Our research, published in Science Advances, shows corals from some of the world’s hottest seas can transfer beneficial genes associated with heat tolerance to their offspring, even when crossbred with corals that have never experienced such temperatures.
Across the world, corals vary widely, both in the temperatures they experience and their ability to withstand high temperatures without becoming stressed or dying. In the Persian Gulf, corals have genetically adapted to extreme water temperatures, tolerating summer conditions above 34℃ for weeks at a time, and withstanding daily averages up to 36℃.
These water temperatures are 2-4℃ higher than any other region where corals grow, and are on a par with end-of-century projections for reefs outside the Persian Gulf.
This led us to ask whether beneficial gene variants could be transferred to coral populations that are naïve to these temperature extremes. To find out, we collected fragments of Platygyra daedalea corals from the Persian Gulf, and cross-bred them with corals of the same species from the Indian Ocean, where summer temperatures are much cooler.
We then heat-stressed the resulting offspring (more than 12,000 individual coral larvae) to see whether they could withstand temperatures of 33°C and 36°C — the summer maximums of their parents’ respective locations.
We found an immediate transfer of heat tolerance when Indian Ocean mothers were crossed with Persian Gulf fathers. These corals showed an 84% increase in survival at high temperatures relative to purebred Indian Ocean corals, making them similarly resilient to purebred Persian Gulf corals.
Genome sequencing confirmed that gains in heat tolerance were due to the inheritance of beneficial gene variants from the Persian Gulf corals. Most Persian Gulf fathers produced offspring that were better able to withstand heat stress, and these fathers and their offspring had crucial variants associated with better heat tolerance.
Conversely, most Indian Ocean fathers produced offspring that were less able to survive heat stress, and were less likely to have gene variants associated with heat tolerance.
Encouragingly, gene variants associated with heat tolerance were not exclusive to Persian Gulf corals. Two fathers from the Indian Ocean produced offspring with unexpectedly high survival under heat stress, and had some of the same tolerance-associated gene variants that are prevalent in Persian Gulf corals.
This suggests that some populations have genetic variation upon which natural selection can act as the world’s oceans grow hotter. Selective breeding might be able to accelerate this process.
We are now assessing the genetic basis for heat tolerance in the same species of coral on the Great Barrier Reef and in Western Australia. We want to find out what gene variants are associated with heat tolerance, how these variants are distributed within and among reefs, and whether they are the same as those that allow corals in the Persian Gulf to survive such extreme temperatures.
This knowledge will help us understand the potential for Australian corals to adapt to rapid warming.
Although our study shows selective breeding can significantly improve the resilience of corals to ocean warming, we don’t yet know whether there are any trade-offs between thermal tolerance and other important traits, and whether there are significant genetic risks involved in such breeding.
Our study was done on coral larvae without the algae that live in close harmony with corals after they settle on reefs. So it will also be important to examine whether the genetic improvements to heat tolerance continue into the corals’ later life stages, when they team up with these algae.
Of course, saving corals from the perils of ocean warming will require action on multiple fronts — there is no silver bullet. Selective breeding might provide some respite to particular coral populations, but it won’t be enough to protect entire ecosystems, and nor is it a substitute for the urgent reduction of greenhouse emissions needed to limit the oceans’ warming.
This includes two remarkable species of gastric-brooding frog. To reproduce, gastric-brooding frogs swallowed their fertilised eggs, and later regurgitated tiny baby frogs. Their reproduction was unique in the animal kingdom, and now they are gone.
Tragically, we have identified an additional three frog species that are very likely to be extinct. Another four species on our list are still surviving, but not likely to make it to 2040 without help.
The 26 most imperilled frogs
The striking yellow-spotted tree frog (in southeast Australia), the northern tinker frog and the mountain mist frog (both in Far North Queensland) are not yet officially listed as extinct – but are very likely to be so. We estimated there is a greater than 90% chance they are already extinct.
The next four most imperilled species are hanging on in the wild by their little frog fingers: the southern corroboree frog and Baw Baw frog in the Australian Alps, and the Kroombit tinker frog and armoured mist frog in Queensland’s rainforests.
The southern corroboree frog, for example, was formerly found throughout Kosciuszko National Park in the Snowy Mountains. But today, there’s only one small wild population known to exist, due largely to an introduced disease.
Without action it is more likely than not (66% chance) the southern corroboree frog will become extinct by 2040.
What are we up against?
Species are suffering from a range of threats. But for our most recent extinctions and those now at greatest risk, the biggest cause of declines is the amphibian chytrid fungus disease.
This introduced fungus is thought to have arrived in Australia in the 1970s and has taken a heavy toll on susceptible species ever since. Cool wet environments, such as rainforest-topped mountains in Queensland where frog diversity is particularly high, favour the pathogen.
The fungus feeds on the keratin in frogs’ skin — a major organ that plays a vital role in regulating moisture, exchanging respiratory gases, immunity, and producing sunscreen-like substances and chemicals to deter predators.
Another major emerging threat is climate change, which heats and dries out moist habitats. It’s affecting 19 of the imperilled species we identified, such as the white-bellied frog in Western Australia, which develops tadpoles in little depressions in waterlogged soil.
Climate change is also increasing the frequency, extent and intensity of fires, which have impacted half (13) of the identified species in recent years. The Black Summer fires ravaged swathes of habitat where fires should rarely occur, such as mossy alpine wetlands inhabited by the northern corroboree frog.
Invasive species impact ten frog species. For the spotted tree frog in southern Australia, introduced fish such as brown and rainbow trout are the main problem, as they’re aggressive predators of tadpoles. In northern Australia, feral pigs often wreak havoc on delicate habitats.
So what can we do about it?
We identified the key actions that can feasibly be implemented in time to save these species. This includes finding potential refuge sites from chytrid and from climate change, reducing bushfire risks and reducing impacts of introduced species.
But for many species, these actions alone aren’t enough. Given the perilous state of some species in the wild, captive conservation breeding programs are also needed. But they cannot be the end goal.
Captive breeding programs can not only establish insurance populations, they can also help a species persist in the wild by supplying frogs to establish populations at new suitable sites.
Boosting numbers in existing wild populations with captive bred frogs improves their chance of survival. Not only are there more frogs, but also greater genetic diversity. This means the frogs have a better chance of adapting to new conditions, including climate change and emerging diseases.
Our knowledge of how to breed frogs in captivity has improved dramatically in recent decades, but we need to invest in doing this for more frog species.
Finding and creating wild refuges
Another vital way to help threatened frogs persist in the wild is by protecting, creating and expanding natural refuge areas. Refuges are places where major threats are eliminated or reduced enough to allow a population to survive long term.
For the spotted-tree frog, work is underway to prevent the destruction of frog breeding habitat by deer, and to prevent tadpoles being eaten by introduced predatory fish species. These actions will also help many other frog species as well.
The chytrid fungus can’t be controlled, but fortunately it does not thrive in all environments. For example, in the warmer parts of species’ range, pathogen virulence may be lower and frog resilience may be higher.
Chytrid fungus completely wiped out the armoured mist frog from its cool, wet heartland in the uplands of the Daintree Rainforest. But, a small population was found surviving at a warmer, more open site where the chytrid fungus is less virulent. Conservation for this species now focuses on these warmer sites.
This strategy is now being used to identify potential refuges from chytrid for other frog species, such as the northern corroboree frog.
No time to lose
We missed the window to save the gastric-brooding frogs, but we should heed their cautionary tale. We are on the cusp of losing many more unique species.
Decline can happen so rapidly that, for many species, there is no time to lose. Apart from the unknown ecological consequences of their extinctions, the intrinsic value of these frogs means their losses will diminish our natural legacy.
In raising awareness of these species we hope we will spark new action to save them. Unfortunately, despite persisting and evolving independently for millions of years, some species can now no longer survive without our help.