HETEROGENEITY OF SPACER LENGTHS IN CIRCLES OF AMPLIFIED RIBOSOMAL DNA OF 2 INSECT SPECIES, DYTISCUS-MARGINALIS AND ACHETA-DOMESTICUS

[摘要] The amplified, extrachromosomal rDNA [DNA containing ribosomal [r] RNA precursors] in the oocytes of 2 taxonomically unrelated species, the water beetle D. marginalis and the house cricket A. domesticus, was studied with 2 different EM techniques. The patterns of transcribed and fibril-covered genes for pre-rRNA (matrix units) and the interspersed, fibril-free apparent spacer intercepts were analysed in spread and positively stained preparations of nucleolar chromatin from manually isolated nuclei. The contour lengths of circular molecules of isolated rDNA from such nuclei as revealed by surface spreading with cytochrome c were measured. The sizes of the pre-rRNA molecules as determined by gel electrophoresis, were compared with the sizes of the matrix units involved in pre-rRNA formation. In both species a significant amount of the rDNA was recovered in closed circles containing from 1-6 pre-rRNA genes. In both species the information content of the transcribed regions exceeded that of the pre-rRNA molecules (2.8 .times. 106 MW) considerably (35% in Dytiscus, .apprx. 100% in Acheta), indicative of an instability of the true primary transcription products. While the matrix units are relatively homogeneous in length (especially in Dytiscus), the apparent spacer intercepts as well as the resulting repeating units showed a pronounced heterogeneity with indications of some preferential subclasses. This heterogeneity could be classified into inter-axial (including intercircular) and intra-axial (including intracircular) heterogeneity of gene and spacer regions. Different circles could consist of different, although for a given circle identical, repeating units. Circles containing repeating units of different lengths were found. The results were discussed in relation to analyses of rDNA gene-spacer patterns in other organisms. The character and the pattern of heterogeneity of functional intercepts in rDNA were not uniform in the rDNA of a specific organism but could differ from 1 group of rDNA genes, or from 1 amplified rDNA molecule, to another. Circles of rDNA were probably derived from different regions of 1 nucleolar organizer.