A project undertaken at The University of Wollongong, and supervised by Emily Howells
Coral reefs are among the most biodiverse ecosystems on the planet, and the services they provide to the environment, economies, and societies are valued at 10 trillion US dollars per year. Their evolutionary success is attributed to productive symbioses between coral animals, beneficial algae, and bacteria, which grow the living foundation of the ecosystem. However, coral symbioses are very sensitive to changes in their environment and are negatively impacted by the effects of climate warming. Unusually warm temperatures cause coral bleaching, a severe stress response involving symbiosis breakdown, coral starvation, and death. Warm water bleaching events have contributed to significant losses of coral from all ocean basins and are increasing in frequency and intensity. For example, Australia’s Great Barrier Reef has experienced mass bleaching events in 5 of the last 9 summers (2016 to 2024).
Coral bleaching and mortality on the Great Barrier Reef (photo: Emily Howells)
Coral bleaching and mortality on the Great Barrier Reef (photo: Emily Howells)
Despite their devastating impacts, these extreme events may also promote evolutionary adaptation to warming provided there is ample heritable variation in thermal tolerance traits for selection to act upon. While thermal tolerance can vary widely among individuals within populations, the heritable component of this variation (i.e., that is due to genetic differences among individuals) is often unresolved due to the unknown relatedness among corals in the wild and challenges in disentangling genetic and environmental contributions to tolerance traits. We set out to address this knowledge gap and provide new information on the potential for coral populations to adapt to warming.
We used a quantitative genetics approach to evaluate heritable genetic variation for thermal tolerance across the Indo-Pacific distribution of a common reef-building coral. Hundreds of families of brain coral (Platygyra daedalea) were bred from parental corals from 10 reef populations, and the survival of more than 56,000 larval offspring was tracked at ambient and elevated temperatures. Survival declined under increasing heat stress exposure but varied considerably among families within all populations, and typically ranged from less than 15% to greater than 90%. We used information on the relatedness among families to determine that additive genetic effects contributed to up to half of the observed variation in heat survival. In most populations, the genetic contribution of individual parents enhanced offspring heat survival by at least 50% and as much as 160% relative to the population average. Similarly, negative genetic contributions of individual parents decreased heat survival by 50% to 230%. These results indicate that the raw genetic material required for corals to adapt to warming is widespread.
Hundreds of families of the brain coral, Platygyra daedalea, were bred from coral spawning events across the Indo-Pacific (photo: Anna Scott)
We then investigated whether coral populations have adapted to recent warming. The rapid rate of warming is expected to impose strong directional selection on thermal tolerance traits and deplete their genetic variation across generations. Therefore, we examined the relationship between heritable genetic variation for heat survival and the history of heatwave conditions that induce coral bleaching across the 10 reef populations. Genetic variation was almost entirely explained by the frequency of heatwave events since 1985 (R2 = 0.90) and was up to 3.5 times lower in populations that experienced the highest frequency of heatwaves. These results provide evidence that recent increases in thermal tolerance of coral populations observed in other studies are at least partially due to selection on standing genetic variation (i.e., evolutionary adaptation) over the past few decades.
We also used DNA sequencing to identify genomic markers associated with heat tolerance traits. This included analysing the genetic makeup of larval corals that survived heat stress in our breeding experiments and adult corals that resisted bleaching during heatwave events across several Indo-Pacific reefs. Further DNA sequencing was performed for additional populations to map the distribution of adaptive genetic variants (i.e., those associated with tolerance traits) and estimate their potential to be dispersed among near and distant reefs. Our Indo-Pacific-wide dataset will also allow us to evaluate whether adaptation to warming has occurred in the same regions of the genome across independent selection events. These analyses are ongoing, and we will provide future updates to communicate new results.
