Fine Structure of Conjugation of the Ciliate Blepharisma japonicum II. Changes of Meiotic and Ameiotic Micronuclei and Development of Meiotic and Ameiotic Macronuclear Anlagen

Abstract

During conjugation of Blepharisma, only some micronuclei enter meiosis. Other micronuclei, called somatomicronuclei, do not degenerate but differentiate directly into macronuclear anlagen (secondary anlagen) without meiosis and karyogamy. Normal (primary) anlagen develop from synkaryon derivatives. We observed these processes at the ultrastructural level.

In early conjugation (0–2 hours after pair formation), all micronuclei swell. This correlates with decondensation of the micronuclear chromatin. At 3 hours, the micronuclei differentiate into somatic and 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 are in pachytene; at the same time, structures in form of loose chromatin patches first appear in somatomicronuclei. The patches gradually condense and become conspicuous at 10–12 hours (stages from diplotene to metaphase I of meiosis). At about 12 hours, the meiotic micronuclei are in metaphase I and display acentric intranuclear spindles with blunt poles and homogenous polar caps; the bivalents have prominent kinetochores. At 16 hours, the somatomicronuclei contain numerous chromatin patches which are possibly subchromosomes, and first nucleoli appear in them. At 16–18 hours, the stage of pronuclei is reached; and other meiotic products start degenerating. The migratory pronuclei show concentration of the chromatin at the centre of the nucleus. At 20, 26, 28 and 34 hours, the fine structure of somatomicronuclei (secondary anlagen) changes little. Their size remains constant (about 5–6 μm). However the nucleoli enlarge at 34 hours. The first division of the synkaryon has protruding poles and no polar caps, unlike meiosis I; though, the nuclear envelope remains intact even at the poles. The synkaryon divisions 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 to 20 μm) and, at early stages of development, their chromatin is so strongly decondensed that the anlagen look “empty”. However later (by 34 hours) loose chromatin patches, small bodies of condensed chromatin 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.