Mycorrhiza-associated sterile fungi in a Mediterranean environment: systematic heterogeneity versus ecological convergence?

Mediterranean ecosystems, featuring high plant diversity and co-dominance, offer an interesting scenario for the study of root-endophytic dark sterile mycelia (DSM) in different hosts. Investigation of ectomycorrhizal Pinus halepensis and endomycorrhizal Rosmarinus officinalis plants in Italy indicated that neighboring healthy individuals of these two hosts harbor several DSM morphotypes. Sequence analyses of nuclear ITS and 18S rDNA regions of the dominant morphotypes allowed their identification as a unique spectrum of DSM, being quite diverse and distinct from those so far described in other host plants investigated. Recognition of systematic affinities was possible with varying degrees of resolution for the different morphotypes, ranging from species-level identification to placement within Ascomycete subclasses. Association of these fungi with both hosts spanned at least eleven years; they thus appear to be true root colonizers coexisting with the accredited mycorrhizal symbionts. In spite of their taxonomic diversity, such DSM displayed some morphophysiological convergence justifying their recognition as an ecological group. One morphotype was identified as Rhizopycnis vagum D.F. Farr, a recently described fungus so far known as a root pathogen of crop plants. Studies of population genetics of this fungus and pathogenicity assays are unraveling the actual range of its ecological plasticity.