Comparison of nuclear ribosomal DNA sequences from Alternaria species pathogenic to crucifers

Abstract

The sequences coding for the nuclear 18 s rRNA, 5·8 s rRNA, and the internal transcribed spacers (ITS1 and ITS2) were amplified by the polymerase chain reaction and sequenced for one isolate each of Alternaria brassicae, A. brassicicola, A. raphani, A. alternata and Pleospora herbarum . The 5·8 s rDNA sequences from the four Alternaria species were identical and differed at only one base pair from that of P. herbarum . The internal transcribed spacer sequences, especially ITS1, were very variable in both base composition and length. The 18 s rDNA sequences were highly conserved, but enough variability was present to distinguish genera clearly. Phylogenetic analysis of the sequence data sets by both parsimony and maximum likelihood methods clearly separated genera and species. All of the Alternaria species were closely related. Pleospora also appeared to be more closely related to Alternaria than to Leptosphaeria .