Comparative study and morphological plasticity of an Italian and a Chinese population of the protist ciliate Pseudokeronopsis erythrina (Hypotrichia, Kentrurostylida) with a mitochondrial genome analysis

Two populations of the kentrurostylid ciliate Pseudokeronopsis erythrina Chen et al., 2011 were isolated from a brackish wastewater treatment plant, Pisa, Italy and from a nutrient-rich freshwater body, the East Lake, Wuhan, China. The populations were thoroughly investigated for their morphology, focusing on cell body plasticity. The study was supplemented with 18S rDNA phylogenetic analysis and mitogenome sequencing, following the integrative taxonomy approach. The Italian population possesses a great variation in terms of the frontal area, and the numbers of left marginal rows, and dorsal kineties. Additionally, the number of left marginal anlagen and dorsal kineties anlagen varied. For instance, additional small anlagen appeared in the left marginal rows, and two dorsal kineties anlagen derived from a single dorsal kinety. The Wuhan population resembled the type population (originally isolated from Guangzhou, China), differing only by the presence of an extra dorsal kinety. The existence of giant individuals characterized both new populations of P. erythrina although with some differences in their respective frequency and features. The 18S rDNA sequences of the Italian, Wuhan, and type populations were identical. Phylogenetic analyses showed that these three populations formed a distinct cluster within the clade containing P. songi, P. parasongi, and P. flava. The structure of the P. erythrina mitochondrial genome is also provided. The content of this genome closely resembled Pseudourostyla cristata, except for the absence of genes nadh3 and nadh6. Our findings suggest that the Wuhan population represents an intermediate form between the type and Italian populations. The greater morphological plasticity observed in the Italian population underscores the importance of molecular data and integrative analyses in species identification.