The meotic prophase in Bombyx mori females analyzed by three dimensional reconstructions of synaptonemal complexes

Serial sectioning followed by three dimensional reconstruction of lateral componenets of the synaptoemal complex have been used to follow chromosome pairing during theprophase of the achiasmatic meiotic division in the silkworm. Bombyx mori. During leptotene and early zygotene, the lateral components become attached to the nuclear envelope at a specific region, thus forming a chromosome bouquet. The attachment of lateral componenets to the nuclear envelope precedes the completion of the components between their attachment points. Synapsis and synaptonemal complex formation start during the period of lateral component organization in the individual nucleus. Telomeric movements on the nuclear envelope occur at two stages of the prophase: the chromosome pairing appears to be initiated by an association of unpaired ends of homologus chromosomes, the nature of this primary attraction and recognition being unknown. Secondly, the paired chromosomes become dispersed in the nucleus by shifting of attachment sites of completed synaptonemal complexes at the end of zygotene. This movement is possible related to a membranes flow occuring during this stage. Membrane material is synthesized at the region of synaptonemal complex attachment. Later, the excess membrane material is shifted to the opposite pole where it protudes into the lumen of the nuclei thus forming vacuoles. Two previously undescribed features of chromosomes paring were revealed. In late zygotene, chromosome pairing and synaptonmal complex formation were frequently observed to be delayed or even prevented over s short distance by interlocking two bivalents, both being attached to the nuclear envelope. Such interlocking of bivalents was not found in pachytene...