Rice Genetics IV - IRRI books - International Rice Research Institute

ility of rice productivity (Lu 1999). As a consequence, the sustainability of worldfood security is threatened. Future increases in rice production will essentially rely onthe use of genetic resources in the rice gene pool, including wild species of rice.Wild species of rice have diversified in a wide range of environments over 40million years (Stebbins 1981) and have become a vast gene pool valuable for thegenetic improvement of rice varieties (Khush 1997). Recent advances in molecularbreeding approaches hold tremendous potential for such improvement through thetransfer of beneficial genes from wild rice species to cultivars (Brar and Khush 1997,Xiao et al 1996, Tanksley and McCouch 1997). However, the effective utility andconservation management of the valuable genetic diversity in the rice gene pool relysignificantly on a clear understanding of the evolutionary relationships of rice speciesand, subsequently, the development of a natural classification of the genus Oryza (Geet al 1999, Lu 1999).In this chapter, we briefly review the historical changes of classification in thegenus Oryza and the related phylogenetic studies, and introduce our recent phylogeneticstudy on the rice genus using sequences of two nuclear Adh genes and onechloroplast gene, matK. We emphasize relationships among genome types and theorigin of the tetraploid species in the genus, and compare the multiple gene phylogenieswith species relationships based on morphology, cytogenetics, and other molecularstudies. Furthermore, we discuss some remaining questions and perspectivesconcerning studies on the phylogeny and evolution of the rice genus.Historical perspectiveIntrageneric grouping and classification of the genusThe genus Oryza L. is classified under the tribe Oryzeae, subfamily Oryzoideae, ofthe grass family Poaceae (Gramineae) (Lu 1999). This genus includes two cultivatedspecies and more than 20 wild species, distributed in tropical Asia, Africa, Australia,and Central and South America (Khush 1997). Since the description of Oryza byLinnaeus (1753), many studies on taxonomy, biosystematics, and phylogeny of theOryza species have been conducted (Prodoehl 1922, Roschevicz 1931, Sampath 1962,Tateoka 1963, Sharma and Shastry 1965, Second 1985, Vaughan 1989, 1994,Morishima et al 1992, Wang et al 1992, Aggarwal et al 1999, Lu 1999). Taxonomistssuch as Roschevicz (1931), Sampath (1962), Tateoka (1963), Sharma and Shastry(1965), Vaughan (1989, 1994), and Lu (1999) have made major taxonomic revisionsof the genus, which gave rise to modern classification systems of the genus Oryza(Table 1).The delimitation of Oryza has varied over time since Roschevicz’s (1931) fundamentalrevision, which recognized 20 species in four sections (Table 1). Two previouslyrecognized Oryza species, O. subulata and O. coarctata, were removed fromOryza to form two monotypic genera, Rhynchoryza and Porteresia, respectively(Tateoka 1965a). Three species, O. angustifolia, O. perrierri, and O. tisseranti, weretransferred to the closely related genus Leersia (Launert 1965). These changes havebeen accepted in the recent taxonomic treatments (Vaughan 1989, 1994, Lu 1999).90 Song Ge et al