Abstract
The Prodiscocephalus-like ciliates, or discocephalines, are cephalized organisms that are traditionally considered to be hypotrichs (sensu lato) but whose precise systematic position has long been uncertain. The main reasons for this are that these organisms exhibit several intermediate morphological and morphogenetic features and that hitherto none has been investigated using molecular methods. In the present study, the cortical development of Prodiscocephalus borrori was observed during binary division and this can be summarized as follows: (i) in the parental adoral zone of membranelles, only the posterior end is renewed by dedifferentiation of the old structures; (ii) the oral primordium in the opisthe occurs de novo on the cell surface as seen in other typical stichotrichs; (iii) in both dividers, the undulating membranes anlage does not split longitudinally in the usual way but, instead, divides transversely to form the paroral and endoral membranes; (iv) usually seven frontoventral transverse cirral anlagen are formed in the primary mode which then divide into two sets, one each for the proter and opisthe; (v) both left and right marginal rows divide into two parts, thus giving rise to a post-lateral marginal segment at the posterior end of each; (vi) invariably five caudal cirri are formed at the posterior end of the three rightmost dorsal kinety anlagen. Thus, it was found that, like other related discocephalines, P. borrori exhibits more similarities to stichotrichs than to euplotids. Based on a combination of morphological and morphogenetic data, a phylogenetic tree was constructed which suggests that the discocephalines group within the stichotrichs and separate from the euplotids. In addition, the complete small-subunit rRNA gene (SSU rDNA) of P. borrori was sequenced and analysed. In the resulting SSU rDNA tree, the discocephalines represent an intermediate group between the euplotids and the Stichotrichia-Oligotrichia-Choreotrichia assemblage, albeit with low bootstrap support. From these data, we conclude that the discocephalines might be a divergent, or possibly an ancestral, group within the Stichotrichia. Furthermore, our findings further support the suggestion that these organisms should be considered as a distinct order, i.e. Discocephalida Wicklow, 1982, in the subclass Stichotrichia Small & Lynn, 1985.