Direct repeat-mediated DNA deletion of the mating type MAT1-2 genes results in unidirectional mating type switching in Sclerotinia trifoliorum.

Liangsheng Xu,Teresa M Jardini,Weidong Chen

doi:10.1038/srep27083

Abstract

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-spores) and self-sterile (small-spores) offsprings in a 4:4 ratio. The present study, comparing DNA sequences at MAT locus of both self-fertile and self-sterile strains, found four mating type genes (MAT1-1-1, MAT1-1-5, MAT1-2-1 and MAT1-2-4) in the self-fertile strain. However, a 2891-bp region including the entire MAT1-2-1 and MAT1-2-4 genes had been completely deleted from the MAT locus in the self-sterile strain. Meanwhile, two copies of a 146-bp direct repeat motif flanking the deleted region were found in the self-fertile strain, but only one copy of this 146-bp motif (a part of the MAT1-1-1 gene) was present in the self-sterile strain. The two direct repeats were believed to be responsible for the deletion through homologous intra-molecular recombination in meiosis. Tetrad analyses showed that all small ascospore-derived strains lacked the missing DNA between the two direct repeats that was found in all large ascospore-derived strains. In addition, heterokaryons at the MAT locus were observed in field isolates as well as in laboratory derived isolates.

Highlights

Fungi have evolved a remarkable diversity of reproductive strategies adapting to the changing environments
The structure of the MAT locus in S. trifoliorum self-fertile L mating type most resembles that of MAT inversion positive strains of S. minor and S. sclerotiorum
The gene order and orientation in the MAT locus of self-fertile isolate mostly resemble those reported for the inversion positive (Inv+)strains of S. minor and S. sclerotiorum, including truncation of the MAT1-1-1 gene

Summary

Introduction

Fungi have evolved a remarkable diversity of reproductive strategies adapting to the changing environments. Several modes of sexual reproductions in ascomycetes have been reported including homothallic (self-fertile), heterothallic (requires a mating partner, one locus with two alleles), pseudohomothallic (a single ascospore with two nuclei of opposite mating type allele)[2,3,4,5], dual mating (isolates can mate with both MAT1-1 and MAT1-2 tester strains), and unidirectional mating type switching (self-fertile strains produce both self-fertile and self-sterile strains)[2,3,6,7]. Compared to the other sexual reproduction systems in fungi, such as homothallism and heterothallism, the mechanisms of unidirectional mating type switching are less studied. They provided the first sequence evidence that direct repeat-mediated deletion results in loss of MAT1-2 genes and in self sterility[14]

Methods

Results

Conclusion