Population studies of native grass-endophyte symbioses provide clues for the roles of host jumps and hybridization in driving their evolution

Fungal endophytes in the genera Epichloe and Neotyphodium, collectively termed the epichloae, have fascinated biologists for decades. These intriguing fungi, also referred to as class 1 or clavicipitaceous endophytes, spend the large majority, or even their entire life cycle, within the tissues of their cool-season grass hosts without eliciting any symptoms of infection. While all epichloae reside within the intercellular spaces of aboveground vegetative grass tissues, the species at the symbiotic extreme are known as Neotyphodium, and the intimacy of their interaction extends to the reproductive (flowering) stage. At this point, fungal filaments (hyphae) nondestructively invade the developing ovaries of their host and are incorporated into perfectly viable, healthy seeds. Thus, these endophytes live solely within the tissues of their host plants and are transmitted maternally from generation to generation. A second life history characteristic of interest is that while all Epichloe and some Neotyphodium species are haploid, a great many of the strictly seed-transmitted Neotyphodium spp. are interspecific hybrids. This phenomenon may be critical for the success of these symbioses over longer spans of evolutionary time and will be discussed in greater detail below. A third characteristic, and one of the primary reasons these grass endophytes have received so much attention over the last three decades, is the strong mutualistic nature these relationships often exhibit. In exchange for photosynthetically derived carbon, the endophytes protect their cool-season grass hosts from grazing herbivores and a variety of abiotic stresses. It has been hypothesized that these three biological phenomena are related (Schardl & Craven 2003), perhaps with the former two driving the third, and it is here that the recent article in Molecular Ecology entitled Genetic diversity in epichloid endophytes of Hordelymus europaeus suggests repeated host jumps and interspecific hybridizations, by Oberhofer & Leuchtmann (2012), provides critical clues to linking these traits together. While the large majority of studies have focused on documenting the ever-increasing list of mutualistic qualities attributed to these fungi, very few have taken an exhaustive population-level approach to document plant and endophyte genotypes within a naturally occurring system (Faeth 2010; Jani 2010; Tintjer & Rudgers 2006). Such information is crucial to more fully elucidate the factors shaping grass-endophyte symbioses and those often driving these relationships to mutualistic extremes.