Double Fertilization in Flowering Plants: Origin, Mechanisms and New Information from in vitro Fertilization

Abstract

One hundred years ago, Sergius Nawaschin (1898), followed a few months later by Léon Guignard (1899) suggested for the first time that fertilization is actually double in Lilium martagon and Fritillaria tenella. Double fertilization is now believed to be ubiquitous among flowering plants. The pollen grain (male gametophyte) germinates on a pistil. It grows a tube in which two male gametes are moved toward an embryo sac (female gametophyte) that contains in particular two female gametes, the egg and the central cells, and two synergids. Once the pollen tube has reached the embryo sac, it releases the two male gametes into a degenerated synergid. One fuses with the egg cell to form a zygote that further develops into the embryo, and the other fuses with the central cell to form a polyploid (generally triploid) cell that develops into the endosperm. Thus, two fertilizations happen simultaneously. This sequence was well documented, especially in the sixties by William Jensen using transmission electron microscopy (Jensen 1964, Jensen and Fischer, 1968). However the underlying mechanisms are not well known, mainly because fertilization takes place inside the embryo sac itself embedded in sporophytic tissues. The isolation of both male and female gametes from the gametophytes (Cass, 1973; Hu et al., 1985; also see review: Theunis et al., 1991; Huang and Russell, 1992a) and their fusion under in vitro conditions (Kranz et al., 1991; Kranz and Lörz, 1993; Faure et al., 1994; also see review: Dumas and Faure, 1995; Kranz and Dresselhaus, 1996) now allows new experimental investigations. The interest in fertilization, the central step of sexual reproduction, has therefore been renewed over the last ten years.