Development of the Male Gametophyte of Ginkgo biloba: A Window into the Reproductive Biology of Early Seed Plants

Abstract

It has been a century since the discovery of zooidogamous reproduction among seed plants by Hirase [1, 2] and Ikeno [3]. The initial observations of motile sperm in Ginkgo and Cycas represented the culmination of progress, beginning with the discovery of the pollen tube by Amici [4], in the field of plant reproductive biology during the nineteenth century. Discovery of zooidogamy in Ginkgo and cycads in 1896 provided a critical connection, in terms of evolutionary history, between the life cycles of non-seed plants (``cryptogams'') with motile sperm and the life cycles of previously described seed plants (conifers and angiosperms) with pollen tubes and non-motile sperm [5]. Moreover, the presence of motile sperm within the male gametophytes of cycads and Ginkgo confirmed a prediction made almost one half century earlier by the renowned biologist Wilhelm Hofmeister [6] that flagellate sperm might be found among representatives of the seed plants [5].