Rice Genetics IV - IRRI books - International Rice Research Institute

MPManual pollination by breeders× FPMolecular switch to apomixisHybridApomictTo farmersApomict= sexual selfing= apomictic selfingFig. 2. Outline of the breeding of apomictic hybrid rice and its reproductionby farmers. FP = female parent, MP = male parent.breeder’s field by selfing (Fig. 2). After the breeder pollinates manually, the resultinghybrid contains the full set of the transgenes for switching to apomixis. When thehybrid plant flowers, it produces apomictic seeds. These seeds may be multiplied inthe breeder’s field to generate adequate amounts of seed for distribution to seed companiesand farmers, both of whom may subsequently reproduce the apomictic seed.Costly field-scale hybridization is not required at any stage.Mapping genes for control of apomixisWhich genes control the naturally occurring forms of apomixis? Little is known aboutthe genetic control of adventitious embryony except that in citrus a single gene controlsthe switch between polyembryonic and monoembryonic states and is dominantfor polyembryony (Iwamasa et al 1967). By contrast, genetic control of apospory anddiplospory is well studied, particularly in members of the Poaceae. Apospory anddiplosory may both be analyzed as a single trait or as a set of component traits.Apomeiosis, parthenogenesis, and facultativeness are some of the component traitsanalyzed genetically in diplosporous apomicts (Savidan 2000). Each component traitoften appears to be under single-locus control and DNA markers have been linked tosome of these loci.Apospory is controlled by a single locus in Brachiaria and in Pennisetum. Thefact that these loci share common DNA markers (Pessino et al 1997) strongly suggeststhat apospory has a common molecular basis in these genera. Pessino et al (1998)developed a fine map of the apospory region in Brachiaria. They identified markerslocated 1.2 and 5.7 cM on either side of the target locus. The markers showed syntenyto regions of maize chromosome 5 and rice chromosome 2. Parthenogenesis inaposporous Kentucky bluegrass (Poa pratensis) and apospory in buffelgrass (Cenchrusciliaris = Poa ciliare) are also controlled by single genetic loci linked to DNA markers(Barcaccia et al 1998, Gustine et al 1997).380 Bennett et al