Environmental factors affect the distribution of two Epichloë fungal endophyte species inhabiting a common host grove bluegrass ( Poa alsodes)

Abstract

Aim: The endophyte Epichloë alsodes, with known insecticidal properties, is found in a majority of Poa alsodes populations across a latitudinal gradient from North Carolina to New York. A second endophyte, E. schardlii var. pennsylvanica, with known insect-deterring effects, is limited to a few populations in Pennsylvania. We explored whether such disparate differences in distributions could be explained by selection from biotic and abiotic environmental factors.

Location: Along the Appalachian Mountains from North Carolina to New York, USA.

Taxon: Fungi.

Methods: Studied correlations of infection frequencies with abiotic and biotic environmental factors. Checked endophyte vertical transmission rates and effects on overwintering survival. With artificial inoculations for two host populations with two isolates per endophyte species, tested endophyte-host compatibility. Studied effects of isolates on host performances in greenhouse experiment with four water-nutrients treatments.

Results: Correlation analysis revealed positive associations of E. alsodes frequency with July Max temperatures, July precipitation, and soil nitrogen and phosphorous and negative associations with insect damage and soil magnesium and potassium. Plants infected with E. alsodes had increased overwintering survival compared to plants infected with E. schardlii or uninfected (E-) plants. Artificial inoculations indicated that E. alsodes had better compatibility with a variety of host genotypes than did E. schardlii. The experiment with reciprocally inoculated plants grown under different treatments revealed a complexity of interactions among hosts, endophyte species, isolate within species, host plant origin, and environmental factors. Neither of the endophyte species increased plant biomass, but some of the isolates within each species had other effects on plant growth such as increased root:shoot ratio, number of tillers, and changes in plant height that might affect host fitness.

Main conclusion: In the absence of clear and consistent effects of the endophytes on host growth, the differences in endophyte-mediated protection against herbivores may be the key factor determining distribution differences of the two endophyte species.

Keywords: ecological factors; endophyte distributions; endophyte‐host compatibility; grass populations; host plant growth; infection benefits; southeastern North America.

Figure 1

(a) Poa alsodes plants inoculates with Epichloë alsodes, A1 and A2 isolates (b, c), and Epichloë schardlii var. pennsylvanica, S1 and S2 isolates (d, e) from the greenhouse experiment with different water‐nutrient treatments

Figure 2

Pairwise comparisons (Means ± SE, Tukey HSD) for Poa alsodes plants from the greenhouse experiment. Seeds originated from North Carolinian (NC) Great Smoky Mountains National Park (GSM) and Pennsylvanian (PA) Elk State Park (EST) populations. Naturally uninfected seedlings were inoculated with endophytes Epichloë alsodes isolates: A1—from GMS, NC, population; A2—from EST, PA, population; E. schardlii var. pennsylvanica isolates: S1—from Chapman State Park, PA, population; S2—from EST, PA, population; E− stayed uninfected. There were 11–13 plants randomly assigned per treatment for each symbiotum combination. Black horizontal lines designate significant differences (p < 0.05) among the same population plants with different infections

Figure 3

Mean (±SE) of the effects of E. alsodes isolates A1 versus A2 on presumably residential versus alien plant hosts from two Poa alsodes populations, North Carolina (NC) and Pennsylvania (PA) placed in HWHN, HWLN, LWHN, LWLN treatments. Fungal isolates were artificially inoculated in naturally uninfected seedlings. Residential for the NC population, A1 isolate, represents the only E. alsodes infection observed there in P. alsodes. A2 isolate represents one of two Epichloë species found in the PA population and thus, it may be considered residential for this P. alsodes population. There were 11–13 plants randomly assigned per treatment for each symbiotum combination. Asterisks indicate significant differences (p < 0.05, one‐way ANOVAs), and for suggestive differences p‐values are provided

Figure 4

Mean (±SE) leaf dry and root dry biomasses for North Carolina (NC) population plants (a, c, respectively) and for Pennsylvania (PA) population plants (b, d, respectively). Naturally uninfected, some NC plants were successfully inoculated with either of two E. alsodes isolates (residential A1 and alien A2) or two E. schardlii var. pennsylvanica isolates (S1 and S2, both alien), or remained uninfected after procedures (E−). Some PA plants were successfully inoculated with either of two E. alsodes isolates (alien A1 and presumably residential A2) or remained uninfected after procedures (E−). After infection status check and cloning, plants were randomly assigned into four treatments—HWHN, HWLN, LWHN, LWLN for 97 days. For each symbiotum combination, there were 11–13 plants per treatment. Letters represent statistically significant differences among infection groups within each treatment (one‐way ANOVAs, p < 0.05)