Rice Genetics IV - IRRI books - International Rice Research Institute

al 1992, Sun et al 1996a,b,c), though the degree of nonrandom association is muchlower than in cultivars.The diphyletic hypothesis postulates that indica and japonica types originatedfrom different lineages of O. rufipogon (Second 1982, Sato 1996). Accumulatedobservations that indica and japonica types consistently showed a clear differencebetween each other and closer affinity with different wild accessions than with eachother seem to support the diphyletic hypothesis. Recent archaeological excavationsin China and analysis of rice remains (phytolith, Wang et al 1998; DNA, Sato et al1995) suggest that Chinese wild rice played an important role in the origin of japonicarice.I am inclined to the view that rice domestication has been a diffused process inboth space and time. During a long period, prototypes of indica and japonica typeshave probably become two dominant groups and have accumulated marked differencesin morphological and adaptive traits, keeping their respective genic constitutioninherited from their founders.Genetic basis of the domestication syndromeMultilocus system in evolutionRelated species or ecotypes are differentiated by a particular pattern of associationbetween states of different characters or between alleles at different loci. Suchmultilocus covariation is called gametic disequilibrium. In rice, variations betweenwild and cultivated types, between perennial and annual types, and between indicaand japonica types are good examples of gametic disequilibrium. Nonrandom associationis caused by various factors such as selection (coadaptation), linkage, pleiotropy,and founder effect, as discussed by Hedrick et al (1978). It is difficult to dissectthe underlying mechanism without elaborate experiments.In rice, linkage blocks harboring genes for internal barriers and fitness traits weredetected by isolating the relevant chromosomal segments using the backcross method(Sano 1992). Key factors for gametic disequilibrium in quantitative traits can be elucidatedto some extent by observing the shift of correlations in the hybrid generations.Parental associations that disappear in the F 2 are mostly due to selection for coadaptedtraits. On the other hand, those that remain in the F 2 or early generations are probablydue to linkage and/or pleiotropy. Many studies showed that parental associations partlypersisted in the F 2 (character coherence) though they were weak but significant. In thecrosses of wild × cultivated, annual × perennial, and indica × japonica rice strains, wehave experienced that parental associations in quantitative traits mostly disappearedin the F 2 . This indicates that those associations are not due to linkage or pleiotropy ofa few major genes.QTL clusters responsible for character associationsAdvances in molecular genetics enabled us to dissect the genetic basis of quantitativedifferences between species or ecotypes, which has been studied only by statisticalgeneticsmethods until recently. Targeting differentiation in the Asian AA genomeEvolution and domestication of rice 71