Hox gene expression during the development of the phoronid Phoronopsis harmeri

Background: Phoronida is a small group of marine worm-like suspension feeders, which together with brachiopods and bryozoans form the clade Lophophorata. Although their development is well studied on the morphological level, data regarding gene expression during this process are scarce and restricted to the analysis of relatively few transcription factors. Here we present a description of the expression patterns of Hox genes during the embryonic and larval development of the phoronid Phoronopsis harmeri. Results: We identified sequences of 8 Hox genes in the transcriptome of P. harmeri and determined their expression pattern during embryonic and larval development using whole mount in situ hybridization. We found that none of the Hox genes is expressed during embryonic development. Instead their expression is initiated in the later developmental stages, when the larval body is already formed. The Hox genes are expressed in the metasomal sac, posterior mesoderm and junction between midgut and hindgut - structures that represent rudiments of the adult worm, which emerges through the process of drastic metamorphosis. Additionally, two Hox genes are expressed in the posterior telotroch, which develops in the later larval stages. Conclusions: The lack of Hox gene expression during early development of P. harmeri indicates that the larval body develops without positional information of the Hox patterning system. Such phenomenon might be a consequence of the evolutionary intercalation of the larval form into an ancestral, direct life cycle of phoronids. Accordingly, the specific actinotrocha larva found only in Phoronida, would represent an evolutionary novelty, for which an alternative molecular mechanism of antrerior-posterior patterning was recruited. Another explanation of the observed Hox gene expression is that the actinotrocha represents a "head larva", which is composed of the most anterior body region that is devoid of Hox gene expression. This implies that the Hox patterning system is used for the positional information of the trunk rudiments and is, therefore, delayed to the later larval stages. Future investigation on head-specific genes expression is needed to test this hypothesis.