Plant-associated microbes have been increasingly recognized for influencing host populations, plant communities, and even herbivores and predators. Thus, understanding factors that affect the distribution and abundance of microbial symbioses may be important for predicting the ecological dynamics of communities. Using endophytic fungi-grass symbioses, we explored how intrinsic traits of the symbiosis, specifically transmission mode, may influence symbiont frequencies in host populations. Combining published literature with new field surveys, we compared Epichloë endophytes, which had mixed horizontal and vertical transmission, with Neotyphodium endophytes, which were exclusively vertically transmitted from host plants to seeds. Exclusively vertical transmission should select against pathogenicity because symbionts depend entirely on hosts for reproduction. Across 118 host species, we found that Neotyphodium hosts had 40-130% higher symbiont frequencies than Epichloë hosts. In field surveys, endophyte frequency was positively correlated with the local density of hosts, but only for Epichloë, suggesting that contagiously spread Epichloë may attain higher frequencies when hosts are more abundant. Epichloë endophytes were also more likely than Neotyphodium to have imperfect vertical transmission; thus, hosts may reduce the transmission of more pathogenic symbionts to seeds. Results are consistent with the conclusion that the evolutionary transition to exclusively vertical transmission can alter patterns of symbiont frequency in nature.
Associations between fungal endophytes and grasses have served as particularly useful systems for exploring the nature and significance of hereditary symbiosis. Here, we propose alternative explanations for recent work by Faeth, in which endophytes were proposed to function as reproductive parasites. Faeth argued that his data demonstrated a symbiosis-induced shift to earlier host reproduction that could generate parasitism through life-history trade-offs with growth/survival. We contend that identifying a symbiosis as mutualistic or parasitic requires studies that incorporate both correlations among demographic pathways and rates of symbiont transmission; such work can advance understanding of the ecology and evolution of symbiosis.
The tawny crazy ant (Nylanderia fulva) is a new invasive pest in the United States. At present, its management mainly relies on the use of synthetic insecticides, which are generally ineffective at producing lasting control of the pest, necessitating alternative environmentally friendly measures. In this study, we evaluated the feasibility of gene silencing to control this ant species. Six housekeeping genes encoding actin (NfActin), coatomer subunit β (NfCOPβ), arginine kinase (NfArgK), and V-type proton ATPase subunits A (NfvATPaseA), B (NfvATPaseB) and E (NfvATPaseE) were cloned. Phylogenetic analysis revealed high sequence similarity to homologs from other ant species, particularly the Florida carpenter ant (Camponotus floridanus). To silence these genes, vector L4440 was used to generate six specific RNAi constructs for bacterial expression. Heat-inactivated, dsRNA-expressing Escherichia coli were incorporated into artificial diet. Worker ants exhibited reduced endogenous gene expression after feeding on such diet for 9 d. However, only ingestion of dsRNAs of NfCOPβ (a gene involved in protein trafficking) and NfArgK (a cellular energy reserve regulatory gene in invertebrates) caused modest but significantly higher ant mortality than the control. These results suggest that bacterially expressed dsRNA can be orally delivered to ant cells as a mean to target its vulnerabilities. Improved efficacy is necessary for the RNAi-based approach to be useful in tawny crazy ant management.
Summary• Here, we examined whether fungal endophytes modulated host plant responses to light availability. First, we conducted a literature review to evaluate whether natural frequencies of endophyte symbiosis in grasses from shaded habitats were higher than frequencies in grasses occupying more diverse light environments. Then, in a glasshouse experiment, we assessed how four levels of light and the presence of endophyte symbioses affected the growth of six grass species.• In our literature survey, endophytes were more commonly present in grasses restricted to shaded habitats than in grasses from diverse light environments.• In the glasshouse, endophyte symbioses did not mediate plant growth in response to light availability. However, in the host grass, Agrostis perennans, symbiotic plants produced 53% more inflorescences than nonsymbiotic plants at the highest level of shade. In addition, under high shade, symbiotic Poa autumnalis invested more in specific leaf area than symbiont-free plants. Finally, shade increased the density of the endophyte in leaf tissues across all six grass species.• Our results highlight the potential for symbiosis to alter the plasticity of host physiological traits, demonstrate a novel benefit of endophyte symbiosis under shade stress for one host species, and show a positive association between shaderestricted grass species and fungal endophytes.
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