Mitosis and early meiosis in Tetrahymena pyriformis and the evolution of mitosis in the phylum Ciliophora.

The micronuclear mitotic spindle of Tetrahymena pyriformis contains several distinctive elements. The 150 or so continuous microtubules (MTs) form a peripheral sheath just inside the inner nuclear membrane while the kinetochore bundles traverse the center of the nucleoplasm. A new set of microtubules, the separation spindle, appears during the 10-fold nuclear elongation which occurs during the late anaphase. Both the separation spindle and peripheral sheath MTs are present in the macronucleus during macronuclear division but there are no definite kinetochore MTs. In the cresent stage of meiotic prophase both peripheral sheath and kinetochore MTs are present in the micronucleus. By using our own, and other workers', data in conjunction with the phylogenetic scheme for the ciliates which has been designed by Corliss (1974, 1975), we have attempted to trace out the evolutionary history of the various elements of the ciliate mitotic spindle. For example, the micronuclear separation spindle can be followed, from its point of origin within the primitive gymnostomes, throughout the phylum. The separation spindle of the macronucleus, by contrast, is lost at the level of the heterotrichs. Similarly, the anaphase breakdown and reconstitution of the nuclear envelope, which occurs in the primitive Loxodes magnus, can be followed up some phyletic branches, such as the heterotrichs, but in the suctorians and some other groups this feature seems to have been lost. Tracing the evolution of the ciliate mitotic spindle is made very difficult both by the incompleteness of the data and by what appear to be a number of cases of secondary reduction and parallel evolution. In general, however, the evolution of mitosis correlates well with the phylogeny of the ciliates constructed by Corliss and we consider this an independent substantiation of the general correctness of his phylogeny.