Morphogenesis of a unique pseudourostylid ciliate, Trichototaxis songi (Ciliophora, Urostylida)

Divisional morphogenesis in the freshwater spirotrichous ciliate, Trichototaxis songi Chen et al., 2007, was investigated. The main morphogenetic events are characterised as follows: (1) the parental oral apparatus is completely renewed by the independently formed oral primordium in the proter; (2) the oral primordium in the opisthe is formed on the cell surface; (3) several left cirri of the midventral pairs participate in the formation of the oral primordium in the opisthe; (4) FVT-anlage I forms the leftmost pair of the bicorona; (5) the macronuclear nodules fuse into many masses rather than a single or branched mass as described in most pseudokeronopsids; and (6) usually, two marginal anlagen develop within each left marginal row separately. However, the number of left marginal anlagen is highly variable, even differing between the proter and opisthe of the same divider. The increase in the number of left marginal anlagen is by de novo generation of small anlagen to the left of the intrakinetal left marginal anlage, whereas the decrease in number is by resorption of the old marginal row(s). We posit that Trichototaxis is an intermediate form between the Pseudourostylidae and Pseudokeronopsidae as it shares morphogenetic features with both. Additionally, as in Uroleptopsis (Uroleptopsis), FVT-anlage I forms the leftmost pair of the bicorona in Trichototaxis indicating these genera may be closely related.