Atypical sexual reproduction of Colletotrichum tanaceti

Colletotrichum is an ascomycete fungal genus with a complex sexual reproduction strategy. Colletotrichum tanaceti is a foliar fungal pathogen of the commercially cultivated pyrethrum (Tanacetum cinerariifolium) in Australia. Although C. has a known sexual stage, the molecular basis of sexual reproduction in C. tanaceti is unknown. Classical mating experiments confirmed the self-sterility of C. tanaceti and revealed that the mating strategy of this species departed from a typical unilocus bi-allelic system, by having more than two mating specificities. A second locus determining fertility, potentially polygenic was hypothesized to regulate mating in C. tanaceti together with the mate recognition locus. The atypical nature of the mating system was further supported by in-sillico analyses conducted using the whole genome sequences of sexually compatible strains (BRIP57314 and BRIP57315) of C. tanaceti. The mating type MAT locus and homologs of other genes known to be involved in mating in fungi were identified from the genomes of both the sexually compatible strains of C. tanaceti contained only the MAT1-2-1 idiomorph. No trace of MAT1-1-1 gene was found, consistent with previous reports on other Colletotrichum species. Comparison of the MAT region of C. tanaceti with other Colletotrichum species revealed the conserved nature of the flanking region and the diverse nature of the idiomorphic region. The homologs of the α-factor pheromone precursor and both of the pheromone receptors were present in both genomes of a sexually compatible cross. Comparative genomics revealed polymorphism in the pheromone precursor duo within different Colletotrichum spp. Hence, the pheromone/precursor system which is independent of the MAT1-1-1 is hypothesized to regulate the mate recognition in Colletotrichum. Orthology to functionally validate mating genes suggested potential functions of the mating genes in C. tanaceti. Future sequencing and reverse genetics studies can be used to validate these hypotheses.