Direct repeat-mediated DNA deletion of the MAT1-2 genes results in unidirectional mating type switching in Sclerotinia trifoliorum
W. CHEN (1), L. Xu (2) (1) USDA ARS, U.S.A.; (2) Washington State University, U.S.A.

The necrotrophic fungal pathogen Sclerotinia trifoliorum exhibits ascospore dimorphism and unidirectional mating type switching - self-fertile strains derived from large ascospores produce both self-fertile (large-spore) and self-sterile (small-spore) offspring. The present study, comparing DNA sequences of the MAT locus, showed that four mating type genes (MAT1-2-1, MAT1-1-5, MAT1-2-1 and MAT1-2-4) were present in the self-fertile strain, but a 2891-bp region was completely deleted from the MAT locus and lost in the genome of small-spore self-sterile strains. This deletion caused the loss of the entire MAT1-2-1 and MAT1-2-4 genes in the self-sterile strain and subsequently results in complete loss of self-fertility in these strains. Meanwhile, two 146-bp direct repeat motifs flanking the deleted DNA region were found in the self-fertile strain, but one copy of this 146-bp motif (a part of the MAT1-1-1 gene) was present in the self-sterile strain. The direct repeat motifs were believed to be responsible for the deletion through homologous recombination during meiosis. Tetrad analyses showed that all small spore-derived strains lacked the missing DNA between the two direct repeats that was found in all large spore-derived strains. In addition, heterokaryons at the MAT locus was observed frequently in field isolates as well as in laboratory derived single-ascospore isolates, suggesting that such direct-repeats mediated deletion may also possibly occur during mitosis.    

Abstract Number: P11-347
Session Type: Poster