Assisted evolution of corals and their symbionts enhances recruit heat tolerance but with complex outcomes

Heat-tolerant coral stock could supplement populations with tolerance-conferring alleles to combat rising sea surface temperatures and marine heat waves. We tested coral selective breeding and Symbiodiniaceae experimental evolution independently and in tandem as interventions for generating heat-tolerant stock. Broodstock from two sites were ranked using Symbiodiniaceae photochemical efficiency under rapid heat stress and crossed to produce offspring from heat-tolerant colonies (top 25th percentile) and control offspring. Offspring were inoculated with heat-evolved or wild-type symbionts and exposed to 28°C (ambient) or 32°C (elevated) for 2 months. Selective breeding using rapid assays enhanced Davies but not Moore Reef recruit survival and growth at 32°C, suggesting that this method does not universally generate heat-tolerant coral due to genetics, maternal effects, and/or acclimation. Heat-evolved symbionts enhanced survival and bleaching resilience at 32°C but reduced growth at 28°C. Combining interventions yielded additive benefits, no enhancement, or resulted in one intervention diminishing the other's impact. These results demonstrate assisted evolution's potential while cautioning against generalizing its outcomes.