II. 37 Cytological localization of ribosomal RNA cistrons in Hordeum - a test for the non-expression of the nucleolar organiser region in interspecific hybrids.
N.C. Subrahmanyam and W. Gerlach, Genetics Department, Research School of Biological Sciences. The Australian National University, A.C.T. 2601, Australia. "R".
From comprehensive cytogenetic and biochemical studies it was evident that cistrons coding for the three ribosomal RNA species (18S, 5.8S and 28S) are tandem, multiple and clustered at the nucleolar organiser regions (NOR). It has been known for over forty years that in interspecific hybrids of Crepis, the NOR of one of the parental species is selectively suppressed. Similar nonexpression has been observed in Hordeum interspecific hybrids (Kasha and Sadasivaiah, 1971). In the hybrid between diploid H. vulgare (4 NOR's) and tetraploid H. bulbosum (4 NOR's) only 2 NOR's of the bulbosum parent are not expressed. Such non-expression may be a consequence of selective loss of the sites for rRNA cistrons and/or repression. To test such possibilities 125I-labelled 18S wheat ribosomal RNA was hybridized to air-dried root-tip squashes of the two parents and their hybrids. Examination of the respective autoradiographs showed complementary sites at each NOR in all the three genotypes and at two additional sites in the hybrids, i.e. 4 complementary sites were detected in each of the 3 genotypes even though only two NOR's were present in the hybrid. In other words, in spite of the presence of complementary sites for rRNA on 4 chromosomes in the hybrid, only two are capable of organising nucleoli. This might suggest that repression is the likely cause of NOR non-expression. It should be pointed out that the level of resolution is not adequate to exclude the possiblity of any loss of rDNA in strains. Therefore, DNA-RNA hybridization studies have been carried out to determine the numbers of rRNA cistrons in each of the genotypes, and a deailed account would be published elsewhere.
Acknowledgements:
Our thanks are due to Dr. A. Appels, Division of Plant Industry, CSTRO, Canberra, for the supply of Iodinated RNA used in the present study.
Reference:
Kasha, K.J. and R.S. Sadasivaiah. 1971. Genome relationships between H. vulgare L. and H. bulbosum L. Chromosoma, 36:263-287.