Scientists have successfully bred corals to improve their heat tolerance

Led by experts at Newcastle University's Coralassist Lab, the study documents the world's first effort to selectively breed adult corals for enhanced heat tolerance, i.e. the ability of adult corals to survive intense marine heatwaves. The breeding effort was a success, showing that it is possible to improve the heat tolerance of adult coral offspring, even in a single generation.

However, the improvement was modest in comparison to future marine heatwaves expected under climate change. The authors stress that rapid reductions of global greenhouse gas emissions are an absolute requirement to mitigate warming and give corals an opportunity to adapt.

The study was published in the journal Nature Communications. It was carried out in partnership with the University of Victoria, Horniman Museum and Gardens, Palau International Coral Reef Center, University of Derby, and the University of Exeter.

The publication is the result of a five-year project which was launched by Dr James Guest with funding from the European Research Council.

Not a silver bullet solution

"This work shows that selective breeding is feasible but not a silver bullet solution and that more research is needed to maximise breeding outcomes," says study lead author, Liam Lachs, a Postdoctoral Research Associate at Newcastle University. He continues, reflecting that "in parallel, rapid reductions of global greenhouse gas emissions are an absolute requirement to mitigate warming and give corals an opportunity to adapt.

Dr Guest, Reader in Coral Reef Ecology at Newcastle University's School of Natural and Environmental Sciences, explains that "the results show that selective breeding could be a viable tool to improve population resilience. Yet, there are still many challenges that need to be overcome. How many corals need to outplanted to benefit wild populations? Can we ensure there are no trade-offs (evidence so far suggests this is not a large risk)? How can we avoid dilution of selected traits once added to the wild? How can we maximise responses to selection?

"Given the moderate levels of enhancement we achieved in this study the effectiveness of such interventions will also depend on urgent climate action."

Successful breeding trial

Selective breeding has been practiced by humans for thousands of years to produce animals and plants with desirable characteristics. Now it is being considered as a tool for nature conservation, particularly for coral reefs. These marine ecosystems are at the forefront of climate change impacts, as reef-building corals are highly sensitive to marine heatwaves. These can trigger mass coral bleaching and mortality events which have already led to considerable reef declines globally.

The experts conducted selective breeding trials for two different traits, either the tolerance to a short intense heat exposure (10 days, reaching +3.5°C) or a less-intense but long-term exposure more typical of natural marine heatwaves (1 month, reaching +2.5°C).

The team found that selecting parent colonies for high rather than low heat tolerance increased the tolerance of adult offspring. This result held for the response to both 10-day and 1-month exposures. Heat tolerance could in theory be enhanced by approximately 1 °C-week within one generation. However, this level of enhancement is likely insufficient to keep pace with unabated warming.

What's next?

Selectively breeding for short-stress tolerance did not show evidence of enhancing the ability of offspring to survive the long heat stress exposure. With no genetic correlation detected, it is plausible that these traits are under independent genetic controls. This would have important implications, as interventions would benefit from cheap and rapid assays that can effectively identify heat tolerant colonies for breeding. However, if these assays do not predict adult colony survival to natural marine heatwaves, it presents a serious challenge for management interventions.

Study lead author, Dr Adriana Humanes, Postdoctoral Research Associate at the Coralassist Lab, Newcastle University, highlights that: "considerable work remains before selective breeding can be successfully implemented. A deeper understanding is needed to determine which traits to prioritize and how these traits are genetically correlated."

Take home message

The authors say that this work is an important proof of concept: selective breeding corals for adult heatwave survival is possible. Now, they call for more research and development to understand how to operationalise breeding interventions and maximise outcomes to hopefully keep pace with the lower levels of warming that can be achieved with concurrent climate action.