Variation in the Number of Nucleoli in Rice Species Having the AA Genome

Abstract

The number of nucleoli newly formed in telophasic cells of root tips was counted in rice species having the AA genome, namely, Oryza sativa, O. perennis, O. sativa f. spontanea, O. glaberrima and O. breviligulata, taking for each species about twenty strains from the stocks kept in the National Institute of Genetics, Japan. The root tips of germinating seeds were fixed in Lewitsky's solution (5% CrO₃3: Formalin 1), and after hydrolysis in warm N-HCl for about 20 minutes squashed with 1% aceto-carmine. The results obtained from the respective strains are given in Tables 1-4, and the distribution of the mean nucleolar number in each species is shown in Table 5. As shown in those tables, the variation in nucleolar number is continuous. If strains with more than 2.1 nucleoli per cell are considered to represent the quadri-nucleolar, and those with less than 2.1 the bi-nucleolar type, all the species investigated can be said to have both types. However, it is found that strains belonging to O. perennis and O. sativa f. spontanea, as well as those of the Indica type of O. sativa, are mostly of quadri-nucleolar type, while strains of O. breviligulata and O. glaberrima are of bi-nucleolar type. Another point of interest was that plants collected from adjacent sites, belonging actually to the same population, showed a marked difference. This indicates that the chromosomes of plants within the same population may differ in nucleolus-forming ability.