Somatic infraciliature in tintinnid ciliates (Alveolata, Ciliophora, Spirotricha): An ultrastructural comparison.

A recent ultrastructural study on the tintinnid ciliate Schmidingerella meunieri displayed unique types of somatic kinetids. The dikinetids (paired basal bodies) have, besides kinetodesmal fibrils and transverse ribbons, some special features, that is, overlapping postciliary ribbons and three extraordinary microtubular ribbons, which together form a conspicuous network in the ciliated anterior cell portion. The distribution of this feature among tintinnids is studied in chemically fixed and ultrathin-sectioned specimens from six genera and five families collected in European coastal waters. The taxa are scattered across the molecular tree. Actually, the somatic kinetids of these six genera share the special features discovered in S. meunieri. Accordingly, the overlapping postciliary ribbons and the three extraordinary ribbons were already present in the early stages of tintinnid evolution, namely in the last common ancestor of tintinnids with hard loricae. Owing to the lack of ultrastructural data in the basally branching Tintinnidiidae with their soft loricae and in aloricate choreotrichids other than the aberrant strobilidiids, the first appearance of the structures is still uncertain. The related oligotrichids do not possess overlapping postciliary ribbons, but show electron-dense material at the sites where the ribbons I-III originate in tintinnids. None of these features is found in any other spirotrich ciliate.