Ribosomal gene spacer length variability in cultivated and wild rice species

Restriction fragment length polymorphism of the rDNA spacer was studied in the genus Oryza using a cloned rice rDNA probe. One-hundred-five accessions, including 58 cultivated rice and 47 wild species with various genome types, were analysed. Seven size classes differing from one another by an "increment" of ca. 300 bp were observed amongst the Asiatic cultivated rice of the species O. sativa. A general tendency from a smaller spacer in the Japonica subtypes to longer ones in Indica is observed. Classification as Japonica or Indica on the basis of rDNA pattern generally agrees with classification based on isozyme patterns. In contrast, African rice of the species O. glaberrima does not display any rDNA size variation. When wild species are considered, extensive variation is observed, but the fragment sizes do not fall into regularly increasing size classes except for O. rufipogon and O. longistaminata. The variation is greater in these species than in the cultivated ones.