Rice Genetics IV - IRRI books - International Rice Research Institute

Interspecific relatedness was also discussed based on the precise analysis of particulargenes including mobile elements (rDNA, Sano and Sano 1990, Cordesse et al 1990;SINE, Hirano et al 1994; CatA, Iwamoto et al 1999; Adh, Ge et al 1999; MITE, Kanazawaet al 2000). The finding of such specific DNA will increase our understanding of thedifferentiation process at the molecular level. It should be noted, however, that genelineage thus revealed does not necessarily reflect speciation.Genetic diversity within taxaThe amount of genetic diversity within a species is important for understanding theevolutionary status of the species. O. sativa has lower genetic diversity at the molecularlevel than O. rufipogon. This contrasts with the variation pattern observed at the phenotypiclevel, in which O. sativa is more diverse than O. rufipogon. The genetic diversityin O. glaberrima is much lower than that in O. sativa. This can be accounted for by thedifference in breeding system of their respective wild ancestors.Among wild taxa, O. rufipogon generally showed the highest diversity, followed byO. longistaminata or O. glumaepatula (Table 1). This may be attributable to its variabilityin perenniality associated with allogamy, which conferred a potential to preservegenetic diversity in the populations (Barbier 1989). Evolution from perennials to annualsand from outbreeders to inbreeders is a general trend found in higher plants. Yet,precise phylogeny in this species complex remains unsolved.Evolutionary trends in the Asian AA genome gene poolRice has two primary gene pools, corresponding to O. sativa and O. glaberrima,which contain cultivated races and their wild and weedy relatives, respectively. Withinthe primary gene pool of O. sativa, four directions of differentiation are recognized(Fig. 2): (1) differentiation from wild to cultivated types, (2) differentiation from perennialto annual types in wild races, (3) geographical differentiation in wild races,and (4) varietal differentiation toward indica and japonica types.Table 1. Genetic diversity within species at isozyme and DNA levels. H and H’ stand for averagegene diversity and diversity index, respectively. (Modified from Morishima et al 1992.)IsozymecpDNASpecies nDNA rDNA mtDNA MITE eH a H b H b H’ c H b H d H b H’ fO. rufipogon 0.50 0.40 0.43 2.21 0.39 1.60 0.24 1.27O. longistaminata 0.26 0.19 0.43 2.07 0.19 0 0.12 1.35O. barthii 0.21 0.15 0.19 0 0.19 0.35 0.05 0O. glumaepatula 0.28 0.18 0.22 0.90 0.24 0.69 0.05 0.38O. meridionalis 0.11 0.18 0.18 0 0.16 0.320.06 0.64aRecomputed from Second (1985). b Akimoto (1999). c Sano and Sano (1990). d Recomputed from Dallyand Second (1990). e MITE = miniature inverted-repeat transposable elements. f Kanazawa et al (2000).66 Morishima