The complete DNA sequence of the nuclear ribosomal RNA gene complex of Verticillium dahliae: Intraspecific heterogeneity within the intergenic spacerregion

The complete sequence of the nuclear ribosomal DNA gene complex of the phyt opathogenic fungus Verticillium dahliae has been determined. The tandemly r epeated unit was 7216 bp long and appears to be the shortest rDNA cluster d escribed so far among filamentous fungi. Primer pairs were designed for amp lification of the region spanning half of the 28S subunit, the intergenic s pacer (IGS), and the 5' end of 18S subunit of a number of Verticillium stra ins, isolated from various hosts and geographic origins. Great heterogeneit y was detected in the amplified products of the IGS region resulting in fra gments varying from 1.6 to 2.0 kb. The majority of Verticillium isolates we re classified into two groups with 1.6- and 1.7-kb amplified products, resp ectively. The former group included 31 V. dahliae, 7 V. longisporum, and 1 V. albo-atrum isolates, whereas the latter included 10 V. dahliae and 1 V. albo-atrum isolates. Sequence analysis of representative PCR products of th e above groups identified a "hot-spot" region harboring most of larger inse rtions, whereas most of the small changes were due to transitions and trans versions, One V. longisporum isolate with a 2.0-kb PCR product contained 13 perfectly conserved tandem repeats of 39 bp long. The presence of similar incomplete sequences in the corresponding regions of V. dahliae, V. longisp orum, and V. albo-atrum isolates revealed a particular standard motif of in sertions in the IGS region of the genus and is discussed. (C) 2000 Academic Press.