Evolutionary rescue occurs when individuals resistant to environmental stress survive and reproduce in a declining population, resulting in trait evolution on ecologically-relevant time scales. This eco-evolutionary response may be a critical component of understanding species resilience in an increasingly stressful environment. Work to date has focused on testing the potential for evolutionary rescue in a single species. However, species interactions, especially tight-knit mutualisms, require us to research evolution in a community context. Specifically, evolutionary rescue by association could occur if a mutualist is able to rapidly evolve and rescue their host from environmental pressures, even in the absence of host evolution. We investigated the potential for evolutionary rescue by association using a model cnidarian-dinoflagellate mutualism: the upside-down sea jelly Cassiopea xamachana and its microalgal endosymbiont, Symbiodinium microadriaticum. To test for standing genetic variation in response to temperature stress, we grew five strains of S. microadriaticum at 26°C, 30°C, and 32°C and measured growth rate, photosynthesis, and respiration in culture. We introduced these strains to polyps to determine if symbiont trait differences affected host fitness at the same three temperatures. We quantified several components of host fitness, including survival, timing of developmental stages, and total ephyra production (asexual reproduction).
Results/Conclusions
Symbiont strains varied in their response to temperature. Growth rate response varied, with some strains growing slowest at 32°C and others growing slowest at 26°C. Most strains had highest respiration and photosynthetic rates at 26°C and lowest at 32°C, although the severity of their temperature response varied among strains; one strain even showed an opposite response to temperature. Different responses to thermal stress suggests that the standing genetic variation required for natural selection to occur exists and could result in rapid evolution in response to warming oceans. We also found that host fitness varied in response to an interaction between temperature and symbiont strain. The timing of development events (time to inoculation, time to strobilation, and time to produce an ephyra) and total ephyra production differed in response to an interaction between temperature and symbiont strain, with different responses to temperature dependent on symbiont strain. This significant interaction suggests the potential for selection on the symbionts that affects host fitness, and therefore the possibility of evolutionary rescue by association. Evolutionary rescue, not only in populations, but in populations of closely associated species, may be a critical component of understanding how species will respond to global change.