PARENT SESSION
Oral Session # 48: Mutualism - Parasitism I: Plants; Modeling.
Presiding: K Jayachandran
Wednesday, August 6. 8:00 AM to 11:30 AM, SITCC Meeting Room 203.

Host plant influences parasite transmission mode in a grass-fungal endophyte association.

Johnston, Tammy^*,1, Clay, Keith¹, ¹ Indiana University, Bloomington, Indiana

ABSTRACT- Theories of the evolution of virulence generally rely on a positive correlation between parasite transmission and virulence. It is particularly important to test the correlation in a system where a range of both vertical and horizontal transmission modes co-occur to critically evaluate how transmission mode affects the evolution of virulence and to explore the potential for interspecific associations to evolve towards reduced virulence and possibly mutualism. Importantly, the degree to which the host versus the parasite controls transmission mode is not well understood. Such an understanding would enlighten models of the evolution of virulence, which often assume that the transmission mode is directly linked to traits of the parasite. The plant/fungus interaction, Elymus hystrix infected with the endophyte Epichloë elymi, exhibits both vertical and horizontal transmission and thus provides an ideal system to explore the relationship between virulence and transmission mode. In a common garden, we detected significant variation in horizontal transmission potential across eight maternal half-sib lines of infected plants and this variation was consistent across years. Vertically infected offspring resembled maternal plants in horizontal transmission. No significant variation was observed in vertical transmission to seeds or in the number of inflorescences produced. While all seeds were infected initially, longevity of the infection in dormant seeds varied across the maternal lines. Thus significant genetic variation in transmission mode exists upon which selection can act. A second experiment using artificial host/fungus combinations revealed significant host, fungus and interaction effects on the number of plant tillers, which plays a significant role in horizontal transmission. These results demonstrate that both the host and fungus shape the expression of parasite transmission. Thus, evidence from a host-parasite interaction that spans a continuum from vertical to horizontal transmission highlights the importance of both plant and parasite in the evolution of transmission and virulence.

Key words: virulence, Epichloë, transmission mode, parasite