Evolution of transmission in symbioses in variable environments
Dr. Alexandra Brown
UC Santa Barbara
From sulfur-oxidizing bacterial symbionts of deep-sea tubeworms to epiphytic fungi on the leaves of trees, symbiosis is ubiquitous in our world. Most intriguing are those symbioses whose effects are neither purely beneficial (mutualism) nor harmful (parasitism), but instead change with the environmental context. For instance, a mutation in the obligate aphid symbiont Buchnera aphidicola greatly increases its aphid host's fertility in cool environments but sterilizes the host in warm ones. Similarly, the effects of the fungus Epichloë amarallians change from positive to negative depending on the soil conditions its grass host is growing in. Hosts in conditional mutualisms are faced with a challenge when it comes to transmitting their symbiont, as they benefit most if they are able to contain the symbiont to the times and places where it is beneficial. We use simulations and analytical models to investigate host transmission of a symbiont whose costs and benefits vary in time and space. We find that when the symbiont affects host lifespan, but not fecundity, horizontal transmission (transmission between neighbors) can be used to contain the symbiont to environments where it is beneficial. On the other hand, synchronicity in environmental state across locations allows vertical (parent-offspring) transmission as method of symbiont containment. We also find an emergent trade-off between horizontal and vertical transmission, suggesting that physiological constraints are not required for hosts to evolve limits on the total amount of transmission.