Assisted evolution: seeding enhanced-performance corals
Currently, there is significant genetic diversity within and among populations of many coral species. Interventions in this group are also known as 'assisted evolution' as they aim to facilitate the sharing of this genetic diversity, to transfer desirable traits between coral species and populations. They aim to strengthen corals’ tolerance to climate change. In principle, any trait of interest could be enhanced by the following methods:
- physiological acclimatisation - controlled exposure to stressful conditions to induce a hardening response in corals
- genetic adaptation - increasing the abundance of temperature-tolerant individuals on reefs to increase the population’s ability to persist under warming conditions.
Assisted gene flow
Assisted gene flow facilitates the natural dispersal of corals, to enhance the adaptation of local populations to expected future conditions. This can be achieved by moving naturally warm-adapted corals (adults, fragments, spawn, larvae or juveniles) to reefs with less heat-resistant individuals.
Corals that survive mass-bleaching events, or those that thrive on naturally-warm reefs, are likely to possess heat-resistant genes. Further, coral stock can also be directly identified with specific genetic markers that signal beneficial traits such as bleaching tolerance. These desirable corals can then be propagated through:
- relocating larval slicks from more heat-resistant reefs
- settling slick-captured larve onto devices in an aquaculture facility and deploying these devices onto damaged reefs
- automated or semi-automated mass-production in shore-based aquaculture facilities using either sexual or asexual (fragmentation) methods.
Stock settled onto devices or mass-produced in aquaculture facilities could also receive additional probiotic or hardening treatments to increase their resistance.
Early studies are promising, however further research is needed to:
- improve quantification of natural rates of adaptation and the distribution and abundance of heat-tolerant corals
- assess the level of enhancement required, numbers, receiving conditions and distributions needed to achieve an impact
- assess the growth, survival and heat-tolerance of introduced corals
- assess the potential to introduce disease or pests and other impacts on both the donor and receiving reefs.
assess the potential to introduce disease or pests; the growth, survival and heat-tolerance of introduced corals; and other impacts on both the donor and receiving reefs.
Photo by Juergen Freund
Interbreeding different coral species can produce novel genetic outcomes. Hybrid individuals typically have an increased performance, compared with their parents, leading to greater ‘fitness’, including health and stress tolerance.
Hybridisation has been used in plant and animal breeding for hundreds of years. Hybridisation can naturally occur in corals and is hypothesised to have played a major role in their evolution.
A negative impact of this method is that some hybrids may be infertile. As individual corals can live for decades or centuries, even infertile hybrids may have benefits on the Reef’s climate resilience for long enough to ‘buy time’, while greenhouse gases are reduced globally, and water temperatures stabilise.
Photo by Marie Roman
Conditioning or hardening
This approach involves exposing adult corals, or their larvae or juveniles, to stressful environmental conditions such as increasing water temperature and ocean acidification. The exposure induces physiological responses that toughen corals, and allow them to better withstand stress.
Hardening responses are not normally genetic, but can be inherited by future generations in a process called ‘transgenerational plasticity’. This can happen through ‘epigenetic’ mechanisms that change how DNA is read, without a change in the genetic sequence.
Further research is needed to fully understand the potential of hardening to influence the stress tolerance of corals, and how this method could be applied in aquaculture-based propagation.
Photo by Christian Miller