SEXUAL CYCLE AND HORIZONTAL TRANSMISSION OF THE GRASS SYMBIONT, EPICHLOE-TYPHINA

Epichloe typhina is a biotrophic symbiont of grasses with its sexual s tate on immature host inflorescences. The ectophytic, presexual struct ure (stroma) of E. typhina produces spermatia and also serves as the f emale structure in mating. After fertilization there is proliferation of a dense mycelium which has been suggested to be heterokaryotic. Per ithecia form in the thickened stroma. We investigated the possible for mation of heterokaryons in matings and the role of ascospores in conta gious spread of E. typhina. In almost all instances the transfer to a stroma of spermatia of opposite mating 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 else where on 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 produ cts. In two instances the female was cultured, and in two the nuclear and mitochondrial haplotypes were primarily of the male. In another in stance mitochondrial haplotypes and mitochondrial plasmids from the fe male became associated with the male nuclear haplotype. These results suggest that, following mating, male hyphae proliferate and heterokary ons may sometimes form and also proliferate. To test how ascospores me diate infection, inflorescences of uninfected perennial ryegrass plant s were surrounded by fertilized stromata from which E. typhina ascospo res were being ejected, seeds were collected and grown, newly infected progeny plants were identified, and isolates from five progeny plants were analysed genetically. As expected for ascospore progeny the rDNA haplotype of the stromal and spermatial parents had segregated, where as all five isolates had the stromal profile of mitochondrial DNA. Thi s observation demonstrated that ascospores mediate infection of new ho st plants.