The Genetic traits of corals that survived recent bleaching events / K. Quigley, B. Ramsby, P. Laffy, V. Mocellin, J. Harris and L.K. Bay

The past five years have seen unprecedented coral bleaching on the Great Barrier Reef (GBR) with three mass bleaching events affecting the full length of the GBR and temperaturescontinue to rise. It is clear that corals need to adapt to increasing temperatures to be able to persist into the future. The potential scope for adaption of natural populations is underpinned by geneticdiversity. Functional or adaptive diversity describes variants of genes that are directly associated with tolerance and survival. The consequences of these major disturbances onhard coral species and their genetic diversity are not yet known. In particular, at present it is poorly understood how severe bleaching events affect the genetics and adaptability of naturalcoral populations and how that information can be used to forecast which corals are likely to survive future climate conditions. The project aimed to identify whether hard corals surviving the 2016 and 2017 bleachingevents had genetic adaptations that enabled them to withstand higher temperatures. To address this, the project investigated the following questions: 1. Has bleaching resulted in a loss of genetic diversity in corals?2. Do surviving populations contain genetic signatures of adaptation to bleaching temperatures? The temperature tolerance of hard corals is also influenced by the genetic identity of the photosymbionts hosted in their tissues. Extreme environments may select for stress tolerant coral individuals possibly through associations with more stress-tolerant symbionts. As most coralsacquire their symbionts anew in each generation, the environmental pool (i.e. the free-living symbiont pool) may thus be an important source of temperature tolerant symbionts. Identifyingthe distribution and abundance of stress-tolerant symbionts (in hospite and free-living) will help identify key hard coral populations for protection, key reefs for resilience management and aspotential breeding stock for future coral reef restoration activities. The project therefore also investigated: 3. What is the prevalence of heat tolerant photo-symbionts within surviving corals and in the environmental pool?4. Are heat tolerant Symbiodiniaceae communities and host genetic variants correlated with bleaching conditions? To understand how bleaching impacts coral populations at a genetic level, high throughput genomic sequencing was used to associate genes with bleaching phenotypes, with theunderlying focus on applying genetic data to management practice (Chapter 2). Bleaching phenotypes are how the coral "appears" when exposed to hot water. For example, vulnerable colonies or species will appear pale or fluorescent after continued exposure to hot water,whereas resistant colonies or species maintain their brown coloration from the maintenance (either in chlorophyll concentration or cell number) of their symbiont populations. (Executive summary,page 1)