The role of seed banks in the dynamics of vertically transmitted plant symbionts

Background/Question/Methods

Heritable symbionts can have important ecological and evolutionary effects on their hosts and understanding the factors that determine the persistence and prevalence of symbionts in host populations is a key goal in the study of symbiosis. Imperfect vertical transmission is thought to play a strong role in the dynamics of heritable symbionts and occurs when offspring of symbiotic parents fail to inherit the symbiont. Current theory simplifies the host life cycle and cannot account for failed symbiont transmission through demographic transitions other than parent to offspring (e.g. symbiont loss during metamorphic stages in arthropods or seed banks in plants). By over-simplifying life cycles, there is also potential to overlook fitness effects of symbionts on non-reproductive host life stages. Motivated by the widespread symbiosis between grasses and vertically transmitted fungal endophytes, we developed new theory to explore how host life history complexity in the form of a seed bank can alter host-symbiont dynamics and symbiont persistence.  We connected the theoretical model to empirical data using four species of endophyte-infected grass hosts. We quantified the loss of endophytes through the host seed bank, the effects of endophytes on seed demography, and how these processes affected endophyte persistence at the population level. 

Results/Conclusions

A model of obligate transition through a seed bank showed that imperfect retention of endophytes during the seed stage can result in endophyte extinction from the host population.  However, beneficial effects of endophytes on seed survival could offset imperfect retention and promote endophyte persistence.  When transition through the seed bank is more flexible (not obligate), imperfect seed bank retention and effects on seed demography have weaker effects on endophyte persistence. In our empirical work, we found that endophytes were lost through time from experimental host seed banks and rates of loss were consistent across species.  We found significant negative effects of endophytes on seed germination and survival in one of host grass species and no detectable effects on the other three. Between species, germination and survival rates differed significantly.  In summary, our theoretical and experimental results indicate that accounting for realistic demographic complexity changes predictions for host-symbiont trajectories and unveils new avenues for symbionts to affect and be affected by their hosts.