Vertically transmitted symbioses, in which symbionts are inherited by offspring, feature as important interactions in evolutionary history. Mitochondria and chloroplasts resulted from the fixation of vertically transmitted symbionts.  Vertically transmitted gut bacteria have allowed animals to occupy niches that would otherwise be inaccessible, and many plants have increased ecological amplitudes due to vertically transmitted microbes.  In many symbioses, vertical transmission occurs only maternally.  Such is the case for fungal endophytes, the focus of our study, as endophytes are only transmitted to seeds, not to pollen.  This transmission mode sets up a potential conflict between the host and symbiont: Endophytes could benefit by diverting host resources away from pollen and toward seed production.  Such conflict is characteristic of other maternally-transmitted symbionts, most notably in “male-killing” Wolbachia in insects. To our knowledge, this question has not been explored for plant-fungal symbioses, but is critical to understanding both the mechanisms of endophyte benefits (and costs) and the consequences of symbiosis for population dynamics.  We grew seeds from experimentally endophyte-disinfected and naturally infected plants of Elymus virginicus (Virginia wild rye) to assess relative investment in pollen production versus seed production. The controlled environment of the greenhouse facilitated careful measurements of pollen investment.  

Results/Conclusions

Preliminary results suggest that endophyte-symbiotic plants have reduced investment in pollen production, although pollen counts are ongoing.  This may skew the ratio of seed to pollen investment toward seeds in endophyte-symbiotic plants , revealing a hidden cost of endophyte symbiosis. Thus, results support our hypothesis of altered sex allocation and provide new insight into the mechanisms and consequences of plant-microbe symbioses.