ORGANIZATION OF RIBOSOMAL DNA IN THE ASCOMYCETE LEPTOSPHAERIA-MACULANS

In the ascomycete Leptosphaeria maculans tandem repeats of ribosomal D NA(rDNA) are restricted to one or two particular chromosomes of the 15 chromosomes of 19 field isolates examined. Ribosomal DNA can account for size differences of 35% between homologous chromosomes in a partic ular tetrad. During crossing, no detectable recombination between bloc ks of tandem repeats, nor changes in their size occur. The organisatio n of rDNA in L. maculans differs from many other haploid fungi. Firstl y, sequence heterogeneity occurs within tandem repeats of rDNA; regula rly spaced Sal 1 sites (0.25 Mb apart) are present within a 1.4 Mb blo ck of tandem repeats. Secondly, individual isolates have different-siz ed rDNA repeats; this variation occurs in the non-transcribed intergen ic spacer region. Thirdly, there is a wide range in the copy number of the rDNA repeat (from 56 to 225) amongst only four field isolates exa mined.