| Method |
To achieve double fertilization characteristic of seed
plants, the development of male and female gametophytes
requires two and three postmeiotic mitotic divisions, respectively.
Male gametogenesis is initiated with the differentiation
and meiotic division of diploid microspore
mother cells, which originate from archespores of another
primordia (McCormick 2004 PMID 15037731, primary source).
After meiosis, four haploid microspores form a tetrad that
later disperses free microspores (Fig. 1). The microspores
undergo asymmetric mitoses, giving rise to an immature
pollen grain with generative and vegetative cells. The vegetative
cell ceases division, whereas the generative cell
undergoes additional mitosis, leading to two sperm cells
required for double fertilization (Fig. 1).
The female gametophyte differentiates from diploid
ovule cells by differentiation of a megaspore mother cell,
which undergoes meiotic division into four megaspores
(primary source fig. 2). More than 70% of flowering
plants, including the model organisms Arabidopsis and
rice, exhibit the polygonum type of gametophyte development
in which three of four megaspores degenerate, leaving
a single meiotic product as the functional haploid
megaspore (Yadegari and Drews 2004 PMID 15075395) (Fig. 1). The megaspore
undergoes three mitotic divisions to produce eight
nuclei of the embryo sack: one of the egg cell, two of the
synergid cells, three of the antipodal cells, and two polar
nuclei that will undergo a fusion to form the diploid nucleus
of the central cell (Fig. 1). |