Evidence for a host role in thermotolerance divergence between populations of the mustard hill coral (Porites astreoides) from different reef environments

Abstract

Studying the mechanisms that enable coral populations to inhabit spatially varying thermal environments can help evaluate how they will respond in time to the effects of global climate change and elucidate the evolutionary forces that enable or constrain adaptation. Inshore reefs in the Florida Keys experience higher temperatures than offshore reefs for prolonged periods during the summer. We conducted a common garden experiment with heat stress as our selective agent to test for local thermal adaptation in corals from inshore and offshore reefs. We show that inshore corals are more tolerant of a 6-week temperature stress than offshore corals. Compared with inshore corals, offshore corals in the 31 °C treatment showed significantly elevated bleaching levels concomitant with a tendency towards reduced growth. In addition, dinoflagellate symbionts (Symbiodinium sp.) of offshore corals exhibited reduced photosynthetic efficiency. We did not detect differences in the frequencies of major (>5%) haplotypes comprising Symbiodinium communities hosted by inshore and offshore corals, nor did we observe frequency shifts ('shuffling') in response to thermal stress. Instead, coral host populations showed significant genetic divergence between inshore and offshore reefs, suggesting that in Porites astreoides, the coral host might play a prominent role in holobiont thermotolerance. Our results demonstrate that coral populations inhabiting reefs <10-km apart can exhibit substantial differences in their physiological response to thermal stress, which could impact their population dynamics under climate change.

Heritable differences in fitness-related traits among populations of the mustard hill coral, Porites astreoides

“…In the Florida Keys, Porites astreoides corals from inshore reefs exhibit elevated temperature tolerance in comparison with corals from offshore reefs (Kenkel et al, 2013a). These populations harbor indistinguishable Symbiodinium phylotypes but hosts are significantly genetically differentiated (Kenkel et al, 2013a) and exhibit divergent gene expression patterns (Kenkel et al, 2013b) suggesting that the coral host has a more prominent role in thermal adaptation in this species.…”

“…Common garden conditions were designed to reflect temperatures commonly experienced by reefs in the Florida Keys, which are known to elicit populationlevel variation in growth and bleaching (Kenkel et al, 2013a). On 28 May 2012, all parent colony halves were buoyant weighted in duplicate (Davies, 1989) and tiles were again photographed to obtain post-rearing measures of growth and survival.…”

“…These populations harbor indistinguishable Symbiodinium phylotypes but hosts are significantly genetically differentiated (Kenkel et al, 2013a) and exhibit divergent gene expression patterns (Kenkel et al, 2013b) suggesting that the coral host has a more prominent role in thermal adaptation in this species. However, it is not clear if observed thermotolerance differences in these populations are the result of heritable genetic variation or long-term acclimatization to their native habitat.…”

“…In addition, high broad-sense heritability estimates for functional traits in Symbiodinium suggests that thermal adaptation of symbionts will have an important role in coral response to climate change (Csaszar et al, 2010), although it must be noted that this heritability was measured in hospite and some of it could have been attributable to the host. Coral thermotolerance limits are affected by host genetic background (Ulstrup et al, 2006;Abrego et al, 2008;Baird et al, 2009;Kenkel et al, 2013a) and holobiont thermal history (Brown et al, 2000(Brown et al, , 2002a(Brown et al, , 2002b. Therefore, it is the adaptive potential of the coral holobiont that will truly determine the ability of coral populations to respond to climate change (Iglesias-Prieto and Trench, 1997;LaJeunesse et al, 2010).…”

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“…1). The east and west banks are Uniquely barcoded individual reads were extracted and trimmed with custom Perl scripts 187 (Data S1) to remove adaptors, barcodes and low quality reads (Kenkel et al 2013;Quigley, KM 188 'unpublished data'). All reads with lengths less than 290 base pairs were removed.…”