Algal symbioses with fire corals demonstrate host genotype specificity and niche adaptation at subspecies resolution

Abstract

AbstractCorals share an intimate relationship with photosynthetic dinoflagellates that contribute to the biology of the emerging metaorganism. While many coral-algal associations exhibit high host fidelity, the extent of this specificity under environmental change remains to be fully understood and is a prerequisite to forecasting the adaptive potential of this obligate symbiosis. Here, we disentangled the contribution of host genotype and environment on governing coral-algae associations by working at subspecies resolution. We used fine-scale genotyping of algal symbionts from 198 fire coral colonies (Milleporacf.platyphylla)that map to ten distinct sexually produced clonal host genotypes across three environmentally distinct reef habitats. Based on microalgal ITS2 genotyping, we show that algal-host specificity extends down to the Symbiodiniaceae subspecies level in a natural reef environment. Closely relatedSymbiodinium(A7)-dominated algal assemblages almost perfectly mapped to fire coral host genotype. Furthermore, identification of host genotype- and habitat-specificSymbiodiniumalga suggest the presence of algal phenotypic diversity even at this taxonomic resolution (i.e., withinSymbiodiniumA7), which may aid environmental niche adaptation of the metaorganism. Our results suggest that the here-identifiedMillepora-Symbiodiniumassociations are co-evolved to match their prevailing environment. Thus, despite the presence of rarer host generalistCladocopiumalgae, scope for environmentally induced modification of the cnidarian-algal association is likely constrained by host genotype.