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The formation of a polyploid endosperm tissue has long been considered a unique and defining feature (autapomorphy) of angiosperms. Contemporaneous with the fertilization of an egg nucleus by a sperm nucleus in Ephedra trifurca (a nonflowering seed plant closely related to angiosperms), a second fertilization event has previously been shown to occur between a second sperm nucleus and the sister nucleus of the egg nucleus. Development of the second fertilization product is now shown to be fundamentally similar to that of endosperm in primitive flowering plants: both are characterized by an initial period of free nuclear proliferation followed by a process of cellularization. In Ephedra, however, the second fertilization product ultimately yields additional embryos. If double fertilization in Ephedra and angiosperms is evolutionarily homologous, it is likely that endosperm evolved from a supernumerary fertilization event that originally produced embryos into one that produced a specialized nonembryo tissue dedicated to the nourishment of the zygotic embryo.
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