Rice Genetics IV - IRRI books - International Rice Research Institute

testing Ka/Ks between the homeologous loci of the allotetraploid and the genes of thediploid species with the corresponding genomes will determine which homeologouslocus has undergone adaptive evolution.Recurrent formation of an allotetraploid genome. Multiple independent formationsof an allopolyploid genome or species from different populations of the diploidprogenitors have been observed in both plants and animals (Soltis and Soltis 2000).The O. officinalis complex with the B, C, and BC genomes is well known for itstaxonomic complexity in Oryza, and the polyploids, in particular, are difficult to group(Vaughan 1989). A recent study using Adh and matK genes suggested that the BCgenome originated independently in Africa and Asia (Ge et al 1999). The BC genomemay have also formed multiple times within each continent. The recurrent formationof the BC genome is probably responsible for the taxonomic difficulty in the speciescomplexes containing the B, C, and BC genomes (Tateoka and Pancho 1963, Tateoka1965b). Because allotetraploids in Oryza involved many genomes and originated fromdifferent lineages and at different times, Oryza should serve as a good model systemfor studying various aspects of polyploid genome evolution.Beyond its agronomic importance, rice has become a model organism for biologicalresearch. Because of its small genome size, Oryza sativa will be the first monocotand the second plant species (following Arabidopsis thaliana) with the entire genomesequenced (Sasaki 1998). The genus Oryza thus offers an excellent system for studiesof plant evolution at the genomic level. This will allow us to examine genomic changesthat are adaptively or agriculturally important. The availability of genomic informationof the selected nuclear gene markers, such as chromosomal locations, will allowus to investigate genome evolution. This will, on the one hand, help address a fundamentalquestion of which genomic consequences have provided adaptive advantagesfor allopolyploids and have made this mode of speciation so prevalent in angiosperms(Wendel 2000). On the other hand, with a robust phylogeny and multiple gene markers,many intriguing questions involving the biogeography of particular species, thegenome, and the whole genus can be addressed.ReferencesAbe T, Edanami T, Adachi E, Sasahara T. 1999. Phylogenetic relationships in the genus Oryzabased on mitochondrial RFLPs. Genes Genet. Syst. 74:23-27.Aggarwal RK, Brar DS, Khush GS. 1997. Two new genomes in the Oryza complex identifiedon the basis of molecular divergence analysis using total genomic DNA hybridization.Mol. Gen. Genet. 254:1-12.Aggarwal RK, Brar DS, Nandi S, Huang N, Khush GS. 1999. Phylogenetic relationships amongOryza species revealed by AFLP markers. Theor. Appl. Genet. 98:1320-1328.Brar DS, Khush GS. 1997. Alien introgression in rice. Plant Mol. Biol. 35:35-44.Cordesse F, Grellet F, Reddy AS, Delseny M. 1992. Genome specificity of rDNA spacer fragmentsfrom Oryza sativa L. Theor. Appl. Genet. 83:864-870.Dally AM, Second G. 1990. Chloroplast DNA diversity in wild and cultivated species of rice(genus Oryza, section Oryza): cladistic-mutation and genetic-distance analysis. Theor. Appl.Genet. 80:209-222.102 Song Ge et al