Nuclear phenomena during conjugation of Heliophrya erhardi (ciliata, suctoria). II. The orientation of postmeiotic and metagamic division spindles isorganized by the cytoskeleton

Whereas the spindles of the meiotic divisions appear in a random orientatio n within the co-conjugants of Heliophrya erhardi, the spindles of the postm eiotic and metagamic nuclei show a distinct fixed position within the conju gational bridge and in the cytoplasm of each partner's cell body In postmei otic divisions, the spindle orientation results in a different functional d etermination of the division products. For their later fate, mostly a conce ntration gradient of "cytoplasmic factors" was postulated [29]. But above a ll, the question arises, what is the mechanistic basis for the spindle orie ntation? We followed the sequential steps of conjugation by time lapse microcinemato graphy and by EM-observation. The result of the maturation divisions are 12 to 24 haploid nuclei [11] from which only one nucleus reaching the conjuga tional 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 d ense microfilament network surrounding the nucleus in a cage-like manner an d anchoring it to the epiplasmic septum separating the co-conjugants. Durin g 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 p ellicle nucleates microtubules (MTs) which penetrate the microfilamentous c age 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 ea ch partner cell. From meta- to telophase the spindles exhibit in most cases a strict central-peripheral orientation, at which the anterior daughter nu cleus (oriented to the cell's center) becomes the macronucleus anlage, whil e the posterior nucleus condenses to the micronucleus. Interestingly, this spindle orientation is held in position by aster-like organized microtubule s 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 r egion 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 th e existence of a cytoskeletal organizing structure, which is comparable to the asters of mitotic spindles in some higher eukaryotes.