An electrophoretic survey of up to forty presumed isozyme loci was carried out in Asian and African cultivated rice (0. sativa and 0. glaberrima) and in weedy and wild forms of African 0. breviligulata on a total of 1,948 strains. Hidden variability was checked by a test of heat stability at thirty isozyme loci in the same species.The mean gene diversity index ("heterozygosity") was high (0.23) in 0. sativa, medium (0.14) in wild 0. breviligulata and low (0.06 and 0.03) in weedy 0. breviligulata and 0. glaberrima.In contrast, a maximum of seven alleles at a single locus could be distinguished in wild 0. breviligulata while only three at the most were found in 0. sativa and two in 0. glaberrima and the weedy forms of 0. breviligulata.Calculations of genetic distances showed that the cultivated, wild and weedy African species formed a genetic group distinct from 0. sativa. Multivariate analysis of the data on an individual strain basis confirmed this fact and showed in turn that most varieties of 0, sativa tended to cluster in two groups which correspond to the so-called Indica and Japonica subspecies. There was however a continuous array of intermediates between the two groups.Multivariate analysis also showed that the endemic African strains with the most genetic affinity to 0. sativa were certain strains of the weedy form of 0. breviligulata.Analysis of Fl pollen sterility among 115 strains of 0, sativa permitted the extraction of one small Indica and one small Japonica group of strains characterized by a high pollen sterility relationship but most strains were intermediate.Each group had little gene diversity, with more than 80% of the loci fixed for one allele. Their isozyme patterns were remarkably complementary in that most of the various gametic associations of alleles found in the numerous intermediate strains could be explained by hybridization between varieties belonging to one and the other group. Consequently, these were assumed to represent the ancestral isozyme patterns of the Indica and Japonica subsp.Similar genetic distances, which point to a divergence time of one to a few millions years ago were found between 0. glaberrima and the ancestral types of Indica and Japonica in the three combinations.Assuming the neutral theory of isozyme polymorphism, the data confirmed 1) This work was completed and the paper was written during a stay at the National Institute of Genetics, Mishima, Shizuoka-ken, 411 (Japan).Present address: Laboratoire de genetique, ORSTOM, 70-74, route d'Aulnay 93140 BONDY (France). 26G. SECOND that 0. glaberrima was domesticated from 0. breviligulata independently of 0, sativa. They also suggest that, at the origin of 0. sativa, the Indica and the Japonica types were also domesticated independently.The large diversity of 0, sativa would result : (i) primarily from introgressions that occurred between the ancestral cultivated types, together with the selection imposed by man, and (ii) secondarily, following the dispersion of cultivars, from the introgression of genes of wi...
Ninety-three accessions representing 21 species from the genus Oryza were examined for restriction fragment length polymorphism. The majority (78%) of the accessions, for which five individuals were tested, were found to be monomorphic. Most of the polymorphic accessions segregated for only one or two probes and appeared to be mixed pure lines. For most of the Oryza species tested, the majority of the genetic variation (83%) was found between accessions from different species with only 17% between accessions within species. Tetraploid species were found to have, on average, nearly 50% more alleles (unique fragments) per individual than diploid species reflecting the allopolyploid nature of their genomes.Classification of Oryza species based on RFLPs matches remarkably well previous classifications based on morphology, hybridization and isozymes. In the current study, four species complexes could be identified corresponding to those proposed by Vaughan (1989): the O. ridleyi complex, the O. meyeriana complex, the O. officinalis complex and the O. sativa complex. Within the O. sativa complex, accessions of O. rufipogon from Asia (including O. nivara) and perennial forms of O. rufipogon from Australia clustered together with accessions of cultivated rice O. sativa. Surprisingly, indica and japonica (the two major subspecies of cultivated rice) showed closer affinity with different accessions of wild O. Rufipogon than to each other, supporting a hypothesis of independent domestication events for these two types of rice. Australian annual wild rice O. meridionalis (previously classified as O. rufipogon) was clearly distinct from all other O. rufipogon accessions supporting its recent reclassification as O. meridionalis (Ng et al. 1981). Using genetic relatedness as a criterion, it was possible to identify the closest living diploid relatives of the currently known tetraploid rice species. Results from these analyses suggest that BBCC tetraploids (O. malampuzhaensis, O. punctata and O. minuta) are either of independent origins or have experienced introgression from sympatric C-genome diploid rice species. CCDD tetraploid species from America (O. latifolia, O. alta and O. grandiglumis) may be of ancient origin since they show a closer affinity to each other than to any known diploid species. Their closest living diploid relatives belong to C genome (O. eichingeri) and E genome (O. Australiensis) species. Comparisons among African, Australian and Asian rice species suggest that Oryza species in Africa and Australia are of polyphyletic origin and probably migrated to these regions at different times in the past.Finally, on a practical note, the majority of probes used in this study detected polymorphism between cultivated rice and its wild relatives. Hence, RFLP markers and maps based on such markers are likely to be very useful in monitoring and aiding introgression of genes from wild rice into modern cultivars.
SummaryThe wild and weedy species of the Sativa group of Oryza were studied for their isozyme polymorphism at 24 loci for 181 plants, each
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