A Contribution to the Morphologic Variable Morphs and Diversity in Microthoracida

The nassophorean ciliates are a key group for understanding morphological diversification, but their phylogenetic relationships remain elusive. Using morphological characteristics, molecular data, and phylogenetic analyses, we attempt to cast light on species relationships within the order Microthoracida Jankowski, 1967. The present multifaceted approach reveals that (1) Leptopharynx costatus costatus Mermod, 1914 exhibits polymorphism, occurring in a large morph (35-55 × 25-40 μm) with a narrow oral basket and an intermediate mean of 228 basal bodies, supporting basal body count as a key diagnostic character; (2) comprehensive morphological and corresponding molecular information for Pseudomicrothorax dubius Maupas, 1883 was provided for the first time; (3) comparisons among different populations and morphs of L. costatus costatus and P. dubius show that the former is more morphologically variable, while the latter exhibits higher molecular variability; (4) SSU rDNA sequences provided in this work cluster with the sequences of conspecific populations, supporting the conclusion that both the families Microthoracidae Wrześniowski, 1870 (containing Leptopharynx Mermod, 1914) and the family Pseudomicrothoracidae Jankowski, 1967 (containing Pseudomicrothorax Mermod, 1914) are monophyletic.