Decrypting corals: Does regulatory evolution underlie environmental specialization of coral cryptic lineages?

Abstract

A recent study has shown that two common Caribbean corals,Montastrea cavernosaandSiderastrea siderea, in the Florida Keys each consist of four genetically distinct lineages. These lineages are strongly specialized to a certain depth and, to a lesser extent, to nearshore or offshore habitat. We hypothesized that the lineages' environmental specialization is at least in part due to regulatory evolution, which would be manifested as the emergence of groups of coregulated genes ("modules") demonstrating lineage-specific responses to different reef environments. Our hypothesis also predicted that genes belonging to such modules would show greater genetic divergence between lineages than other genes. Contrary to these expectations, responses of cryptic lineages to natural environmental variation were essentially the same at the genome-wide gene coexpression network level. Moreover, none of the identified coregulated gene modules exhibit elevated between-lineage divergence. The environmental specialization of cryptic lineages must, therefore, come from relatively subtle adjustments to gene function or regulation that are not detectable at the gene network level and/or involve other constituents of the coral holobiont rather than the coral host.