What is successive and simultaneous type of division during micro sporogenesis?
Answers
Explanation:
Microsporogenesis or male meiosis is the earliest step in pollen ontogeny. It consists of nuclear divisions associated with cytoplasmic divisions or cytokinesis. This process starts with microsporocytes or pollen mother cells enclosed in a callose envelope within which meiosis takes place. Cytokinesis takes place through the formation of intersporal walls composed of callose. Once meiosis is completed, the four microspores form a tetrad embedded within the callose wall of the pollen mother cell, until the callose is digested by an enzyme called callase. In most species, apertures are already visible at the late tetrad stage, suggesting that aperture pattern (shape, number and distribution of apertures on the pollen grain surface within the tetrad) is determined during microsporogenesis.
Two basic types of cytokinesis occur in angiosperms, successive or simultaneous (only a few cases of intermediate cytokinesis have been recorded). In successive cytokinesis, the cytoplasm is successively partitioned after each meiotic division. A dyad stage is thus observed which consists of two cells embedded within the pollen mother cell wall and separated by a callose wall. In simultaneous cytokinesis, intersporal callose walls are formed only after both nuclear divisions have taken place. These two types of cytokinesis result in different tetrad morphologies. Tetrads obtained through successive cytokinesis can be tetragonal, decussate, T-shaped, Z-shaped and linear, whereas tetrads resulting from simultaneous cytokinesis can be tetrahedral, rhomboidal, tetragonal and decussate.