Characterization of mating type ( <i> <scp>MAT</scp> </i> ) alleles differentiated by a natural inversion in <i>Sclerotinia minor</i>

Abstract

A recent study on fungal mating type genes revealed two MAT alleles within homothallic Sclerotinia sclerotiorum differentiated by an inversion, Inv− (inversion negative) and Inv+ (inversion positive). An analysis of mating type in closely related S. minor was conducted to shed light on the evolution of this MAT inversion. Inv− and Inv+ MAT alleles were identified in S. minor and were characterized. Both MAT alleles in S. minor were flanked by APN 2 and SLA 2 , and consisted of two idiomorphs fused as in other homothallic ascomycetes. However, in the Inv+ MAT , the 3·6 kb MAT region was inverted relative to the Inv− MAT . Except for the inversion, both Inv− and Inv+ MAT in S. minor were equal in size and identical in nucleotide sequence. The MAT inversion in Inv+ S. minor was at exactly the same place as in Inv+ S. sclerotiorum and affected three of four MAT genes: MAT 1‐1‐1 was truncated and MAT 1‐2‐4 and MAT 1‐2‐1 were inverted. Unlike S. sclerotiorum , expression of MAT genes did not differ between Inv− and Inv+ S. minor . The 250 bp inverted repeat motif that flanked the inverted MAT region in S. sclerotiorum and believed responsible for the MAT inversion was also found in S. minor , but was 256 bp. Depending on the MAT genes, 93–96% nucleotide identity was observed between Sclerotinia species. Both Inv+ and Inv− MAT S. minor and S. sclerotiorum isolates were commonly found in lettuce fields of Arizona along with MAT heterokaryons.