Flowering plants have evolved a unique reproductive process called double fertilization, whereby two dimorphic female gametes are fertilized by two immotile sperm cells conveyed by the pollen tube. The two sperm cells are arranged in tandem with a leading pollen tube nucleus to form the male germ unit and are placed under the same genetic controls. Genes controlling double fertilization have been identified, but whether each sperm cell is able to fertilize either female gamete is still unclear. The dynamics of individual sperm cells after their release in the female tissue remain largely unknown. In this study, we photolabeled individual isomorphic sperm cells before their release and analyzed their fate during double fertilization in Arabidopsis thaliana. We found that sperm delivery was composed of three steps. Sperm cells were projected together to the boundary between the two female gametes. After a long period of immobility, each sperm cell fused with either female gamete in no particular order, and no preference was observed for either female gamete. Our results suggest that the two sperm cells at the front and back of the male germ unit are functionally equivalent and suggest unexpected cell-cell communications required for sperm cells to coordinate double fertilization of the two female gametes.
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