Deletion of the MAT- 2 mating-type gene during uni-directional mating-type switching in Ceratocystis

Abstract

Ceratocystis eucalypti is strictly heterothallic, with single ascospore strains representing one of two opposite mating types. Most other Ceratocystis species, including C. virescens, C. pinicola, and C. fimbriata, are homothallic. In the homothallic species, the MAT-2 strains are self-fertile, while MAT-1 strains are self-sterile and grow more slowly than MAT-2 strains. The current hypothesis is that self-fertility of MAT-2 strains is due to the deletion of the MAT-2 mating-type gene, resulting in the expression of the MAT-1 mating type. These mutant MAT-1 strains are able to cross with MAT-2 strains. Part of the MAT-2 mating-type gene in C. eucalypti, C. pinicola, and C. fimbriata was amplified using degenerate primers designed from the conserved MAT-2 HMG DNA-binding motif. The expected approximately 300-bp PCR products were cloned and sequenced. Specific primers were designed that amplified 210-bp fragments only in MAT-2 isolates of C. eucalypti, C. virescens, C. pinicola, and C. finbriata. These fragments were present in self-fertile field isolates and self-fertile progeny but were absent in the self-sterile (MAT-1) progeny from selfings of C. virescens, C. pinicola, and C. fimbriata, thus supporting the hypothesis that the MAT-2 mating-type gene is deleted during uni-directional mating-type switching. A Southern-blot analysis was performed to confirm the deletion of MAT-2 gene in self-sterile progeny. The DNA sequence data for the C. eucalypti MAT-2 mating-type gene was increased to 1371-bp using TAIL-PCR and uneven PCR, representing a portion of the complete MAT-2 gene DNA sequence.

Mating-type heterokaryosis and selfing in Cryphonectria parasitica

“…If this were case, it would be a form of switching unlike those described previously in ascomycetes (Haber, 1998;Harrington and McNew, 1997;Perkins, 1987;Witthuhn et al, 2000), and instead might be more similar to parasexual recombination involving mitotic crossing over (Pontecorvo, 1956;Roper, 1966). For example, Ellingboe (1963) reported an unusual recombination event in the basidiomycete Schizophyllum commune, involving the transfer of only a mating-type gene.…”

“…Other species include self-fertile isolates that are MAT heterokaryons but without a regular mechanism to maintain heterokaryosis through ascospores (reviewed in Faretra et al, 1988). Finally, mating-type switching is a third mechanism in which both MAT idiomorphs are required in the same individual for self-fertility (Haber, 1998;Harrington and McNew, 1997;Perkins, 1987;Witthuhn et al, 2000).…”

Genetic variation in the wattle wilt pathogen Ceratocystis albofundus

“…Each isolate originated from a different tree and was transferred from a single drop of ascospores on one perithecium. It has been shown for the closely related C, fimbriata that the progeny of a single perithecium differs only in mating type (Harrington and McNew, 1997;Witthuhn et al, 2000). …”

Scientific Opinion on the pest categorisation ofCeratocystis platani(Walter) Engelbrecht et Harrington

“…The pathogen has two mating types (MAT-1 and MAT-2) and is able to outcross, but the MAT-2 mating type is self-fertile (homothallic) through unidirectional mating type switching (Witthuhn et al, 2000). Because of selfing, an introduced strain of the fungus could spread via both asexual and sexual spores in an essentially clonal manner (Engelbrecht et al, 2004).…”