Abstract
Determining the adaptive potential of foundation species, such as reef-building corals, is urgent as the oceans warm and coral populations decline. Theory predicts that corals may adapt to climate change via selection on standing genetic variation. Yet, corals face not only rising temperatures but also novel diseases. We studied the interaction between two major stressors affecting colonies of the threatened coral, Acropora cervicornis: white-band disease and high water temperature. We determined that 27% of A. cervicornis were disease resistant prior to a thermal anomaly. However, disease resistance was largely lost during a bleaching event because of more compromised coral hosts or increased pathogenic dose/virulence. There was no tradeoff between disease resistance and temperature tolerance; disease susceptibility was independent of Symbiodinium strain. The present study shows that susceptibility to temperature stress creates an increased risk in disease-associated mortality, and only rare genets may maintain or gain infectious disease resistance under high temperature. We conclude that A. cervicornis populations in the lower Florida Keys harbor few existing genotypes that are resistant to both warming and disease.
Highlights
Genetic diversity within a population leads to varying levels of stress tolerance among individuals (Sorensen et al, 2001), and is critical for species survival and persistence in a changing climate (Hoffmann and Sgro, 2011)
The objectives of the present study were to (1) determine the relative abundance of genotypes of Acropora cervicornis from the lower Florida Keys that were resistant to disease, (2) characterize the Symbiodinium strains within each host and explore the potential relationship between the algal symbionts and disease susceptibility, and (3) quantify the relative change in disease risk when corals were bleached
Our results suggest that disease resistance and temperature tolerance evolve independently within A. cervicornis of the lower Florida Keys
Summary
Genetic diversity within a population leads to varying levels of stress tolerance among individuals (Sorensen et al, 2001), and is critical for species survival and persistence in a changing climate (Hoffmann and Sgro , 2011). Tolerance to stress may be a result of different symbiotic algal species (Symbiodinium spp.) or even of different genotypes (i.e. strains) of certain Symbiodinium species that reside within the coral host (Gregoire et al, 2017; Parkinson and Baums, 2014; Fabricius et al, 2004). Coral populations showing resilience to high water temperatures constitutively frontload the expression of genes related to heat tolerance in concert with several genes influencing the host innate immune response (Barshis et al, 2013), suggesting these corals may have evolved the general ability to tolerate a multitude of threats. A critical question is whether certain coral genotypes, existing within the same environment, are generally more stress
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