Rice Genetics IV - IRRI books - International Rice Research Institute

come and may use imprinting to achieve that influence. As inbreeding involves onlya single parent, the absence of parental conflict should abolish the need for imprinting.Any imprinting that remains in inbreeding species may reflect either an evolutionaryvestige from an outcrossing ancestor (Scott et al 1998) or an additional rolefor imprinting.If rice is strongly imprinted for endosperm production, maintenance of the 2m:1pgenotype will be essential and can be achieved by retaining fertilization of the centralcell by one sperm nucleus from pollen. If rice is not strongly imprinted for endospermproduction, autonomous endosperm formation (2m:0p) may be acceptable. The factthat rice is an inbreeder would suggest that imprinting may be weaker than in maize.However, our focus on hybrid rice and heterosis may restore the importance of imprinting.DNA methylation may be the molecular link between heterosis (Tsaftarisand Polidoros 2000) and imprinting (Luo et al 2000, Grossniklaus et al 2001). Methylationof maternal and paternal alleles controls the expression of zein and tubulingenes in the maize endosperm (Lund et al 1995a,b).Is apomixis safe nonsex?According to evolutionary biological theories, a dominant apomixis gene will rapidlybecome fixed in an outcrossing sexual population (Van Dijk and Van Damme 2000).Therefore, in theory, apomixis transgenes could have unconditional advantages thatmight result in their uncontrollable spread. A synthetic apomict that displays bothautonomous endosperm formation and autonomous embryo formation should be ableto dispense with the apparatus of pollination entirely and would not pose a threat tobiodiversity. However, a synthetic apomict that retained self-pollination for the sakeof triploid endosperm formation might release pollen to the environment and wouldhave to be designed to be safe (Savidan 2000).Our approach to achieving synthetic apomixis should satisfy this requirementbecause it includes a mechanism to ablate the sexual embryo (Fig. 1B). This ablationmechanism was devised principally to avoid competition between the adventitiousembryo and the sexual embryo but it serves the additional purpose of preventing geneflow by ablating the zygotic embryo in any plant to which pollen of the apomictmight spread from a farmer’s field.Our approach is also safe when viewed from breeders’ fields (Fig. 2). Breederswill need to grow very small numbers of the male and female parents of the hybrid.The feasibility and safety of our approach lies in the fact that we distribute thetransgenes required for synthetic apomixis between the two parents of the hybrid insuch a way that neither parent carries the apomixis trait and neither parent ablates thesexual embryo (see Fig. 3). Only when breeders manually cross the two parents is thefull complement of transgenes assembled together. Thus, the two parents will notablate their own embryos and pollen escaping from them will not carry the apomixistrait to neighboring fields. Pollen on apomictic hybrids in breeders’ fields will be justas safe as pollen on the apomict grown by farmers: the zygotic embryo of the recipientplant will be ablated and gene flow will be immediately arrested.386 Bennett et al