Rice Genetics IV - IRRI books - International Rice Research Institute

asexual reproduction through apomixis, the disincentive of buying expensive hybridseed annually would be removed. Apomixis would render hybrid seed productionmore flexible and less costly, and farmers could replant their seed indefinitely(Koltunow et al 1995, Calzada et al 1996, Savidan 2000). Even poor farmers couldenjoy the greater productivity of hybrid rice.An overview of apomixisSexual reproduction in the ovaryFigure 1A summarizes the normal pathway of sexual reproduction in the ovary ofcereals. The product of the pathway is a developing seed consisting of a diploid embryo,a triploid endosperm, and a seed coat. The double fertilization event leading toformation of the embryo and the endosperm occurs in the embryo sac after the deliveryof two sperm nuclei by a pollen tube. The embryo sac and the pollen are bothproducts of meiosis. If the flower in question belongs to an F 1 hybrid plant derivedfrom divergent inbred parents, recombination between the parental chromosomes willoccur during both male and female meiosis, producing an endosperm, an embryo, andhence an F 2 plant that are genetically distinct from the hybrid and the two parents.Types of apomixisApomixis has been found in more than 300 species of plants, including close relativesof cereals such as maize, wheat, Pennisetum, and sorghum (Asker and Jerling 1992,Sharma and Thorpe 1995, Peel et al 1997, Huang and Chen 1999). The asexual embryoof apomictic seed is formed without passage through a normal meiosis (Koltunow1993). This is illustrated in Figure 1A for the form of apomixis known as adventitiousembryony; the embryo is genetically identical to maternal tissue. The other majorforms of apomixis are known as diplospory and apospory.In all three forms of apomixis, male sporogenesis and gametogenesis are generallynormal, but female sporogenesis and gametogenesis are altered (diplospory) orsuffer competition from an asexual embryo (apospory and adventitious embryony).The diplosporous embryo arises from an unreduced cell of the sexual embryo sac,whereas the aposporous embryo arises from an embryo sac formed from a singledifferentiated nucellar cell adjacent to the sexual embryo sac. By contrast, adventitiousembryos arise directly from cells in maternal tissue, most frequently from thenucellus (Fig. 1A).In the family Poaceae, which includes the cereals, apomixis is predominantlyeither diplosporous or aposporous, with the endosperm formed by pseudogamy, i.e.,the fertilization of the polar nucleus or nuclei by a reduced male gamete. We areattempting to introduce into hybrid rice a synthetic form of apomixis that is unusualin its combination of adventitious embryony in the nucellus with pseudogamous endospermformation (Fig. 1B). If successful, this approach will generate a seed containingan embryo that has bypassed meiosis completely. The embryo will be geneticallyidentical to the F 1 plant on which it forms; the accompanying endosperm, bycontrast, will be formed by the normal sexual pathway and will remain triploid. Be-378 Bennett et al