The active spread of adaptive variation for reef resilience

Abstract

The speed at which species adapt depends partly on the rates of beneficial adaptation generation and how quickly they spread within and among populations. Natural rates of adaptation of corals may not be able to keep pace with climate warming. Several interventions have been proposed to fast‐track thermal adaptation, including the intentional translocation of warm‐adapted adults or their offspring (assisted gene flow, AGF) and the ex situ crossing of warm‐adapted corals with conspecifics from cooler reefs (hybridization or selective breeding) and field deployment of those offspring. The introgression of temperature tolerance loci into the genomic background of cooler‐environment corals aims to facilitate adaptation to warming while maintaining fitness under local conditions. Here we use research on selective sweeps and connectivity to understand the spread of adaptive variants as it applies to AGF on the Great Barrier Reef (GBR), focusing on the genus Acropora. Using larval biophysical dispersal modeling, we estimate levels of natural connectivity in warm‐adapted northern corals. We then model the spread of adaptive variants from single and multiple reefs and assess if the natural and assisted spread of adaptive variants will occur fast enough to prepare receiving central and southern populations given current rates of warming. We also estimate fixation rates and spatial extent of fixation under multiple release scenarios to inform intervention design. Our results suggest that thermal tolerance is unlikely to spread beyond northern reefs to the central and southern GBR without intervention, and if it does, 30+ generations are needed for adaptive gene variants to reach fixation even under multiple release scenarios. We argue that if translocation, breeding, and reseeding risks are managed, AGF using multiple release reefs can be beneficial for the restoration of coral populations. These interventions should be considered in addition to conventional management and accompanied by strong mitigation of CO2 emissions.

Assessing the role of historical temperature regime and algal symbionts on the heat tolerance of coral juveniles

“…The production and reseeding of cool reefs with corals that have a comparatively high thermal tolerance can facilitate adaptation to climate change. This technique has thus been proposed as a way of increasing the likelihood of successful reef-restoration initiatives (Anthony et al, 2017;Quigley et al, 2019;van Oppen et al, 2014van Oppen et al, , 2015. Despite the small sample size of reproductive colonies, this study demonstrates that crosses consisting of two parents from the warmest reef provided increased growth and survival coupled with decreased bleaching under experimentally elevated temperature in coral juveniles.…”

Signatures of selection in the coral holobiont reveal complex adaptations to inshore environments driven by Holocene climate change

“…We apply our approach to identify signatures of selection and profile symbiont communities in Acropora tenuis populations from inshore reefs of the GBR. This species is widespread throughout the Indo-Pacific where it is common on both inshore and offshore reefs, and has become an important model for research into water quality stress 25,26 , adaptive changes in symbiont composition 27 and the feasibility of assisted evolution approaches 28,29 . The reefs sampled in this study provide an ideal context for local adaptation because they were recolonised between 14 and 6Kya when sea levels rose resulting in marine flooding of the GBR shelf 30 .…”

Interventions to help coral reefs under global change—A complex decision challenge

“…Such delayed action represents a lost opportunity as interventions take time to develop, and because damage-prevention and restoration are now both needed to sustain ecosystems [82,83]. For example, coral populations in the northern Great Barrier Reef (GBR) are adapted to 1-2˚C higher temperatures than populations in the central section [84], but the North-to-South larval spread is limited by diverging currents [47,85]. Under expectations of escalated GBR-wide warming [86], building resilience in the central and south using warm-adapted coral stock from the north will be a race against time as both donor reefs and receiving reefs are at risk.…”

“…While natural processes of physiological acclimation may improve coral heat tolerance [43,44] genetic adaptation generally acts on longer timescales [45]. Warm-adapted traits may not spread fast enough in most coral species to keep up with the rate of global warming, even under strong carbon mitigation [14,[46][47][48].…”