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Double Fertilization in Gnetales: Implications for Understanding Reproductive Diversification among Seed Plants

William E. Friedman and Jeffrey S. Carmichael
International Journal of Plant Sciences
Vol. 157, No. 6, Supplement: Biology and Evolution of the Gnetales (Nov., 1996), pp. S77-S94
Stable URL: http://www.jstor.org/stable/2475210
Page Count: 18
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Double Fertilization in Gnetales: Implications for Understanding Reproductive Diversification among Seed Plants
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Abstract

The coupled processes of double fertilization and postfertilization endosperm formation have long been viewed as important and synapomorphic features of flowering plants. Recent developmental studies of fertilization in the nonflowering seed plants Ephedra and Gnetum clearly document a regular process of double fertilization. The condition for Welwitschia remains unknown. Unlike angiosperms, the product of the second fertilization event in Ephedra and Gnetum is diploid and expresses the developmental program of an embryo. Explicit criteria for the evaluation of evolutionary homology indicate that the processes of double fertilization in Gnetales and angiosperms are homologous, having first evolved in a common ancestor of these two lineages. It is hypothesized that the second fertilization product initially yielded a supernumerary embryo genetically identical to the normal embryo. This rudimentary process is expressed in relatively unmodified form in Ephedra. Other reproductive features of Ephedra that are conserved from the common ancestor of angiosperms and Gnetales are a monosporic female gametophyte with two or more eggs and the partial allocation of maternal resources, for subsequent embryo nourishment, to a large cellular female gametophyte in advance of fertilization. In Gnetum, evolutionary modification of reproduction resulted in a paedomorphic female gametophyte that is fertilized at a "juvenilized" free nuclear stage of development. In Gnetum gnemon, egg cells are not formed and maternal provisioning of the embryo-nourishing female gametophyte takes place entirely after fertilization. The biological significance of double fertilization that does not form endosperm, in Ephedra and Gnetum, is currently unknown. This process may be biologically neutral. However, due consideration must be given to the hypothesis that the second fertilization product in Ephedra and Gnetum, while expressing a structural/developmental program of an embryo, may "behave" as a rudimentary and cryptic endosperm and assist with the development of the normal embryo.

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