Division and reorganization in the genus Psevdokeronopsis and relationships between urostylids and oxytrichids (ciliophora, hypotrichida)

Abstract

Summary The cortical development of Pseudokeronopsis rubra, P. carnea , and P. flava was investigated using the protargol method. The overall pattern of the morphogenetic events is similar in all strains. In the opisthe new basal bodies originate close to several intact left midventral cirri. The primordia of the adorai membranelies (AMP), the undulating membranes (UMP), and the fronto-midventral-cirri (FMA) develop from the growing anachic field. In the proter, proliferation occurs de novo on the surface of the buccal cavity generating the AMP and UMP. The FMA originate apokinetal to the right of the parental paroral membrane. The new undulating membranes generate the first frontal cirrus. The longitudinal series of oblique streaks can be divided in three segments: (1) about 32 anterior anlagen which become frontal or midventral cirri form two cirri each; (2) the next 4–6 anlagen build three cirri, each generating pairs of midventral cirri and transverse cirri at their posteriormost ends; (3) the two posteriormost anlagen produce 4 cirri each — transverse, ventral, and frontoterminal cirri. In P. flava the posteriormost and even the anteriormost anlagen from supernumerary (if compared with the interphase cells) cirri which are resorbed later. Marginal cirral rows and dorsal kineties develop within the parental structures. The numerous macronuclear segments divide without prior fusion. Reorganization corresponds to the divisional processes of the opisthe, but only slight deformations of the macronuclear segments occur. This investigation provides ontogenetic criteria for the families Pseudokeronopsidae and Bakuellidae. The single frontal cirral arch developing from the undulating membrane primordium in the Keronidae is termed “monocorona”, whereas the “bicorona” consisting of an anterior and a posterior row of arched cirri in the frontal region of several urostylids originates, apart from the first frontal cirrus, in the same way as the midventral primordia. Several characters suggest a common ancestor of urostylids and oxytrichids. The most important synapomorphies are the frontoterminal cirri and the few ventral cirri close to the transverse cirri.