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Animals are both radially and bilaterally symmetrical: Accommodating seemingly mutually exclusive paradigms

Animal symmetry is a fundamental issue in biology. The two main symmetries that characterize the animal world are radial and bilateral symmetry. The term radial symmetry indicates that the animal is symmetrical around its longitudinal axis. This is the anteroposterior axis of polarity in most animals, and the oral-aboral axis in Cnidarians and Ctenophores. The members of this latter phylum show biradial symmetry with two planes of rotational symmetry. Bilateral symmetry is created when a second axis – the dorsoventral axis – also forms, perpendicular to the anteroposterior axis. The process is due to the asymmetrical expression of developmental genes along this axis. The phylogenetic relationship between the two symmetries has long been a major question in biology. According to the classical view, bilateral symmetry arose from radial symmetry (e.g. [1]). This view has become largely outdated. For example, in the case of Cnidaria – animals traditionally grouped with Radiata – it is known that the radial body, which also contains bilaterally symmetrical elements, is organized by a complex genome including several bilaterian gene orthologs (e.g. [2, 3]). For this reason, it has been proposed that Cnidarians be considered originally bilaterally symmetrical animals [4, 5]. What is more, it also has been ques-