Genetic variation in responses to a settlement cue and elevated temperature in the reef-building coral Acropora millepora

Abstract

Reef-building corals are threatened by increasing sea surface temperatures resulting from global climate change. Whether corals can adapt to increasing temperatures over the course of generations will depend in part on heritable variation in thermal physiology and dispersal potential, which may serve as the raw material for natural selection. To investigate whether such variation exists in coral populations, and build a framework for identifying the coral-specific genetic factors involved, we performed controlled crosses between 3 genetically distinct colonies of the branching coral Acropora millepora. We compared the families of larvae (which in this species naturally lack symbionts) for several physiological traits, and observed between-family differences in nearly every case. Using larvae cultured at standard and elevated temperatures, we measured the developmental decrease in protein content and the expression of candidate heat response genes. We used an in vivo assay for mitochondrial enzyme activity to evaluate the metabolic response to temperature changes in individual larvae. We also compared the responsiveness of larvae from different families to a nat- ural settlement cue to gain insights into long-range dispersal potential. Partitioning the components of total phenotypic variance confirmed the existence of additive genetic effects for settlement rates and βγ-crystallin expression, while variance in mitochondrial Q10 and the expression of actin and Hsp16 were driven by non-additive effects. The phenotypic variance observed among the small num- ber of families analyzed here suggests the existence of considerable heritable variation in natural coral populations, which supports the possibility of effective adaptive responses to climate change.