Post-Pollination Ovule Development inDendrobiumOrchids: III. Fine Structure of Meiotic Prophase I.

Abstract

Some of the ultrastructural modifications which occur during meiotic prophase I in three hybrids of Dendrobium orchids have been examined by electron microscopy. In early prophase I the enlarged nucleus has no annulate pores in its envelope. The onset of nuclear break down is evidenced by the reappearance of pore-like discontinuities in a small region of the envelope. During this stage the chromosomes exhibit synaptinemal complexes. Continued nuclear envelope destruction occurs as the number of pores increases with new pores being formed in regions of the envelope adjacent to those pores already present. Initially, the cytoplasmic organelles are distributed throughout the megasporocyte. Subsequently they are concentrated in the perinuclear cytoplasm and with the progressive break down of the nucleus all the membrane-bound organelles of the megasporocyte are degenerated. It is proposed that marked decrease in all cytoplasmic components during prophase I accounts for the increase in numbers of phragmosomes which may be formed by the coalescence of degraded organelles. Very few identifiable organelles remain in meiotic metaphase I; however, mitotic metaphase figures in proliferating integument exhibit no comparable organelle destruction. Several persisting ultrastructural features of the megasporocyte which are described in earlier studies are also mentioned. These include the presence of rhopheocytosis, lipid micro-droplets, and small Golgi-apparati; and the absence of intercellular plasmodesmata. Developmental patterns are established and discussed.