Unusual features of the Botrytis cinerea mating system

Botrytis cinerea is a heterothallic ascomycete with two mating types, MAT1-1 and MAT1-2. Fragments of the MAT1- 2-1 and MAT1-1-1 genes were detected bordering idiomorphs of MAT1-1 and MAT1-2 isolates, respectively. Both these fragments encode truncated, non-functional proteins. B. cinerea has probably evolved from a homothallic ancestor containing all genes, with MAT1-1 and MAT1-2 arising from the loss of HMG and alpha-domain sequences, leaving the disabled gene fragments present in current loci. Two ORFs, designated MAT1-1-5 and MAT1-2-3, have not previously been reported from other fungi. In a cross of a MAT1-1-5 knockout mutant with a wild type MAT1-2 strain, the stipe develops normally but transition to the differentiation of a cup is blocked. Most B. cinerea isolates act in a standard heterothallic fashion, but some isolates can mate with both MAT1-1 and MAT1-2 isolates and are referred to as ‘dual maters’. Some dual mater isolates can self-fertilize and are truly homothallic. The MAT locus of five dual mater isolates was analysed. Four of those contain a MAT1-2 locus, without any part of the MAT1-1 locus being detected, whereas one homothallic isolate contains a MAT1-1 locus, without any part of the MAT1-2 locus being detected. We conclude that dual mating and homothallism are controlled by factors other than the MAT locus