| Introgressive hybridization is thought to be widespread and important in the evolution of reef-building corals because of high hybridization potential provided by mass spawning and suggested in laboratory crosses. Broad sympatric ranges of hybridizing reef corals provide a unique opportunity to study how hybridization, introgression, and selection shape the genetic architecture of coral species over large geographic ranges. Using multilocus sequence data, I show that two sympatric species of Caribbean staghorn coral--- Acropora cervicornis and A. palmata---hybridize and generate a morphologically diverse suite of F1 hybrids historically called A. prolifera. One-way backcrossing of hybrids with A. cervicornis allows unidirectional gene flow from A. palmata into A. cervicornis. Introgression was detected at three loci---mtDNA, Calmodulin and PaxC---but not at a MiniCollagen locus. Bayesian coalescent analyses indicated introgression was rare, and variable among genes. Mitochondrial introgression was 13 times higher than nuclear introgression. Nuclear introgression ranged from one event per five generations at PaxC to completely absent at MiniCollagen. Differential introgression demonstrates that selection limits gene flow at some genes, while permitting other parts of the genome to be porous. This suggests a genic view of coral introgression where selection acts to preserve the genetic integrity of corals across key parts of their genomes.;Genetic data also revealed strong population structure in A. cervicornis throughout the Caribbean. Introgressed alleles from A. palmata obscured this signal of population structure over broad spatial scales, but highly localized introgression signatures generated fine-scale structure among reefs separated by as little as 2km. Results suggests long-distance larval dispersal is uncommon in Caribbean staghorn corals, implying that disturbed coral reefs will require local source populations for rapid recovery. This finding is especially important for the Caribbean staghorn corals which have suffered unprecedented declines due to white band disease and widespread anthropogenic disturbance. |
