Distribution of hybrid fungal symbionts and environmental stress.

Most asexual fungal symbionts of grasses in the genus Neotyphodium occurring in nature are of hybrid origin. Most hybrid Neotyphodium species result from interspecific hybridization events between pathogenic Epichloë species or co-occurring non-hybrid Neotyphodium species. Current hypotheses for the prevalence of hybrid Neotyphodium species include reduction of mutation accumulation and increased adaptive response to environmental extremes. We tested the adaptive response hypothesis by characterizing the distribution of uninfected, hybrid, and non-hybrid Neotyphodium endophytes in 24 native Arizona fescue host populations and abiotic parameters at each locality. Infection was high in all host populations (>70%), but the majority of host populations were infected by non-hybrid Neotyphodium (>50% on average). Principal component analysis indicates the frequency of plants infected with hybrid fungi is negatively related to soil nutrients and positively correlated with early spring moisture. Non-hybrid infected hosts are positively associated with soil nutrients and show a complex relationship with soil moisture (negative in early spring moisture, positive with late summer soil moisture). These results suggest the frequency of uninfected, hybrid, and non-hybrid infected plants is related to resource availability and abiotic stress factors. This supports the hypothesis that hybridization in asexual fungal symbionts increases host adaptability to extreme environments.