Rice Genetics IV - IRRI books - International Rice Research Institute

Wild(O. rufipogon)Geographical differentiationPerennialAnnualWeedy typeJaponicaIndicaCultivated(O. sativa)Fig. 2. Differentiation within the primary gene pool ofO. sativa.Differentiation from wild to cultivated types: domesticationIn seed crops, the cultivated type is characterized by nonshedding of seeds, rapid anduniform germination, efficient seed production, and determinate growth in comparisonwith the wild type. At the incipient stage of domestication, planting harvestedseeds by man automatically selected this “adaptive syndrome of domestication” (Harlan1975). This holds true in rice.Oryza sativa and O. rufipogon are genetically very close in spite of their clearphenotypic difference, and barely distinguishable by molecular markers. Wild andcultivated plants easily interbreed if grown nearby. Gene flow is mainly from predominantlyinbreeding cultivated races to partially outbreeding wild races (outcrossingrate ranging from 10% to 60%, Oka 1988). Gene flow might have played animportant role in diversification of the domesticates as in many other crops. Evennow, natural hybridization between wild and cultivated rice occurs frequently andhybrid derivatives are found abundantly as weed types. We rarely find truly wildpopulations without introgression of genes from cultivated rice in tropical rice-growingareas.Differentiation from the perennial to annual type in wild racesPerennial and annual types exhibit contrasting life-history traits that characterize fecundity/survivalschedules of the individuals. The perennial (polycarpic) type showsvigorous vegetative growth, low seed productivity, late flowering, and a high outcrossingrate, whereas the annual (monocarpic) type shows the opposite characteristics(Oka 1988, Sano and Morishima 1982).Evolution and domestication of rice 67