Epichloid endophytes are common symbionts of cool season grasses. These endophytes live within their host’s above ground tissue and reproduce asexually by extending their hyphae into the developing seeds of their hosts. Given the tightly linked nature of their life histories, natural selection might be expected to favor mutualistic interactions. And while interactions between some specific grass and endophyte species are highly mutualistic, other species combinations exhibit more variable outcomes. This variation is caused by differences in the interaction between specific host and endophyte genotypes, as well as differences in the local environments in which the interactions occur. As such, understanding any geographical structuring of genetic variation in both the epichloid endophyte and its host may provide insight into the variation in the interaction outcomes.
Canada wildrye (Elymus canadensis), a grass native to North America, commonly interacts with the endophyte Epichloë canadensis. Canada wildrye seeds were collected from populations ranging from Texas to Minnesota. The seeds were germinated on the Indiana University Kokomo campus and grown for 6 to 8 weeks before DNA was extracted from ~100 mg of their above ground tissue. Genetic variation in the endophyte was assessed using microsatellite loci and genetic variation in the host plant was assessed using microsatellites in nuclear DNA and DNA sequence variation in chloroplast genes.
Results/Conclusions
Microsatellite loci in the Epichloë canadensis are significantly linked, as would be expected in an asexual organism. The endophyte exhibits a significant amount of intrapopulational diversity, however, mostly due to a large number of private haplotypes. There is significant interpopulational genetic structuring as shown by pairwise FST values between most of sampled populations, but these differences do not correlate with geographic distances, or differences in average temperature or precipitation. Pairwise FST values between Canada wildrye populations also describe significance structuring between these populations. While the ecological consequences of this structuring is not yet known, it does suggest local adaptations may be important in understanding the variation seen in the symbiosis between Canada wildrye and its fungal endophyte.