Nuclear phenomena during conjugation of the suctorian Heliophrya erhardi: I. EM-Observations of micronuclear meiosis

Summary We investigated micronuclear meiosis in the conjugating suctorian Heliophrya erhardi by time lapse cinemicrography of living cells, by indirect immunofluorescence (IIF) with monoclonal antibodies against modified tubulins, and by transmission electron microscopy, respectively high voltage electron microscopy (HVEM) of ultrathin and semithin sections. The fine structure of mid meiotic prophase reveals moderately condensed chromatin threads forming a “parachute” arrangement. Since only extremely short and also somewhat atypically structured synaptonemal complexes (SCs) are first discernible as early as diplotene, when partial chromosomal disjunction is seen, it is suggested that we are dealing here with achiasmatic meiosis. The spindle axis is established during prometaphase I by parallel alignment of polar MTs (pMTs) and the congression of chromosomes towards the equator is brought about by outgrowing kinetochore MTs (kMTs) becoming increasingly parallel. Metaphase I is indicated by a complete depolymerization of pMTs, revealing a truncated spindle type consisting exclusively of kMTs which never reach the presumptive poles before the metaphase/anaphase transition, and, as also seen by IIF, several of the small chromosomes converge with their kMTs forming numerous “subspindles”. These are focused on electron dense bodies located at half the distance to the presumptive poles. At late metaphase stages spindles become stabilized by their anchorage at the lamin-like sheets of the inner membrane of the nuclear envelope. Anaphase I is clearly subdivided into anaphase A and into anaphase B. While chromosome segregation in anaphase A I is not conspicuous an enormous interzonal spindle elongation that contributes substantially to chromosomal segregation is formed at anaphase B I. The mean velocity of pole separation of 0.5 μm/min corresponds rather well with the anaphase AI poleward migration. The telophase spindle develops from a centrally located torsional zone of the stembody in anaphase B by separation into two twisted areas which push the chromosomal fronts into opposite directions. During the dumbbell-shaped telophase stage, the daughter nuclei are pinched off by rotational movements of the terminalized contorted regions of the spindle. The nuclear events leading to meiosis II are highly similar to meiosis I.