Rice Genetics IV - IRRI books - International Rice Research Institute

Search for the immediate ancestor of the domesticatesThe four directions of differentiation mentioned above are conceptually independentfrom each other. Yet, they have proceeded, probably interacting with each other in theprocess of domestication. The evolutionary role of perenniality, geographical variation,and indica versus japonica differentiation will be discussed below in relation tothe domestication process.Which is the immediate ancestor of domesticates, the perennial or annual type? Ithas been a subject of discussion whether the perennial or annual type is the ancestor ofO. sativa. Sano et al (1980) inferred that the perennial-annual intermediate type is mostprobably the immediate ancestor. “Intermediate type” implies the population that ishabitually clonal and partially outbreeding but that can propagate sexually if seed propagationis advantageous. Such populations are now mostly the secondary products ofnatural hybridization between perennial wild and cultivated plants. When the primitiveperennial population was exposed to disturbed or dry conditions, the population genotypeprobably shifted toward wild annuals in natural habitats or a primitive cultivatedtype in man-made habitats through the “intermediate type.” A trade-off which constrainsenergy allocation into sexual and asexual reproduction might have moved the plantstoward higher seed yielders.Which geographical races of wild rice evolved into incipient domesticates? Olderrice remains excavated to date are concentrated in the middle and lower basin of theYangtze River in China. Though O. rufipogon is not distributed at present in thisarea, some evidence suggests the existence of wild rice in the past (Sato et al 1991).It seems difficult, however, to determine the exact place of origin of cultivated riceuntil more archaeological evidence, particularly on wild rice, will accumulate.For genetic relationships between particular wild and cultivated rice, the followingsubjects are worth considering. First, Chinese wild rice strains, in particularthose collected in the northern fringe of distribution of O. rufipogon, have somejaponica-specific genes and characters (Second 1985, Sano et al 1989, Morishimaand Gadrinab 1987, Nakano et al 1992, Cai et al 1995). Second, some indigenouscultivars grown in deepwater areas in Bangladesh carry particular isozyme allelessuch as Est10-4 and Amp5-4, which are rare in cultivars but not rare in wild populations(Cai and Morishima 2000a). Third, the annual wild rice distributed on thewest coast of India and primitive cultivars growing nearby showed high geneticsimilarity in isozymes, and both showed similarity with the japonica type in cpDNAas well as in rDNA according to Lolo and Second (1988).Are indica and japonica types monophyletic or diphyletic? Oka and his groupconsidered that indica and japonica types have diverged as domestication proceeded.This view is mainly based on the fact that wild rice has a potential to evolve intoindica as well as japonica types (Oka and Morishima 1982) and that indica versusjaponica differentiation was not found in O. rufipogon. A recent collection of O.rufipogon obtained from a broader geographical range, however, yielded a slightlydifferent variation pattern. A tendency toward indica-japonica differentiation in termsof particular association among genes or characters was observed among wild strains(Second 1985, Morishima and Gadrinab 1987, Dally and Second 1990, Nakano et70 Morishima