Currently, there is significant genetic diversity within and among populations of many coral species. Assisted evolution aims to facilitate the sharing of this genetic diversity, to transfer desirable traits between coral species and populations. ‘Assisted evolution’ interventions being considered in this program aim to strengthen corals’ tolerance to climate change. In principle, however, 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 leading to an increase in 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 either through fragmentation methods or bred in an aquaculture facility before being placed onto receiving reefs.
Early studies are promising, however further research is needed to 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 for hardening to influence the stress tolerance of corals, and how this method could be applied in aquaculture-based propagation.
Photo by Christian Miller