Fine structure and cytochemistry of the nuclei of the primitive ciliateTracheloraphis crassus (Karyorelictida)

Summary The nuclei ofTracheloraphis crassus were studied using light and electron microscopy combined with Bernhard's RNP staining and pronase digestion. The nuclear apparatus of this species consists of a longitudinal row of 11–43 macronuclei and 4–16 micronuclei. Like in all karyorelictids, the macronuclei are unable to divide and become segregated during cytokinesis; their number is supplemented in every cell cycle by differentiation of several new macronuclei from micronuclei.Each adult macronucleus contains a single compact endonuclear aggregate of several large chromocenters, readily destained with EDTA, and several RNP containing nucleoli. There is continuity between the material of the chromocenters and the decondensed DNP fibrils in the nuclear matrix. The nucleoli contain NORs in the form of fibrillar centers. The endonuclear aggregate includes also groups of RNP granules which are especially resistant to EDTA destaining. A microfibrillar sphere, usually localized at the periphery of the aggregate, contacts one or several nucleoli. The sphere is not bleached with EDTA, and only its periphery becomes digested with pronase. The macronuclear matrix consists of both protein fibrils and pronase-resistant fibrils, the latter being localized at the nuclear periphery.Developing macronuclear primordia contain loose strands of decondensed chromatin; only later they form chromocenters and nucleoli.The micronuclei reproduce by mitosis with typical chromosomes (2n=66). During interphase, they are filled with condensed chromatin which can be bleached with EDTA; they form no nucleoli. Ring-like lamellae, existing in the cavities of the chromatin mass, stain for RNA (after Bernhard) and are pronase-sensitive. These lamellae resemble the kinetochore material conserved during interphase in another karyorelictid ciliate,Trachelocerca geopetiti.