Assisted evolution of corals and their symbionts enhances recruit heat tolerance but with complex outcomes (RRAP ECT 2.1)

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. This research makes substantial contributions to the development of assisted evolution interventions that might be used in the conservation of biodiversity.

Gravid, adult Acropora spathulata colonies were collected from the reef crest and upper reef slope from Davies and Moore Reefs on the central and northern Great Barrier Reef prior to the December full moon in 2022 (Great Barrier Reef Marine Park Authority permit G22-47337.1). Rapid heat stress assays were conducted on the broodstock corals following Naugle et al. 2024 and Denis et al. 2024. Colony performance from the rapid heat stress test informed our selective breeding.

Coral spawning occured at the National SeaSimulator facility following the December full moon (07/12/2022). The egg-sperm bundles of the colonies were combined in bulk crosses according to broodstock heat tolerance (based on ED50 rankings) to produce four offspring groups with 3 – 9 parental colonies for each reef: 1) offspring of the most heat-tolerant broodstock with rankings in the top 25 percentile (RR), 2) offspring of the moderately heat-tolerant broodstock with rankings between the 37.5 and 62.5 percentiles (MM), 3) offspring of the most thermally sensitive broodstock with rankings in the bottom 25 percentile (SS), and 4) offspring produced from a bulk cross containing gametes from all colonies that spawned on the night when the most colonies spawned in synchrony (Bulk).

The larvae were settled and their crosses were compared to assess whether Fv/Fm ED50 phenotyping under rapid heat stress assays could be used to enhance recruit tolerance under ecologically meaningful levels of heat stress. To investigate whether recruit heat tolerance was enhanced via the inoculation of the recruits with HE Symbiodiniaceae of C. proliferum, a wildtype (WT) strain (WT10, SCF055-01.10) and HE strain (SS8, SCF055-01.08) were used for inoculations and compared during the anlyses. Overall, the stress test was conducted to assess the effect of selective breeding and inoculation with HE symbionts on the heat tolerance of coral recruits under ecologically relevant levels of thermal stress.