In angiosperms, development of the male gametophyte starts out with meiotic division of the microspore mother cell generating four haploid microspores. Subsequently, the haploid microspores undergo two mitotic divisions, referred to as pollen mitoses. The first pollen mitosis involves an asymmetric division of the haploid microspore resulting in a large vegetative cell and a small generative cell. The vegetative cell receives most of the cytoplasm from the microspore and completely envelopes the generative cell (Fig. 2). The generative cell then gives rise to two sperm cells by another mitotic division (second pollen mitosis). During fertilization, the two sperm cells move towards the ovule through the growing pollen tube. One of them fuses with the egg cell giving rise to the zygote, whereas the other fuses with the central cell to produce the precursor cell of the endosperm (double fertilization).
The Lycopersicon type of maternal plastid inheritance involves plastid exclusion during the first pollen mitosis. The extremely asymmetric division of the microspore results in a vegetative cell that contains all plastids and a generative cell that is free of plastids (Fig. 2). Consequently, also both sperm cells lack plas-tids. It is generally assumed that plastid inheritance in the majority of angiosperm species follows the exclusion mechanism of the Lycopersicon type (Hagemann and Schröder 1989; Hagemann 2002; Table 2).
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