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

Abstract

The rate of coral reef degradation from climate change is accelerating and, as a consequence, a number of interventions to increase coral resilience and accelerate recovery are under consideration. Acropora spathulata coral colonies that survived mass bleaching in 2016 and 2017 were sourced from a bleaching-impacted and warmer northern reef on the Great Barrier Reef (GBR). These individuals were reproductively crossed with colonies collected from a recently bleached but historically cooler central GBR reef to produce pure and crossbred offspring groups (warm-warm, warm-cool and coolwarm). We tested whether corals from the warmer reef produced more thermally tolerant hybrid and purebred offspring compared with crosses produced with colonies sourced from the cooler reef and whether different symbiont taxa affect heat tolerance. Juveniles were infected with Symbiodinium tridacnidorum, Cladocopium goreaui and Durusdinium trenchii and survival, bleaching and growth were assessed at 27.5°C and 31°C. The contribution of host genetic background and symbiont identity varied across fitness traits. Offspring with either both or one parent from the northern population exhibited a 13-to 26-fold increase in survival odds relative to all other treatments where survival probability was significantly influenced by familial cross identity at 31°C but not 27.5°C (Kaplan-Meier P=0.001 versus 0.2). If in symbiosis with D. trenchii, a warm sire and cool dam provided the best odds of juvenile survival. Bleaching was predominantly driven by Symbiodiniaceae treatment, where juveniles hosting D. trenchii bleached significantly less than the other treatments at 31°C. The greatest overall fold-benefits in growth and survival at 31°C occurred in having at least one warm dam and in symbiosis with D. trenchii. Juveniles associated with D. trenchii grew the most at 31°C, but at 27.5°C, growth was fastest in juveniles associated with C. goreaui. In conclusion, selective breeding with warmer GBR corals in combination with algal symbiont manipulation can assist in increasing thermal tolerance on cooler but warming reefs. Such interventions have the potential to improve coral fitness in warming oceans. This article has an associated First Person interview with the first author of the paper.

Neighboring colonies influence uptake of thermotolerant endosymbionts in threatened Caribbean coral recruits

“…We observed a significant negative correlation between the proportion of D. trenchii and polyp size after 5 months, consistent with previous studies reporting reduced growth in juvenile and adult corals hosting D. trenchii (Little et al 2004; Cantin et al, 2009; Jones & Berkelmans 2010; Pettay et al 2015; but see Yuyama & Higuchi 2014; Quigley et al 2020). For instance, Little et al (2004) found that acroporid recruits infected with Cladocopium grew 2-3 times faster than those hosting Durusdinium .…”

Variability in Fitness Trade-Offs Amongst Coral Juveniles With Mixed Genetic Backgrounds Held in the Wild

“…Studies using juveniles produced from the hybridisation of gametes sourced from different coral populations have shown either negligible (Quigley et al, 2016) or strong effects of local adaptation (i.e., no survival benefits) (van Oppen et al, 2014), in which the difference in effect size may be driven by the outplant environment (northern × central hybrids to central site or central × southern hybrids to southern site). In the laboratory, several-fold benefits in survival and growth were recorded in hybrid juveniles with at least one warm-adapted dam and in symbiosis with Durusdinium symbionts (Quigley et al, 2020a). Taken together, results in corals from reciprocal translocations and population hybridisation show promise in understanding their use as intervention strategies to enhance the adaptive responses of corals to increasingly warm oceans.…”

Drivers of adaptive capacity in wild populations: implications for genetic interventions

“…Although encouraging, due to the costs and risks associated with human interventions, it is important that future models are based on empirical biological and environmental data explicit for specific populations targeted for intervention. This requires that further experimental studies, like those currently underway 20,[24][25][26][27] continue to fill in the knowledge gaps on the genomic underpinnings of traits related to climate change adaptation in corals.…”

“…Therefore, a detailed understanding of the genetic architecture of fitness related traits, including epistasis and linkage maps; as well as the trade-offs among traits is critical for improved evolutionary modelling efforts in the future. Additionally, and especially important on coral reefs, the contribution of microbial partners to the phenotypes of their hosts is increasingly recognised 37 , and the interplay between the evolution of the host organism and its microbial associates cannot be neglected in coral adaptation models 23,25,38,39 .…”