Corals already have the genes to adapt to warmer oceans

26 June, 2015
Australian Institute of Marine Science, Media releases

Townsville and Texas researchers discover a genetic basis to temperature tolerance in coral. And it likely depends on ‘mum’s genes’.

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A team of Australian and US scientists have discovered that corals already have the genes to tolerate global warming. It may only be a matter of shuffling them to where they are most needed.

Far Northern Great Barrier Reef. Reefs around the world are threatened by climate change but if corals can evolve higher temperature tolerance their future outlook is much improved. Credit: Line K Bay, AIMS

Far Northern Great Barrier Reef. Reefs around the world are threatened by climate change. A new study shows that some corals have the genes to adapt to warmer oceans. Credit: Line K Bay, AIMS

Migration and breeding may allow coral to adapt to hotter oceans, and offer a path for smarter reef conservation and restoration, according to a paper published today in Science by researchers from the Australian Institute of Marine Science (AIMS) and the University of Texas at Austin.

The team crossed individuals of branching coral Acropora millepora (pictured on right) from the far northern Great Barrier Reef with members of the same species at Orpheus Island, 540 km further south. They found that corals living in warmer waters on the north of the Great Barrier Reef pass on higher temperature tolerance to their offspring.

The study also suggests that ‘Mum is best’: coral larvae had up to a ten-fold increase in their chances of surviving heat stress, of which five-fold was contributed by the mother. A possible explanation was provided by genetic analysis: tolerant larvae had altered expression of genes working in mitochondria, the powerhouses of the cells that are inherited solely from mothers.

“This discovery adds to our understanding of the potential for coral on the Great Barrier Reef to cope with hotter oceans,” says Dr Line Bay, an evolutionary ecologist from AIMS in Townsville and joint senior author on the paper (picture shows Line at work in the Far Northern Sector of the Great Barrier Reef).

“Averting coral extinction can begin with something as simple as exchange of coral immigrants across latitudes, which will happen naturally through larval dispersal but can be jump-started by humans moving adult corals,” says Mikhail Matz, a professor of integrative biology at the University of Texas at Austin and co-lead author. “This is occasion for hope and optimism about coral reefs and the marine life that thrive there.”

Potential applications include:

  • Choosing heat-tolerant corals for artificial propagation and reef restoration
  • Identifying reefs with heat-tolerant coral communities and protecting them so that they naturally repopulate other reefs

Before such steps are considered we need to learn much more about coral adaptability to be able to discriminate between ‘winners’ and ‘losers’ in the game of climate change. One of the first tasks would be to confirm the role of mitochondria in heat tolerance, which was suggested for the first time by this study.

This study is part of a broad body of work at AIMS into the ability of coral and coral communities to adapt to a changing climate and to cumulative impacts of stress factors such as heat, ocean acidity, sediment, and water quality.

This study was supported by funds from the US National Science Foundation and the Australian Institute of Marine Science.

For further information visit www.aims.gov.au and https://cns.utexas.edu.
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Video: Michelle Jonker © AIMS

Background information

Genetic rescue can save corals from warming oceans

Coral bleaching is a serious threat to the future of coral reefs, according to marine researchers and the Great Barrier Marine Park Authority. It occurs when high sea temperatures cause the loss from corals of the symbiotic algae that provide most of their food and, incidentally, their colour. While bleaching can be temporary, at best it causes long-term reductions in health, and at worst widespread mortality.

Rising sea temperatures precipitated by global warming are likely to cause further declines in coral cover, and extinctions of local populations – and possibly of species – within this century. The good news is that even modest change in stress tolerance (i.e. through the process of adaptation) can vastly improve the outlook for reefs.

The fundamental building block for adaptation is biological variation, and temperature tolerance is a variable response. Coral species suffer to different degrees, and within species some individuals and some locations are less affected than others. Corals from naturally warmer reefs, for example, can cope with warmer temperatures than corals from cooler reefs. It might be possible to avoid bleaching and subsequent mortality from projected temperature increases on cooler reefs through the introduction of heat-tolerant corals from naturally warmer reefs. But this can only work if the variation in heat tolerance is heritable – meaning that offspring resemble their parents and not the environment in which they are born.

In the study published today in Science, researchers crossed corals from naturally warmer and cooler locations on the Great Barrier Reef (GBR) more than 500 km apart and distinguished by an average two degrees in temperature. The work provides new insights into the mechanisms that underpin temperature tolerance in corals.

The researchers demonstrated up to a ten-fold increase in the odds of survival of coral larvae under heat stress when their parents came from a warmer lower-latitude location. This directly demonstrates that temperature tolerance is heritable: parents that tolerate higher temperatures have offspring that do so too. This means, a potential solution to the problem of coral bleaching already exists in natural populations. If more tolerant corals can spread their genes to locations impacted by bleaching, either naturally through larval dispersal or with the help of humans moving adult corals, then some of the negative effects of global warming can be averted. This mechanism of adaptation based on existing genetic variation is termed ‘genetic rescue.’

It has been suggested previously that coral thermal tolerance can be triggered by a pre-emptive stress response that makes an individual ‘primed’ for stress. The results of this study do not support this idea. On the contrary, the researchers showed that more tolerant individuals exhibit less, not more, of a stress response before the actual stress is applied.

Coral larvae inherited expression of more than 2700 genes directly from their parents, equally from mother and father. Still, larval heat tolerance was mostly due to mother’s influence: of the ten-fold overall increase observed in the experiments, five-fold came from mother. By analysing the heat-adapted offspring further, the researchers identified two regions of the genome that were important for heat tolerance, which happened to encode proteins destined to work in the internal cellular powerhouses known as mitochondria. Since mitochondria are inherited solely from the mother, this provided a reasonable explanation of elevated maternal influence on heat tolerance. The role of mitochondria in coral heat tolerance has not been considered previously and the authors’ next task will be to verify this novel hypothesis.

In summary, the study shows that variation in thermal tolerance of corals across latitudes has a strong genetic basis that can serve as raw material for natural selection, and hence, evolution.

The researchers crossed individuals of the common branching coral, Acropora millepora from Princess Charlotte Bay on Cape York Peninsula, 180 km northwest of Cooktown with those from Orpheus Island, about 75 km north of Townsville. There is about 5° of latitude and 540 km between the two.

Media resources including background information, photos, and a copy of the paper available at: www.scienceinpublic.com.au/marine

Abstract

Genomic Determinants of Coral Heat Tolerance Across Latitudes

Groves B. Dixon1, Sarah W. Davies1, Galina A. Aglyamova1, Eli Meyer2, Line K. Bay3* and Mikhail V. Matz1*

1 Department of Integrative Biology, University of Texas at Austin, 205 W 24th St. C0990, Austin, Texas 78712, USA

2 Department of Integrative Biology, Oregon State University, 3106 Cordley Hall, Corvallis, OR 97331, USA

3 Australian Institute of Marine Science, PMB 3, Townsville MC, Queensland 4810, Australia

* Authors for correspondence, l.bay@aims.gov.au, matz@utexas.edu

As global warming continues, reef-building corals could avoid local population declines through “genetic rescue” involving exchange of heat-tolerant genotypes across latitudes, but only if latitudinal variation in thermal tolerance is heritable. Here we show up to ten-fold increase in odds of survival of coral larvae under heat stress when their parents come from a warmer lower-latitude location. Elevated thermal tolerance was associated with heritable differences in expression of oxidative, extracellular, transport and mitochondrial functions that indicated a lack of prior stress. Moreover, two genomic regions strongly responded to selection for thermal tolerance in inter-latitudinal crosses. These results demonstrate that variation in coral thermal tolerance across latitudes has a strong genetic basis and could serve as raw material for natural selection.

About the organisations

The Australian Institute of Marine Science (AIMS) is Australia’s national tropical marine research institute in Australia.  It is based in Townsville. www.aims.gov.au

The University of Texas at Austin Department of Integrative Biology has its base in one of the largest college of natural sciences in the United States. It is home to a top-ten ranked program in ecology, evolution and behaviour. www.cns.utexas.edu.