Variation in host-symbiont compatability among Cassiopea-algal symbioses
Surprisingly few empirical studies have addressed the evolutionary ecology of mutualisms. In particular, there are few data available that address the following crucial questions: 1) what factors align the interests of symbiotic partners? 2) what is the degree of ecological and genetic variation in symbionts across multiple populations of a single host species? and, 3) what evolutionary mechanisms drive variation in host-symbiont compatibility where it exists? Although there is no general theory of mutualism, conventional wisdom suggests that mutualisms are best defined as reciprocal exploitations that provide net benefits to the partners involved. Contemporary theory regarding the evolution of virulence has identified several factors that help align host and symbiont interests. However, the extent to which natural systems conform to these theoretical expectations and what factors are most responsible for maintaining cooperative symbioses remains unclear. I used Cassiopea xamachana to address what evolutionary and ecological factors influence endosymbiotic mutualisms. Cassiopea, like many marine invertebrates, harbors endosymbiotic algae within its tissues. Algal symbionts are acquired each generation via horizontal transmission. In chapter 3, I examined variation in host-symbiont compatibility by performing a series of cross-infection experiments using Cassiopea larvae and algal symbionts collected from a single medusa at ten sites in the Florida Keys. Results reveal significant differences among Cassiopea-algal combinations for both host survival and growth. In chapters 4 and 5, I quantify the observed variation by increasing the number of polyp lineages used per site. Results indicate that the observed variation among Cassiopea-algal combinations is geographically structured. Additionally, significant host-symbiont interaction effects suggest that the algal symbionts are locally adapted to jellyfish hosts within a given site. In chapter 5, I re-examine variation in host-symbiont compatibility by using seawater to infect Cassiopea hosts. The results roughly mimic the results obtained in chapters 3 and 4. In chapter 6, I investigate the population genetic structure of the algae inhabiting Cassiopea using RFLP and ISSR markers. Results indicate that the algal symbionts are members of the same species, Symbiodinium microadriaticum. Further, there is marked intraspecific symbiont variation within this species. Overall, host-symbiont compatibility plays a vital role in the symbiotic outcome.