Detection in primitive gymnosperms of proteins and glycoproteins of possible significance in reproduction

Abstract

THE modern cycads are the remnant of a gymnosperm group that has been in existence for at least 200 Myr¹. Cycads are dioecious, with separate male and female plants, and fertilisation is effected by motile male gametes—so-called zooidogamous reproduction. Zooidogamy is found also in Ginkgo, another dioecious gymnosperm of considerable antiquity¹. Reproduction in all other gymnosperms and in flowering plants is siphonogamous. Here fertilisation occurs through a pollen tube which conveys the non-motile male gamete to the egg. A pollen tube is produced in the zooidogamous gymnosperms, but it is not involved in gamete transfer; it simply anchors the male gametophyte to the ovule during gametogenesis (Fig. 1). The ripe male gametes are released to swim freely in a pool of fluid bathing the openings of the egg-containing archegonia (Fig. 1). It is not known how the gametes that carry out fertilisation are selected in these plants. Dioecism ensures outcrossing, but there is presumably some form of intraspecific screening. The male gametophyte or male gamete could, for example, identify itself to the parent of its prospective mate, or the male gamete to the egg cell at the threshold of syngamy. I report here the occurrence in cycads of proteins and glycoproteins at sites in the pollen grain and ovule where they might be involved in reproduction.