Nuclear phenomena during conjugation of Heliophrya erhardi (Ciliata, Suctoria). II. The orientation of postmeiotic and metagamic division spindles is organized by the cytoskeleton

Summary Whereas the spindles of the meiotic divisions appear in a random orientation within the co-conjugants of Heliophrya erhardi , the spindles of the postmeiotic and metagamic nuclei show a distinct fixed position within the conjugational bridge and in the cytoplasm of each partner’s cell body. In postmeiotic divisions, the spindle orientation results in a different functional determination of the division products. For their later fate, mostly a concentration gradient of “cytoplasmic factors” was postulated [29]. But above all, the question arises, what is the mechanistic basis for the spindle orientation? We followed the sequential steps of conjugation by time lapse microcinematography and by EM-observation. The result of the maturation divisions are 12 to 24 haploid nuclei [11] from which only one nucleus reaching the conjugational bridge survives and starts the postmeiotic division. The position of this gonal nucleus is fixed on the left marginal side of the bridge by a dense microfilament network surrounding the nucleus in a cage-like manner and anchoring it to the epiplasmic septum separating the co-conjugants. During the following division, the spindle extends parallel to the septum fixed by cytoskeletal elements during anaphasic spindle elongation. Additionally, at metaphase, a basal body appearing at the spindle pole proximal to the pellicle nucleates microtubules (MTs) which penetrate the microfilamentous cage and contact also the nuclear envelope. The fixation of the spindle axis parallel to the separating septum by cytoskeletal elements guarantees the positioning of one pronucleus as the future stationary nucleus on the left side, and the other — the presumptive migratory nucleus — on the right side of the conjugational bridge. After exchange of the migratory nuclei and subsequent fusion with formation of the diploid synkaryon, one metagamic division follows immediately in each partner cell. From meta- to telophase the spindles exhibit in most cases a strict central-peripheral orientation, at which the anterior daughter nucleus (oriented to the cell’s center) becomes the macronucleus anlage, while the posterior nucleus condenses to the micronucleus. Interestingly, this spindle orientation is held in position by aster-like organized microtubules radiating out to a microfilamentous cap situated at the anterior spindle pole. As an organizing center for this cytoskeletal structure again a basal body is responsible, which is localized between nuclear envelope and cap region from meta- to telophase. It is evident, that in both divisions, the orientation of the spindle axis and consequently the fate of the division products depends completely on the existence of a cytoskeletal organizing structure, which is comparable to the asters of mitotic spindles in some higher eukaryotes.