Introduction e gametes of owering plants are formed by discrete haploid gametophyte structures consisting of only a few cells that develop within the diploid reproductive oral organs. During spermatogenesis, each single haploid microspore divides asymmetrically to produce a larger vegetative cell that eventually gives rise to the pollen tube and a smaller germ, or generative, cell (reviewed in ,). In contrast to germline cells in metozoans , angiosperm male germ cells do not undergo regenerative stem cell divisions, but divide once to form a pair of sperm cells. ese sperm cells are delivered to the embryo sac via the pollen tube, where they fuse with egg and central cells to produce embryo and endosperm respectively. is process of double fertilization depends upon two functional sperm cells and is considered one of the major advances in the evolutionary success of owering plants. Despite this importance, the molecular mechanisms underlying many component processes, including the production of both male and female gametes, remain largely unknown.