Heterogeneity of spacer lengths in circles of amplified ribosomal DNA of two insect species, Dytiscus marginalis and Acheta domesticus

Abstract

The amplified, extrachromosomal rDNA † in the oocytes of two taxonomically unrelated species, the water beetle Dytiscus marginalis and the house cricket Acheta domesticus, has been studied with two different electron microscopic techniques. (1) The patterns of transcribed and fibril-covered genes for pre-rRNA (matrix units) and the interspersed, fibril-free apparent spacer intercepts have been analysed in spread and positively stained preparations of nucleolar chromatin from manually isolated nuclei. (2) The contour lengths of circular molecules of isolated rDNA from such nuclei as revealed by surface spreading with cytochrome c have been measured. (3) In addition, the sizes of the pre-rRNA molecules, as determined by gel electrophoresis, have been compared with the sizes of the matrix units involved in pre-rRNA formation. In both species a significant amount of the rDNA has been recovered in closed circles containing from one to six pre-rRNA genes. In both species the information content of the transcribed regions exceeds that of the pre-rRNA molecules (2·8×106 molecular weight) considerably (35% in Dytiscus, about 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 show a pronounced heterogeneity with indications of some preferential subclasses. This heterogeneity can be classified into inter-axial (including intercircular) and intra-axial (including intracircular) heterogeneity of gene and spacer regions. Different circles can consist of different, although for a given circle identical, repeating units. In addition, circles containing repeating units of different lengths are also found. The results are discussed in relation to analyses of rDNA gene-spacer patterns in other organisms. The data show that the character and the pattern of heterogeneity of functional intercepts in rDNA are not uniform in the rDNA of a specific organism but may differ from one group of rDNA genes, or from one amplified rDNA molecule, to another. They further strongly suggest that circles of rDNA can be derived from different regions of one nucleolar organizer.