MUTANTS OF NECTRIA-HAEMATOCOCCA CREATED BY A SITE-DIRECTED CHROMOSOMEBREAKAGE ARE GREATLY REDUCED IN VIRULENCE TOWARD PEA

Pathogenicity of Nectria haematococca toward Pisum sativum is determin ed by a 1.6 million base pair (Mb) supernumerary chromosome. Repeats o f the simple telomeric DNA sequence (TTAGGG) and a gene (PDA1) from th e 1.6-Mb chromosome were used to direct a chromosomal break at the PDA 1 locus, Mutant strains contained a novel 1.5-Mb chromosome that appea red to result from an approximately 100-kb deletion from the 1.6-Mb ch romosome, These transformants (but not others that retain an apparentl y intact and mitotically stable 1.6-Mb chromosome) were essentially no npathogenic on common pea, a host for the original isolate. Mutants co ntaining the 100-kb deletion grow at a rate similar to the original st rain in culture and retain the ability to colonize host tissue, Based on these results, our hypothesis is that genetic determinants of patho genicity toward pea reside on the deleted portion of the 1.6-Mb chromo some, To our knowledge, this is the first reported use of site-directe d chromosome breakage in filamentous fungi, Telomere-mediated chromoso me breakage may speed genetic analysis of asexual fungi and be valuabl e for determining the consequences of duplicated and/or unique dispens able chromosomes in other pathogenic fungi.