FINE-STRUCTURE OF CONJUGATION OF THE CILIATE BLEPHARISMA-JAPONICUM - II - CHANGES OF MEIOTIC AND AMEIOTIC MICRONUCLEI AND DEVELOPMENT OF MEIOTIC AND AMEIOTIC MACRONUCLEAR ANLAGEN

During conjugation of Blepharisma, only some micronuclei enter meiosis . Other micronuclei, called somatomicronuclei, do not degenerate but d ifferentiate directly into macronuclear anlagen (secondary anlagen) wi thout meiosis and karyogamy. Normal (primary) anlagen develop from syn karyon derivatives. We observed these processes at the ultrastructural level. In early conjugation (0-2 hours after pair formation), all mic ronuclei swell. This correlates with decondensation of the micronuclea r chromatin. At 3 hours, the micronuclei differentiate into somatic an d meiotic (leptotene) ones: the latter develop bundles of microtubules . The somatomicronuclei remain homogeneous and lack microtubules. At 8 -9 hours meiotic micronuclei display synaptonemal complexes and thus a re in pachytene; at the same time, structures in form of loose chromat in patches first appear in somatomicronuclei. The patches gradually co ndense and become conspicuous at 10-12 hours (stages from diplotene to metaphase I of meiosis). At about 12 hours, the meiotic micronuclei a re in metaphase I and display acentric intranuclear spindles with blun t poles and homogenous polar caps; the bivalents have prominent kineto chores. At 16 hours, the somatomicronuclei contain numerous chromatin patches which are possibly subchromosomes, and first nucleoli appear i n them. At 16-18 hours, the stage of pronuclei is reached; and other m eiotic products start degenerating. The migratory pronuclei show conce ntration of the chromatin at the centre of the nucleus. At 20, 26, 28 and 34 hours, the fine structure of somatomicronuclei (secondary anlag en) changes little. Their size remains constant (about 5-6 mu m) Howev er the nucleoli enlarge at 34 hours. The first division of the synkary on has protruding poles and no polar caps, unlike meiosis I; though, t he nuclear envelope remains intact even at the poles. The synkaryon di visions give rise to new micronuclei and primary macronuclear anlagen. The primary (meiotic) macronuclear anlagen differentiate in number of 2-4 at 22-24 hours. They are much larger than secondary anlagen (up t o 20 mu m) and, at early stages of development, their chromatin is so strongly decondensed that the anlagen look ''empty''. However later (b y 34 hours) loose chromatin patches, small bodies of condensed chromat in and nucleolar primordia appear in them, like in somatomicronuclei, and the primary anlagen at 34 hours show additionally a karyosome-like central condensation of the chromatin.