Taxonomy, Ontogenesis and Evolutionary Relationships of the Algae-Bearing Ciliate Bourlandella viridis () comb. nov., With Establishment of a New Genus and New Family (Protista, Ciliophora, Hypotrichia).

Hypotrichs are the most complex and highly differentiated ciliate lineages and have great potential for evolutionary novelties. Problems in hypotrich systematics are mainly due to discordance between the morphological and genetic data (mainly small subunit rRNA gene). Species with morphologies that are characteristic of two or more higher rank taxa are probably a major contributing factor to these conflicts. The present study describes a Chinese population of a poorly known organism with numerous endosymbiotic zoochlorellae, the morphology of which corresponds well with the type population of Limnoholosticha viridis (Kahl, 1932) Li et al., 2017. Newly obtained information shows this species has a zigzag midventral cirral pattern that is diagnostic of urostylids, whereas the dorsal ciliature shares features (presence of dorsomarginal kinety and dorsal kinety 3 fragmentation) that are typical of oxytrichids. Molecular phylogenetic analyses reveal a close relationship with oxytrichids. An integrative approach combining morphological, morphogenetic, phylogenetic and ecological analyses indicates that L. viridis represents a new genus and new family which might be an intermediate form between uorstylids and dorsomarginalians. Thus, Bourlandellidae fam. nov. and Bourlandella gen. nov. are here established. Lastly, we speculate that phenotypic convergence and mixtrophy might confer on the new combination, Bourlandella viridis (Kahl, 1932) comb. nov., the ability to adapt to a wide range of environmental conditions.