Evidence of sweepstakes reproductive success in a broadcast-spawning coral and its implications for coral metapopulation persistence.

Processes governing genetic diversity and adaptive potential in reef-building corals are of interest both for fundamental evolutionary biology and for reef conservation. Here, we investigated the possibility of "sweepstakes reproductive success" (SRS) in a broadcast spawning coral, Acropora hyacinthus, at Yap Island, Micronesia. SRS is an extreme yearly variation in the number of surviving offspring among parents. It is predicted to generate genetically differentiated, low-genetic-diversity recruit cohorts, containing close kin individuals. We have tested these predictions by comparing genetic composition of size classes (adults and juveniles) at several sites on the island of Yap. We did see the genome-wide dip in genetic diversity in juveniles compared to adults at two of the four sites; however, both adults and juveniles varied in genetic diversity across sites, and there was no detectable genetic structure among juveniles, which does not conform to the classical SRS scenario. Yet, we have identified a pair of juvenile siblings at the site where juveniles had the lowest genetic diversity compared to adults, an observation that is hard to explain without invoking SRS. While further support for SRS is needed to fully settle the issue, we show that incorporating SRS into the Indo-West Pacific coral metapopulation adaptation model had surprisingly little effect on mean rates of coral cover decline during warming. Still, SRS notably increases year-to-year variation in coral cover throughout the simulation.