Program Book - 27th Fungal Genetics Conference

CONCURRENT SESSION ABSTRACTSThe Sclerotinia sclerotiorum mating type locus (MAT) contains a 3.6-kb region that is inverted in every generation. Patrik Inderbitzin 1 , PeriasamyChitrampalam 2 , Karunakaran Maruthachalam 1 , Bo-Ming Wu 3 , Krishna Subbarao 1 . 1) Department of Plant Pathology, University of California-Davis, Davis,CA, USA; 2) Department of Plant Sciences, University of Arizona, Tucson, AZ, USA; 3) Department of Plant Pathology, China Agricultural University, 2 WestYuanmingyuan Rd., Haidian District, Beijing, China.Sclerotinia sclerotiorum is a filamentous ascomycete in the Sclerotiniaceae (Pezizomycotina) and a necrotrophic pathogen of more than 400 hostsworldwide, including many important agricultural crops. In California, the biggest lettuce producer in the United States, S. sclerotiorum is a causal agent oflettuce drop that reduces overall annual lettuce yield by 15%. Little is known about the details of sexual reproduction in S. sclerotiorum, but the structureof the S. sclerotiorum mating type locus MAT, the master regulator of sexual reproduction in ascomycetes, has previously been reported. As in otherhomothallic (self-fertile) ascomycetes, S. sclerotiorum MAT contains both idiomorphs (divergent alleles) fused end-to-end at a single locus. Using 283isolates from lettuce in California and from other states and hosts, we investigated the diversity of S. sclerotiorum MAT, and identified a novel version ofMAT that differed by a 3.6-kb inversion and was designated Inv+, as opposed to the previously known S. sclerotiorum MAT that lacked the inversion andwas Inv-. The inversion affected three of the four MAT genes: MAT1-2-1 and MAT1-2-4 were inverted and MAT1-1-1 was truncated at the 3’-end.Expression of MAT genes differed between Inv+ and Inv- isolates. In Inv+ isolates, only one of the three MAT1-2-1 transcript variants of Inv- isolates wasdetected, and the alpha1 domain of Inv+ MAT1-1-1 transcripts was truncated. Both Inv- and Inv+ isolates were self-fertile, and the inversion segregated ina 1:1 ratio regardless of whether the parent was Inv- or Inv+. This suggested the involvement of a highly regulated process in maintaining equalproportions of Inv- and Inv+, likely associated with the sexual state. The MAT inversion region, defined as the 3.6-kb MAT inversion in Inv+ isolates and thehomologous region of Inv- isolates, was flanked by a 250-bp inverted repeat on either side. The 250-bp inverted repeat was a partial MAT1-1-1 thatthrough mediation of loop formation and crossing over, may be involved in the inversion process. Inv+ isolates were widespread, and in California andNebraska constituted half of the isolates examined. We speculate that a similar inversion region may be involved in mating type switching in thefilamentous ascomycetes Chromocrea spinulosa, Sclerotinia trifoliorum and in certain Ceratocystis species.27th Fungal Genetics Conference | 71