent Valley National Park, Kerala, India, were investigated using live observation, protargol staining and scanning electron microscopy. The new species differs from its congeners by the following combination of features: cell size in vivo 85–110 × 35–50 μm, on average 84 × 37 μm in protargol preparations; four ellipsoidal macronuclear nodules; 31 adoral membranelles; 17 frontal-ventral-transverse cirri consisting of three frontal, four frontoventral, one buccal, three ventral, two pretransverse and invariably four transverse cirri; resting cyst with separate macronuclear nodules. Sterkiella tetracirrata differs from the similar species S. terricola in the number of transverse cirri (invariably 4 vs. 3) and in the number of adoral membranelles (24–35 vs. 22 or 23). Morphogenesis resembles that of its congeners S. nova and S. histriomuscorum. Phylogenetic analyses based on SSU rRNA gene sequences consistently place the new species within the stylonychine oxytrichids, clustering closer to Gastrostyla steinii than to either S. cavicola or S. histriomuscorum. The analyses support the morphological evidence (e.g., similarity in the oral apparatus and the dorsal kinety pattern) that Gastrostyla and Pattersoniella evolved from a Sterkiella-like ancestor.