Turning Hox “signatures” into synapomorphies

SUMMARY It has recently been shown that the three metazoan superphyla that are recognized on the basis of 18S rDNA phylogenies—ecdysozoans, lophotrochozoans, and deuterostomes—each have characteristic Hox genes. This observation has been taken further, and these “signature” Hox genes have been looked for in taxa of uncertain affinity such as the mesozoa, in order to link them to one of the three superphyla. Here I point out that, in the absence of an out‐group, these so‐called signature Hox genes are unpolarized characters and, as such, should not be used in this cladistic sense to determine phylogeny. Taking the example of the mesozoans, which have the Lox5 gene in common with the lophotrochozoans, I show that it is possible to polarize this character using paralogous Hox genes as proxy out‐groups; however, due to the impossibility of reliable alignment outside the homeobox, only two residues of the Lox5 peptide are susceptible to this method. With this in mind, I find slim evidence for an association between mesozoans and lophotrochozoans. I demonstrate that the lophotrochozoan genes Lox2 and Lox4 would provide many more reliable residues that are truly indicative of lophotrochozoan affinity. Finally, I point out the potential problems in using unpolarized signatures to address the question of the position of the acoel flatworms.