Microalgal symbionts in the Symbiodiniaceae are important symbionts on coral reefs. The breakdown in the mutualism with coral reef species that occurs as a result of increasingly warmer ocean temperatures is a major threat to coral reefs around the globe. Given their short generation times, symbionts have been the focus of attempts at assisted evolution aimed at reef conservation. We found significant variation among algal genotypes in the species Breviolum antillogorgium, suggesting that laboratory selection experiments might produce phenotypes adapted to different water temperatures. So we conducted a 2 year (>500 generations) selection experiment at ambient and warmed temperatures to examine whether symbionts could adapt to warmer water temperatures. We then introduced different strains of algae into host polyps to determine whether different genotypes conferred heat tolerance to the holobiont.
Results/Conclusions
The lengthy laboratory selection experiment did not result in local adaptation to high temperatures associated with bleaching. Further, introduction of symbionts into polyp hosts greatly depressed the extent of genetic variation in photosynthetic rate. Mutualistic traits, such as growth rate and photosynthetic rate, measured in culture may not reflect the expression of those traits within hosts. In part, this may be due to differences in bacterial communities between cultures and hosts, which greatly affect the physiological efficiency of the holobiont. Different rates of selection inside and outside of hosts means that efforts at assisted evolution will depend on how much of the algal life cycle occurs inside and outside of hosts.