THE NUCLEAR-APPARATUS OF SOME PRIMITIVE CILIATES, THE KARYORELICTIDS - STRUCTURE AND DIVISIONAL REORGANIZATION

This review considers the peculiarities of the generative nuclei (micr onuclei) and the somatic nuclei (macronuclei) of the presumably primit ive ciliates of the order Karyorelictida. Their nuclei display the ten dency to form nuclear clusters. Whereas the karyorelictid micronuclei are of the type usual for other ciliates and divide by mitosis, die ma cronuclei never divide and belong to the vesicular type of nuclei, bei ng rather poor in condensed chromatin, and near-diploid as regards DNA content. The macronuclei also contain nucleoli 2nd often nuclear bodi es in the form of either spheres or crystalloids containing protein an d sometimes RNA. The division of the ciliate is always accompanied by nuclear reorganization involving repeated mitoses of the micronuclei a nd differentiation of some excess ones into new macronuclei, . the old ones being segregated without division. The development of the new ma cronuclei involves one round of DNA replication but no signs of furthe r polytenization and/or polyploidization. However, an amplification of the rDNA is likely to occur in developing and aging macronuclei. Acco rding to sequence analysis, this type of nuclear apparatus is primitiv e or at least early emerging.