Abstract
Ontogenesis in Sultanophrys arabica Foissner & AL-Rasheid, 1999, a trachelocercid karyorelictid ciliate, was investigated using live observation, silver impregnation, and scanning electron microscopy. Division is homothetogenic and occurs in freely motile (non-encysted) condition. The parental oral apparatus does not reorganise and cell shape is maintained. Stomatogenesis is parakinetal, that is, the anlage for the opisthe oral apparatus is derived directly from the first ordinary somatic ciliary row right of the glabrous stripe and has no connection with parental mouth structures. The oral primordium appears slightly subequatorially and consists of an anarchic field of basal bodies, from which many dikinetidal kinetofragments differentiate. The kinetofragments migrate centrifugally and assemble to a circumoral kinety and three minute brosse kineties. The somatic kineties, the bristle kinety, and the lateral kinety divide without anlagen formation, that is, are disrupted by cytokinesis. Thus, morphogenesis of trachelocercid karyorelictids is distinctly different from that of loxodid karyorelictids, which generate the oral primordium buccokinetally. This shows that different stomatogenic modes developed early in ciliate evolution, which is emphasised by the heterotrichs, whose parakinetal stomatogenesis is rather different from that of the trachelocercids. Nevertheless, our data give some support for the subphyletic division suggested by Lynn (1996), but do not corroborate any of the hypotheses on evolution of ciliate cytoarchitecture. Both, loxodid and trachelocercid karyorelictids have conspicuous ontogenetic similarities (scutica-like structure, "director meridian") with oligohymenophoreans. This is sustained by the trachelocercid brosse kineties, which resemble oligohymenophorean and prostomatid adoral membranelles. Whether these traits evolved convergently or have a deeper meaning needs further investigations.