Neotyphodium infection and hybridisation as a function of environmental variation

Neotyphodium is an asexual, vertically transmitted, obligate fungal endosymbiont infecting cool-season grasses such as Arizona fescue. The relationship between Neotyphodium and several native grass hosts ranges from antagonistic to mutualistic. One theory that may explain how Neotyphodium infection is maintained despite inconsistent mutualistic benefit to the host is the bounded hybrid superiority hypothesis. This hypothesis argues that hybrids are more fit than non-hybrids in response to some environmental stresses. Neotyphodium infects hosts in both hybrid and non-hybrid forms. We tested the possibility of hybrid superiority in depauperate habitats (low soil water and nitrate) by quantifying the types and frequency of host infections (uninfected, hybrid-infected and non-hybrid-infected), and the quality of resources available between three host populations. A second theory, the geographic mosaic theory of coevolution, may also explain different symbiotic outcomes at the population level in response to variation in abiotic and biotic population characters. We provide cursory support for both hypotheses. Keywords: geographic mosaic theory of coevolution, hybrid, Festuca, Neotyphodium, symbiosis, mutualism, bounded hybrid superiority