Distribution Of Repetitive Sequences In The Genus Oryza And Its Relation With The Differentiation Of Rice

The genus Oryza has two cultivated species ( O. sativa L. and 0. glaberrima Sted.) and more than 20 wild species distributed throughout the tropics and subtropics. Repetitive DNA sequences are present in all higher plants and account for about 50 percent in rice genome. The genomic distribution of repetitive sequences is an important character for studying the origin and differentiation of different species in the genus. Two tandem repetitive DNA sequences, 45S rDNA and Os48, were used in this study. 45S rDNA is the encoding sequences for ribosomal RNA synthesis. Os48 is the subtelomeric tandem repeat sequence specific to rice AA genome. Chromosomes with large 45S rDNA arrays were recognized as the chromosomes with nucleolus organizing regions (NOR chromosomes). The distribution of the two repetitive sequences in different rice species and cultivars was identified using the fluorescence in situ hybridization (FISH) techniques.In cultivated rice, one pair of NOR chromosomes and two or three pairs of Os48 loci were found in Oryza sativa ssp. Japanica. The NOR was located at the end of short arm of chromosome 9. Wide compatibility variety 02428 has one pair of NOR chromosomes and three pairs of Os48 loci, which are very close to those in japonica rice. Two pairs of NORs and five to seven pairs of Os48 loci were detected in Oryza sativa ssp. indica by chromosome FISH. Widecompatibility varieties, Dular and Aus373, have two pairs of NORs and five pairs of Os48 loci, indicating that they are very close to indica rice.One or two pairs of NORs and two to five pairs of Os48 loci were observed in O. nivara. Among them, O. nivara with one pair of NORs and two pairs of Os48 loci was similar to that of japonica rice.In general, two pairs of NORs and seven or eight pairs of Os48 loci were detected in O. rufipogon. According to the number and intensity of signals of the two repetitive sequences, O. rufipogon was the ancestor of O. sativa ssp. indica. Moreover, one type of O. rufipogon with one pair of NORs and nine or ten pairs of Os48 loci located at the end of eight or nine pairs of chromosomes was first reported in Oryza genus.There are one pair of NORs and two, four or ten pairs of Os48 loci in O. glaberrima. O. barthii, a putative ancestor of O. glaberrima, has one pair of NORs and four pairs of Os48 loci. Two pairs of NORs and five pairs of Os48 loci were identified in O. longistamina.O. glumaepatula from Colombia has one pair of NORs and no Os48 loci, which is the only exception without Os48 loci among rice AA species.None Os48 FISH signal was detected from the 11 Oryza species with the BB, BBCC, CC, CCDD, EE and FF genomes. Three pairs of NORs were identified in O. punctata. There are two strong signals of NOR at the distal end of the chromosomes, and one weak signal of NOR at the proximal end of the chromosome in O. officinalis. Three or five pairs of NORs were identified in O. minuta. Two or five pairs of NORs were detected in O. latifolia. Species with E and F genomes showed one pah- of NOR chromosomes.