SUB-REPEAT SEQUENCES IN THE RIBOSOMAL-RNA INTERGENIC REGIONS OF VERTICILLIUM-ALBOATRUM AND V-DAHLIAE

Most eukaryotic organisms have sub-repeat sequences in the intergenic regions (IGRs) of their rRNA gene clusters but such sub-repeats are unusual in fungi. Verticillium alboatrum and V. dahliae (hyphomycetes) have been shown to have imperfect sub-repeat sequences in their IGRs which probably did not arise by simple reiteration. Although the consensus sequences derived from the sub-repeats of V. alboatrum, of haploid isolates of V, dahliae and of diploid isolates of this species were all related, the overall structures of the sub-repeat regions in the three were clearly distinct. Considerable variation in the total length of these regions was found between haploid isolates of V. dahliae but this was not correlated with any identified character, including restriction fragment length polymorphism (RFLP) group. Earlier work on RFLPs, using random genomic clones as probes, showed that the isolates of V. dahliae studied most of which came from the U.K., could be divided into clearly defined groups. Within-isolate length variation was seen only in a few cases. A single base difference, occurring at the same position in each repeat, was characteristic for V. dahliae RFLP group B or groups A and M combined. A second difference gave similar differentiation but was not present in all repeats. No length variation was seen between V. alboatrum isolates or between two diploid isolates of V. dahliae. The extent of the differences in structures of sub-repeat regions suggest that haploid and diploid isolates of V. dahliae would be better regarded as separate species, along with V. alboatrum, but the lack of correlation between sub-repeat length and any other known character means that sub-repeat length cannot be used for rapid identification of sub-specific taxa.