Reef-building corals often possess high levels of intraindividual and intraspecific ribosomal DNA (rDNA) variation that is largely polyphyletic between closely related species. Polyphyletic rDNA phylogenies coupled with high intraindividual rDNA variation have been taken as evidence of introgressive hybridization in corals. Interpreting the data is problematic because the rDNA cluster evolves in a complex fashion and polyphyletic lineages can be generated by a variety of processes — such as incomplete lineage sorting and slow concerted evolution — in addition to hybridization. Using the genetically characterized Caribbean Acropora hybridization system, we evaluate how well rDNA data perform in revealing patterns of recent introgressive hybridization in contrast to genetic data from four single-copy loci. While the rDNA data are broadly consistent with the unidirectional introgression seen in other loci, we show that the phylogenetic signature of recent introgressive hybridization is obscured in the Caribbean Acropora by ancient shared rDNA lineages that predate the divergence of the species.
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