In angiosperm fertilization, the pollen tube enters the female gametophyte via one of two synergids, in which the pollen tube ceases elongation and releases its contents, including two sperm cells. Whether the entrance of the pollen tube into a synergid occurs at random or is determined in advance is currently unclear. We examined two dimorphic synergids of Allium tuberosum (Amaryllidaceae). The smaller of the two cells becomes a degenerative synergid; the pollen tube enters this smaller cell and gains access to the female gametophyte for sperm discharge. The larger cell becomes a persistent synergid, which can persist for up to four days after anthesis. We further confirmed the dimorphism of the two synergids of A. tuberosum by isolating the egg apparatus from living, dissected ovule. The size difference between synergids increased with development. We also examined the calcium distribution in developing synergids, finding that the small synergid accumulated more calcium precipitates than the large one, which may reflect the initiation of its degenerative process or attraction of the pollen tube. The dimorphism in morphology, structure and function between the two synergids in A. tuberosum provides an experimental platform for investigating the regulatory mechanisms underlying both the development and function of these structures.
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Vol. 53 • No. 3–4