Rice Genetics IV - IRRI books - International Rice Research Institute

Issues related to achieving synthetic apomixisIs apomixis compatible with diploidy?Our goal is to introduce apomixis into diploid hybrid rice, and yet the overwhelmingmajority of apomicts are polyploid. Is our project therefore doomed to failure becauseapomixis requires polyploidy? Or are apomixis and polyploidy associated for otherreasons: perhaps both mechanisms help plants to survive and reproduce in adverseenvironments.Two reports on maize-Tripsacum hybrids suggest that apomixis may indeed becompatible with diploidy (Leblanc et al 1996, Grimanelli et al 1998b). In Tripsacum,all polyploids reproduce through diplospory and all diploids are sexual. In an effort totransfer apomixis from diplosporous tetraploid Tripsacum (2n = 4x = 72) into maize(2n = 20) through conventional backcrosses, Leblanc et al (1996) produced polyhaploidplants combining one complete set of chromosomes from each genus. Thesepolyhaploid plants were totally male sterile but viable seeds were produced apomicticallywhen they were pollinated using maize. Apomictic reproduction in suchpolyhaploids, which show a diploid-like chromosomal complement, suggested thatdiplosporous apomixis and polyploidy are not totally linked and that apomixis mightbe compatible with diploidy.Grimanelli et al (1998b) used RFLP markers linked to diplospory to analyze variousgenerations of maize-Tripsacum hybrids and backcross derivatives and to derivea model for the inheritance of diplosporous reproduction. The results suggested thatthe gene or genes controlling apomixis in Tripsacum are linked with a segregationdistorter-type system promoting the elimination of the apomixis alleles when transmittedthrough haploid gametes. This model offers an explanation of the relationshipbetween apomixis and polyploidy and suggests that the evolutionary importance ofthe segregation distortion system is to protect the diploid level from being invaded byapomixis.Until recently, all of the apomictic biotypes studied so far in the genus Hieraciumsubgenus Pilosella had been described as polyploids. However, Bicknell (1997) identifieda diploid apomictic form of H. aurantiacum reproducing through apospory.Seed set was low, apparently because of the presence of competing megagametophyteswithin each ovule. This report not only provides encouragement for efforts tointroduce apomixis into diploid rice but confirms the importance of ablating the sexualembryo to prevent competition with the apomictic embryo. Savidan (2000) reviewsother reports of diploid or dihaploid apomicts and concludes that, “the absence ofapomixis at the diploid level, as generally observed in the wild, is not a problem ofexpression but rather of transmission constraints.”Should the triploidy of the endosperm be preserved?One way of preventing competition between the apomictic and sexual embryos is toeliminate the sexual process entirely. Such an approach would, of course, have theimportant consequence of preventing normal endosperm formation and necessitatethe triggering of an autonomous (fertilization-independent) process. In autonomous384 Bennett et al