Molecular genetics of Epichloë typhina and Acremonium coenophialum

The close associations between grasses and endophytes, whereby the fungi constitute maternally inherited components of the symbiotic entities (symbiota), make genetic engineering and other genetic manipulations of the endophytes an attractive approach to improvement of forage, pasture and turfgrass cultivars. A number of protective alkaloids are produced by the endophytes. Although insecticidal, nematocidal and antimicrobial metabolites produced by the mycosymbionts are desirable agronomic characteristics, anti-mammalian activities which are problematic for livestock farmers need to be mitigated. As a necessary prerequisite for endophyte genetic engineering, the authors have developed a transformation protocol for the tall fescue symbiont Acremonium coenophialum Morgan-Jones et Gams. The approach used was to introduce the antibiotic resistance gene, hph, which was obtained from the bacterium Escherichia coli and modified in vitro to allow expression in fungi. Successful incorporation of this 'chimeric' gene into the fungal genome was detected by resistance of the transformants to hygromycin B and by DNA hybridization analysis. The wide variation in the amounts and/or types of protective alkaloids produced by different isolates of Epichloë typhina (Fr.) Tulasne (Acremonium typhinum Morgan-Jones et Gams), and other Acremonium spp., raises the prospect of employing the sexual cycle of E. typhina as an alternative genetic system for development of new endophyte strains. However, there is a lack of phenotypically variable characters to follow in Mendelian crosses. Therefore, restriction fragment length polymorphisms should be identified and used for genetic mapping. Preliminary studies of this approach are presented. Genetic manipulations and other approaches to cultivar development based on the endophytes should be done with an understanding of their evolutionary biology and the basis for host preference. Previous studies have added much to the understanding of the alkaloid production potential of different isolates, and the degree of host specificity of both choke pathogens and non-pathogenic endophytes. This paper presents a phylogenetic analysis using DNA sequence information. The results support four conclusions: (1) the endophytes have evolved from E. typhina; (2) the endophytes do not have a strict coevolutionary relationship with their grass hosts; (3) loline production, characteristic of certain symbiota, is not a good indicator of phylogenetic affinities of the fungal symbionts; (4) there is a close relationship between isolates of A. coenophialum from tall fescue, and Acremonium lolii Latch, Samuels et Christensen from perennial ryegrass and a non-pathogenic A. typhinum from red fescue.