Wellcome Trust Sanger Institute, Hinxton, United Kingdom; European Molecular Biology Laboratories-European Bioinformatics Institute, Hinxton, United Kingdom
Harald Gruber-Vodicka
Max Planck Institute for Marine Microbiology, Bremen, Germany
Michael G Hadfield
Kewalo Marine Laboratory, Pacific Biosciences Research Center and the University of Hawaii-Manoa, Honolulu, United States
Vicki B Pearse
Institute of Marine Sciences, University of California, Santa Cruz, United States
Ana Riesgo
Invertebrate Division, Life Sciences Department, The Natural History Museum, London, United Kingdom
Wellcome Trust Sanger Institute, Hinxton, United Kingdom; European Molecular Biology Laboratories-European Bioinformatics Institute, Hinxton, United Kingdom; Cancer Research UK Cambridge Institute, University of Cambridge, Cambridge, United Kingdom
Gonzalo Giribet
Museum of Comparative Zoology, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, United States
The phylogenetic placement of the morphologically simple placozoans is crucial to understanding the evolution of complex animal traits. Here, we examine the influence of adding new genomes from placozoans to a large dataset designed to study the deepest splits in the animal phylogeny. Using site-heterogeneous substitution models, we show that it is possible to obtain strong support, in both amino acid and reduced-alphabet matrices, for either a sister-group relationship between Cnidaria and Placozoa, or for Cnidaria and Bilateria as seen in most published work to date, depending on the orthologues selected to construct the matrix. We demonstrate that a majority of genes show evidence of compositional heterogeneity, and that support for the Cnidaria + Bilateria clade can be assigned to this source of systematic error. In interpreting these results, we caution against a peremptory reading of placozoans as secondarily reduced forms of little relevance to broader discussions of early animal evolution.
