Rate of Neotyphodium endophyte vertical transmission imposed by host grass fitness threshold

GUNDEL, P. E.; GARCIA PARISI, P.; CASAS, C.; OMACINI, M.; MARTÍNEZ-GHERSA, M. A.; GARIBALDI, L. A. ; GHERSA, C. M.

Congreso; Joint Meeting of the Mycological Society of America (MSA) and of the International Symposium on Fungal Endophytes of Grasses (ISFEG); 2010

The nature of the symbiosis between vertically transmitted Neotyphodium endophytic fungi and grasses may depend on the ecological context. It has been found that the endophyte may depress host fitness under restrictive growth conditions, which has been interpreted as a cost for the host in maintaining the symbiont. Whether this cost may impact the endophyte fitness is unknown. Endophyte fitness is determined by host seed production and the proportion endophyte-infected seeds (transmission rate). While the former has been widely studied, the latter has received very little attention. Here, we explore the relationship between endophyte transmission rate and seed production considering individual spike and whole plant levels using Lolium multiflorum annual grass and Neotyphodium occultans as a study model. We analyzed two data sets. The first one includes the transmission rate and seed production per plant of 94 individual plants. Variability in seed production per plant was obtained by subjecting the plants to different resource and stress levels. The second set includes the transmission rate and seed production per spike of 74 individual spikes. Variability in seed production per spike was obtained by subjecting monospecific patches to simulated grazing (defoliation and trampling). We found a positive correlation between transmission rate and seed production for both, individual plants (rho=0.27, P<0.009) and spikes (rho=0.43, P<0.001) data sets. Transmission rate was highest (≈1) and invariant in plants or spikes with high seed production. On the other hand, transmission rate was variable in low seed production plants and spikes. Our results suggest that there could be a threshold of host fitness value (in terms of seed production) below which the endophyte-grass symbiosis could be at risk.