Rice Genetics IV - IRRI books - International Rice Research Institute

South Asia (particularly on the west coast of India), Southeast Asia (including the eastcoast of India), and China tend to differentiate (Cai and Morishima 1996). Recently,similar geographical differentiation was observed in organellar markers (Akimoto1999). The geographical differentiation in O. rufipogon is accounted for by “isolationby distance” and adaptation to local environments that occurred during the expansionof this species in Asia and Oceania. A reproductive isolation barrier does not seem todevelop within O. rufipogon, though a more extensive survey is needed.Indica versus japonica differentiation in cultivated racesThe indica-japonica problem has been argued repeatedly as reviewed by Oka (1988).Various molecular studies consistently showed a distinct difference between thesetwo major varietal groups in nuclear (Second 1982, Glaszmann 1987, Wang andTanksley 1989, Nakano et al 1992) and organellar genomes (Kadowaki et al 1988,Dally and Second 1990, Second and Wang 1992, Ishii et al 1993, Chen et al 1993).When typical indica and japonica types were compared, many genes and charactersshowed nonrandom association with each other to clearly separate the two types.When a large number of primitive cultivars are analyzed without a priori criteria,indica versus japonica differentiation is the principal variation, but some varietiesthat do not belong to either of the two types are also found. Such atypical cultivarsare not necessarily intermediate on the axis distinguishing indica and japonica typesor a recombined type, but seem to vary on a different variation axis. Himalayanhilly areas are known as the homeland of the atypical varieties (Sano and Morishima1992). Further, some cultivars grown in low-lying deepwater areas in the Bengaldelta are also atypical types (Glaszmann 1987, Hakoda et al 1990). A recent studyby Cai and Morishima (2000a) demonstrated that primitive cultivars collected indeepwater areas in Bangladesh included a japonica-like group and an atypical group,which did not belong to either type (Table 2). It was further suggested that geographicaldifferentiation could precede seasonal ecotype differentiation (aman, aus,and boro).Table 2. Classification of indigenous cultivars collected in Bangladesh deepwaterareas. Numbers in parentheses show the number of cultivars collected in KhulnaDistrict. (From Cai and Morishima 2000a.)Seasonal ecotypeIsozyme cluster a Boro Aus T. aman B. aman Rayada Ashina1 (japonica) 1 32 1 1 1 2(2)3 1(1) 10 (4) 4 (4)4 3(3) 1(1) 1(1)5 1 5(2 )6 (indica) 1 28 2aBased on cluster analysis of 8 polymorphic isozymes.Evolution and domestication of rice 69