Rice Genetics IV - IRRI books - International Rice Research Institute

marked by a deletion/chromosomal rearrangement, where f = 1/number of targets andN = number of mutations surveyed. With 10,000 mutants screened, the probability ofdetecting a mutation in a specific gene is about 52%.Figure 1 summarizes the status of the IR64 mutant collections produced by threemutagens—DEB, fast neutron, and gamma ray (cobalt 60). It should be emphasizedthat we deliberately chose to induce as many mutations as the genome can tolerate.We reason that the background mutations will mostly be silent. Once an interestingphenotype is identified, genetic control can be determined by cosegregation analysis.In the first phase of mutant production, we maintained the pedigree by extracting 6–10 plants from an M 2 family. Although it was useful to have sister lines to infer inheritance,this advantage is outweighed by the burden of maintaining a large number oflines with redundant mutations. Thus, in the second stage, we advanced all M 1 linesby single-seed descent.Phenotypic characterizationMorphological variationsWe used several obvious mutant phenotypes with known genetic control to comparethe spectrum of mutations produced by different mutagens (Table 1). Approximately6% of the mutant population shows morphological variants. Most of the morphologicalmutants have obvious phenotypes that affect the vegetative and reproductive stages.It appears that the frequencies of inducing variability by three mutagens are similar(0.2% to 0.05%). We have not made special attempts to examine quantitative varia-IR64 breeder seedsDiepoxybutane0.006%Fast neutron33 GYGamma ray250 GY3,200 M 2 families 7,300 M 2 families 7,900 M 2 familiesAbout 18,400 M 3 /M 4 families (target 40,000)Systematic phenotyping• morphological (vegetative and reproductive)• disease and insect responses• water stresses (too much and too little) and salinity• mineral deficiency toleranceFig. 1. Development of IR64 mutants using three mutagens: diepoxybutane(DEB), fast neutron (FN), and gamma ray (GR). Treatment doses are indicated.Both FN and GR treatments were performed at the InternationalAtomic Energy Commission, Vienna, Austria. Gy = Gray, 1 joule kg –1 oftarget specimen.242 Leung et al