An unusual repetitive element from highly virulent isolates of Leptosphaeria maculans and evidence of its transfer to a weakly virulent isolate.

Abstract

A 5,238-bp repetitive DNA element from a highly virulent isolate of Leptosphaeria maculans has been cloned and sequenced. The element is present in approximately 80 copies per haploid genome and hybridizes to every chromosome resolved by pulse field gel electrophoresis. The sequence is composed of 66% A+T and has numerous, very short, direct and inverted repeats. No RNA complementary to the element was detected in log phase cultures, and no open reading frames of significant length are present in the sequence. It has no structural similarity to other repetitive elements or significant homology to database sequences. We have designated the element LMR1. Southern blot hybridization indicated that the element is present in all isolates of L. maculans that are highly virulent to Brassica napus and B. rapa. The general structure of the element was conserved among isolates of different mating type, pathogenicity group, and geographic origin, as determined by both Southern blot analysis and primer-directed DNA amplification. LMR1 did not hybridize to DNA from weakly virulent strains of L. maculans, with the exception of one isolate. Phylogenetic analyses of restriction fragment length polymorphism and rDNA sequence indicated that the highly virulent and weakly virulent strains of L. maculans are not monophyletic. Therefore, the presence of the LMR1 element in a weakly virulent isolate may indicate that a rare transfer event has occurred. Surprisingly, the weakly virulent isolate that contains LMR1 is more pathogenic on B. napus and B. juncea than a similar isolate that lacks the element.