Epichloë typhina is a biotrophic symbiont of grasses
its sexual state on immature host inflorescences. The ectophytic, presexual
structure (stroma) of E. typhina produces spermatia and also serves
female structure in mating. After fertilization there is
proliferation of a dense mycelium which has been suggested to be heterokaryotic.
Perithecia form in the thickened stroma. We
investigated the possible formation of heterokaryons in matings and the
ascospores in contagious spread of E. typhina. In
almost all instances the transfer to a stroma of spermatia of opposite
type leads to thickening of the stroma; however,
ascospores were produced only if the parents were of the same mating population.
Fertilization of part of a stroma by one
spermatial parent often inhibited fertilization by another strain elsewhere
the stroma depending upon the spermatial isolates used.
Tests for heterokaryon formation were performed by culturing stromata after
matings that produced no ascospores, to avoid
analysing meiotic products. In two instances the female was cultured, and
two the nuclear and mitochondrial haplotypes were
primarily of the male. In another instance mitochondrial haplotypes and
mitochondrial plasmids from the female became associated
with the male nuclear haplotype. These results suggest that, following
male hyphae proliferate and heterokaryons may
sometimes form and also proliferate. To test how ascospores mediate infection,
inflorescences of uninfected perennial ryegrass plants
were surrounded by fertilized stromata from which E. typhina ascospores
were being ejected, seeds were collected and grown, newly
infected progeny plants were identified, and isolates from five progeny
were analysed genetically. As expected for ascospore
progeny the rDNA haplotype of the stromal and spermatial parents had segregated,
whereas all five isolates had the stromal profile
of mitochondrial DNA. This observation demonstrated that ascospores mediate
infection of new host plants.