Digestive System Development and Posterior <i>Hox</i> / <i>Parahox</i> Gene Expression During Larval Life and Metamorphosis of the Phoronid <i>Phoronopsis harmeri</i>

Phoronida is a small group of marine animals, most of which are characterized by a long larval period and complex metamorphosis. As a result of metamorphosis, their body changes so much that their true anterior and posterior ends are very close to each other, and the intestine becomes long and U-shaped. Using histology and electron microscopy, we have shown that the elongation and change in shape of the digestive tract that occurs during metamorphosis in Phoronopsis harmeri larvae is accompanied by the formation of new parts and changes in ultrastructure. At the same time, our in situ hybridization data suggest that the posterior markers Cdx and Post2 are expressed in posterior tissues at larval stages, during metamorphosis, and in juveniles, and that changes in their expression correlate with remodeling of the posterior parts of the digestive tract. Our data may shed light on the evolution of body patterning in animals undergoing complex metamorphosis.