Phylogenetics and morphological evolution of Scleractinian corals
Barbeitos, Marcos Soares
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Scleractinian corals are modular organisms of great ecological and economic importance that may have solitary or colonial growth forms. In spite of the rich fossil record, the evolutionary history of the group is poorly understood and its taxonomy is highly artificial because many of the features commonly employed in systematic studies of the group exhibit large amounts of plasticity. Morphological variation in corals is so large that it hinders the application of traditional morphometric techniques, which normally depend on assessment of homologous landmarks. I developed a homology-free metric based on information theory that captures a specific but biologically relevant aspect of colonial complexity. The results suggest that total complexity in corals can be partitioned between modular and supra-modular levels of morphological organization, which likely respond independently to selective pressures, and also that colonial morphogenesis displays a surprisingly high level of developmental integration. The distinction between solitary and colonial species is not only morphological but also taxonomic, since evolution of coloniality is thought to have proceeded in step with evolution of coral reefs while solitary species are thought to have evolved in non-reef environments. I conducted comprehensive phylogenetic reconstructions of the group using partial sequences from a nuclear and a mitochondrial marker (rRNA genes 12S and 28S, respectively). Bayesian and parsimony analyses recovered a monophyletic Scleractinia clade, rejecting the "naked coral hypothesis" and also recovered well supported clades made up by combinations of solitary and colonial species as well as species that occur in opposite sides of the Atlantic. The large morphological disparity between colonial and solitary species within clades contrasts with their small genetic distances. I hypothesize that this pattern can be explained by loss of coloniality via heterochronic processes, which allow for substantial morphological change with minimal genetic reprogramming and that such losses may have been adaptive or exaptive in the context of global change. This hypothesis was corroborated by Bayesian reconstruction of ancestral character states. These results suggest that evolution of coloniality in corals may have been much more dynamic than previously suspected. The link between reef and non-reef coral species points towards the need for more extensive taxonomic sampling of the latter in future phylogenetic reconstructions of the order.