Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2004
Publication Date: 3/1/2004
Citation: Moon, C.D., Craven, K.D., Lenchtmann, A., Clement, S.L., Schardl, C.L. 2004.Prevalence of interspecific hybrids amongst asexual fungal endophytes of grasses. Molecular Ecology. v. 13. p. 455-1467. Interpretive Summary: Fungal endophytes (Epichloe and their asexual Neotyphodium forms) of temperate grasses form one of the better know plant-microbe associations. Host grasses harboring these endophytes show no outward signs of infection. Grass breeders and seed companies recognize the benefits (drought tolerance, insect resistance) associated with endophyte infection, with the result that seed companies market endophyte-infected seed for golf courses, lawns and playfields for better stand persistence and insect resistance. This paper addresses the dearth of information about the evolution of these fungal endophytes by researching the prevalence of interspecific hybrids among a wide range of fungal endophytes of grasses, including endophytes from wild barley. Knowledge about the evolution and prevalence of these grass-microbe associations is prerequisite to fully capitalizing on their existence for forming novel endophyte-grass associations for expanding commercial purposes.
Technical Abstract: Epichloe endophytes are fungal symbionts of grasses that span a continuum including asexual mutualists (vertically transmitted), obligately sexual pathogens (horizontally transmitted), and mixed-strategy symbionts with both mutualistic and pathogenic capabilities. This paper shows that the processes of genome evolution differ markedly for the different symbiont types. Genetic and phylogenetic analysis was conducted of a broad taxonomic, ecological and geographical sample of isolates, in which were identified and sequenced alleles of genes for B-tubulin and translation elongation factor1-', and microsatellite alleles were identified by length polymorphisms. The majority of asexual isolates had two or three alleles of most loci, but every sexual isolate had only single alleles for each locus. Phylogenetic analysis indicated that in all instances of multiple alleles in an isolate, the alleles were derived from different sexual species. It is concluded that, whereas horizontally transmissible species had haploid genomes and speciation occurred cladistically, most of the strictly seedborne mutualists were interspecific hybrids with heteroploid genomes. Furthermore, the phylogenetic evidence indicated that, in some instances, hybridization followed rather than caused evolution of the strictly seedborne habit. Therefore, the abundance of hybrid species among grass endophytes, and their prevalence in many host populations suggests a selective advantage of hybridization for the mutualistic endophytes.