ABSTRACT
Cytoplasmic preparation for the unequal first mitosis in non-vacuolate pollen of moth orchids (Pbalaenopsis) includes reorganization of the microtubular cytoskeleton and nuclear migration. Following meiotic cytokinesis, both microtubules and F-actin are unpolarized in microspores of persistent tetrads. Microtubules radiate from the centrally located nucleus and F-actin forms a reticulate pattern in the cytoplasm. Polarization of the microspores is marked by a dramatic reorganization of microtubules while the pattern of F-actin remains unchanged. We describe a novel system of microtubules at the generative pole (GPMS), which forms a polar structure at the distal surface and marks the path of nuclear migration prior to pollen mitosis. The GPMS consists of numerous microtubules that extend between the plasma membrane and nuclear envelope. The nucleus becomes displaced toward the generative pole and flattened in association with microtubules of the GPMS. Initiation of the GPMS is marked by a localized proliferation of ER and clearing of large organelles from the generative pole.
