Some reflections on double fertilization, from its discovery to the present.

The fusion of one sperm with the egg cell to form the embryo and of the other sperm with the polar fusion nucleus to give rise to the endosperm ('double fertilization') was discovered by Nawaschin in 1898 in the liliaceous plants, Lilium martagon and Fritillaria tenella. The occurrence of two fusion events analogous to double fertilization has recently been described in some gymnosperm species although the product of the second fusion is a transient embryo, rather than the endosperm as in angiosperms. Recent investigations in angiosperms describe the cell biology and nuclear cytology of double fertilization and the successful in vitro demonstration of the two fusion events using isolated egg cells, central cells, and sperm cells and the development of the fusion products into the embryo and endosperm. Molecular and genetic studies on the component elements of double fertilization have focused on the identification of mutants of Arabidopsis thaliana that display developmental patterns in the seed that result in autonomous endosperm development and even partial embryogenesis in the absence of fertilization. Characterization of the genes and their protein products has provided evidence for a predominant effect of maternal gametophytic genes and of silencing of paternal genes during double fertilization. Contents Summary 565 I. Introduction 566 II. Discovery of double fertilization 566 III. Seed development without double fertilization 568 IV. A case for double fertilization in gymnosperms 570 V. Structural and cytological perspectives on double fertilization 571 VI. In vitro double fertilization 575 VII. Genetic and molecular perspectives 576 VIII. Concluding comments 578 Acknowledgements 579 References 579.