Ascospore germination patterns revealed ascomycetous biotrophic mycoparasite specificity to Fusarium hosts

Abstract

Sphaerodes mycoparasitica is an ascomycetous mycoparasitic taxon commonly found growing and sporulating on Fusarium pathogens in Canada. Ascospores of S. mycoparasitica failed to germinate under different standard laboratory conditions (sterile distilled water, water agar, and commercially available media), even after heat- and cold-shock treatments. In contrast, spore germination was obtained on commercial potato dextrose agar medium amended with Fusarium filtrates. Significant improvement in the percentage of spore germination was obtained for the spores suspended in Fusarium filtrates. Fusarium avenaceum and Fusarium oxysporum filtrates induced the highest germination, whereas Fusarium sporotrichioides and Fusarium proliferatum triggered lower germination frequency. Filtrates of beneficial fungal inoculants, including Trichoderma harzianum (RootShield®), Penicillium bilaii (JumpStart®), and Chaetomium globosum , had no impact on germination. Ascospore suspensions showed a double-polar germination pattern in F. avenaceum filtrate and single-polar germination when challenged with F. oxysporum filtrate. Sphaerodes mycoparasitica grown on F. oxysporum maintained the same spore germination patterns when transferred on F. avenaceum for three subsequent offspring generations, indicating a stable genome-regulated expression. Preliminary phylogenetic analyses based on large subunit rDNA showed the evolution of a group of mycoparasitic Sphaerodes species from nonmycoparasitic ancestors, but no clear evidence of coevolution or cospeciation between known Sphaerodes and their Fusarium hosts.