Ultrastructure of meiosis inChlamydomonas reinhardtii

Ultrastructural details of meiosis in Chlamydomonas reinhardtii are presented. Zygotes undergo a resting stage prior to meiosis, during which a multi-layered zygote wall is deposited. Basal bodies reappear at interphase. Heterochromatin is not visible in the interphase nucleus. Leptotene is characterized by the appearance of axial cores and the condensation of chromonemata. By early zygotene, axial elements associated with the condensing chromatin associate with the nuclear envelope. Tubular elements appear in the perinuclear space adjacent to the associated chromosomes. Bivalents begin to associate with the nucleolus. By pachytene, typical synaptinemal complexes have formed. The synaptinemal complexes possess two lateral elements associated with the chromosomes and separated by a central space of about 130 nm. A central element may form midway between the lateral elements and be joined to them via transverse elements. Separated synaptinemal complexes appear in the cytoplasm by diplotene-diakinesis. At this stage the chromosomes become most condensed and chiasmata occur. The nucleolus disappears. The later stages of meiosis are described. By metaphase I the nuclear envelope is fenestrated and a spindle develops. Daughter nuclei reform at telophase. Basal bodies are associated with a ribosome-free zone adjacent to the nuclear membrane. The interzonal spindle degenerates. Cleavage occurs via a phycoplast. At the initiation of the second division, basal bodies are present at the incipient cleavage furrow. Basal body replication appears to occur at pre-prophase. By metaphase II, the basal bodies migrate to the poles of the fenestrated spindle. Completion of the second meiotic division results in the formation of haploid zoospores.