Gene Family Complexity and Expression Divergence as a Mechanism of Adaptation In Coral

Gene family complexity and its influence on expression dynamics has long been theorized to be an important source of adaptation in natural systems through providing novel genetic material and influencing gene dosage. There is now growing empirical support for this theory; however, this process has only been demonstrated in a limited number of systems typically using recently diverged species or populations. In particular, examples of how this process operates in basal animals with deeper species splits has not been well explored. To address this issue, we investigated the evolution of gene family complexity in five species of common Caribbean coral. We demonstrate widespread divergence in gene repertoires owing to slow rates of gene turnover occurring along deep species splits. The resulting differences in gene family complexity involve numerous biologic processes, shedding light on to the selective forces that have influenced the evolution of each species. By coupling these findings with gene expression data, we show that increased gene family complexity promotes increased expression divergence between species, indicating an interplay between gene family complexity and expression divergence. Finally, we show that immune genes are evolving particularly fast demonstrating the importance of interactions with other organisms in the evolutionary history of Caribbean corals. Overall, these findings provide support for gene copy number change as an important evolutionary force in Caribbean corals, which may influence their ability to persist in a rapidly changing environment.