Skip to main content

The Grass is Greener in Kentucky

Symbiont Transmission and the Mutualism-Parasitism Spectrum

By Thomas S. Nühse

The Grass is Greener in Kentucky



Kentucky 31 is a widely grown variety of tall fescue grass that owes its extreme stress resistance, but also its toxicity, to an endophytic fungus. It is difficult to decide where to place this symbiosis along the mutualism-parasitism spectrum. In this case study, students explore a variety of  influencing factors, including partner choice, defense vs. accommodation, fitness effects on the host, and transmission of symbionts. The first three sections of the case offer lower-level undergraduate biology students an opportunity to practice hypothesis formation, scientific reasoning, and experimental planning. Students are asked to speculate about the cause of fescue toxicity before the symbiont is revealed, which then leads to an examination of defense versus partner recognition as plant responses to microbes. The fourth section of the case is appropriate for an intermediate ecology course and involves short group presentations investigating a broader range of symbioses with different transmission modes (vertical/horizontal) and microbial “services” that enhance host fitness. The final section, targeted to advanced students in research-led courses who are familiar with molecular lab techniques, requires students to write a research proposal to further investigate the nature of the fescue-endophyte interaction.


Date Posted




  • Use scientific reasoning to form hypotheses about the cause of tall fescue toxicity, and to propose simple experiments to test these hypotheses.
  • Use ecological observations to judge if a symbiont is mutualistic or parasitic.
  • Describe symbiont recognition and host response to mutualists versus parasites in simple terms; discuss the link to symbiont transmission.
  • Compare and contrast the fescue-endosymbiont interaction with other host-microbe interactions; discuss the range of fitness outcomes for the host.
  • Develop a research question and outline an experimental approach to further dissect the fescue-endosymbiont interaction.


Endophyte; grass; symbiosis; symbiont; mutualism; parasitism; fitness; hypothesis testing; tall fescue; Kentucky 31; fungi; fungus; microbiome; holobiont; optimal virulence


Subject Headings

Biology (General)
Botany / Plant Science
Evolutionary Biology


Undergraduate lower division, Undergraduate upper division



PDF, PowerPoint












Asset 2