Origin and impacts of ploidy variation in the coral Pocillopora acuta in Hawai'i

Differences in ploidy impact fitness, shaping adaptation and evolutionary trajectories. Although polyploidy has been documented in corals, the mechanisms that give rise to polyploidy and the phenotypic consequences of this variation are unknown. We established a model population of 326 Pocillopora acuta colonies in Kāne'ohe Bay, Hawai'i, and used RAD sequencing of these samples and historical collections to define clonal lineages, ploidy state, and mechanistic origin of polyploidy in each colony. We also measured growth, morphometrics, thermal tolerance, and fecundity in common-gardened corals. Here, we show that triploidy in P. acuta likely originated via allopolyploidy via hybridization between P. acuta and its sister species, P. damicornis. Triploids were present in this population before and after the 2014/2015 bleaching events, becoming the dominant ploidy on contemporary reefs (>60% of genotypes). These bleaching events did not select for triploid genotypes; however, relative triploid abundance may have increased in part due to higher growth rates compared to diploids. Both ploidies released larvae on the same lunar cycle and there was no difference in photosynthetic efficiency during heat stress, but triploid colonies experienced greater mortality at lower levels of stress, suggesting a host-derived fitness consequence influenced by ploidy state. This work highlights a cryptic source of variance in coral fitness, with important implications for understanding their ecology and evolution.