How selective breeding could improve coral heat tolerance during marine heatwaves

Coral selective breeding and marine heatwaves

Our research group has bred corals that are better able to survive marine heatwaves. The work, now published in Nature Communications, demonstrates that coral heat tolerance can be improved within just one generation.

We achieved this through selective breeding, a technique people have used for thousands of years to produce plants and animals with desirable traits. It is the method by which humans transformed wolf-like dogs into St Bernards, chihuahuas and all the breeds in between.

Selective breeding is now being considered as a conservation tool, particularly for coral reefs. The Coralassist Lab, of which we are members, and the Palau International Coral Reef Center have focused specifically on helping corals survive heatwaves. Our latest findings are the result of seven years of work.

Marine heatwaves cause mass coral bleaching and death, and 2023–2024 was declared the fourth global mass bleaching event. Assisted evolution methods, such as selective breeding, aim to accelerate natural adaptation and buy corals time as the climate changes.

However, the improvement in heat tolerance among our selectively bred corals was modest compared with the intensity of future marine heatwaves. Although selective breeding is possible, it is unlikely to be a panacea. We will still need to address the cause of mass coral bleaching by reducing greenhouse gas emissions, limiting warming and giving assisted evolution programmes time to work.

How to breed corals for heat tolerance

Our first step was to assess the heat tolerance of many potential parent corals on the reef. We then selected particular individuals to produce two separate families of offspring, bred for either high or low heat tolerance. The offspring were raised for three to four years, until they reached reproductive maturity, before we tested their heat tolerance.

We ran selective breeding trials for two different traits: tolerance of a short, intense period of heat, at temperatures 3.5°C above normal for ten days, and tolerance of a less intense but longer exposure, at 2.5°C above average for a month, which is more typical of natural marine heatwaves. This allowed us to estimate each trait’s heritability, its response to selective breeding and whether the two traits shared a genetic basis.

For both traits tested, choosing parents with high rather than low heat tolerance improved the tolerance of their adult offspring.

Measuring coral heat tolerance

Heritability was approximately 0.2 to 0.3 on a scale of 0 to 1. This means that about a quarter of the variation in the offspring’s heat tolerance was attributable to genes inherited from their parents. Put another way, the traits have a substantial genetic basis that can be acted on by natural and artificial selection.

We use degree-heating weeks (°C-weeks) to measure cumulative heat stress and tolerance, taking account of both the temperature reached and its duration. Based on the variation in these traits among the particular corals studied, heat tolerance could theoretically be increased by about 1°C-week in a single generation.

Even that improvement, however, may not be sufficient to keep up with increasingly intense heatwaves. Depending on climate action, heatwave intensity is expected to increase by around 3°C-weeks per decade in the coming decades, outpacing the improvement achieved in our study.

Interestingly, corals selectively bred for high rather than low tolerance of short heat stress were no better at surviving prolonged heat exposure. We found no genetic correlation, making it plausible that separate sets of genes drive these traits. Corals that survive short, sharp heat stress well are therefore not necessarily the best survivors of longer marine heatwaves.

This could have important implications. Work of this kind would benefit from inexpensive, rapid tests that reliably identify heat-tolerant colonies for breeding. But if such tests cannot predict which coral colonies will survive month-long heatwaves, that presents a serious challenge.

Scaling up selective breeding

Now that selective breeding for greater coral heat tolerance has proved possible, the next step is to carry out large-scale trials in the wild. This will probably involve deploying considerable numbers of selectively bred corals, perhaps by seeding coral larvae directly onto reefs or planting corals raised in an aquaculture facility.

For the approach to work, outplanted corals must themselves reach reproductive age and contribute to the gene pool of wild populations. Achieving this on a very large scale will be challenging, although it may not be necessary to replenish coral coverage over large areas.

Instead, a smaller network of strategically located larval production hubs may be enough. These would contain selectively bred corals at high densities to maximise fertilisation success, seeding other reefs and potentially supplying further broodstock for targeted action.

Much more research and development is needed, and many critical questions remain. How many corals must be outplanted to achieve the desired effect? Can we ensure there are no trade-offs that might compromise populations, although evidence so far suggests this is not a large risk? How can selected traits be prevented from being diluted once introduced into the wild? How can responses to selection be maximised?

Given how quickly the ocean is warming, assisted evolution will need to be optimised and implemented soon if it is to have a chance of succeeding, even at small scales. Above all, coral reef survival still depends on urgent climate action.

Liam Lachs, Postdoctoral Research Associate in Climate Change Ecology and Evolution, Newcastle University; Adriana Humanes, Postdoctoral Research Associate in Coral Reef Ecology, Newcastle University, Newcastle University, and James Guest, Reader in Coral Reef Ecology, Newcastle University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.