Expression of segment polarity genes in brachiopods supports a non-segmental ancestral role of engrailed for bilaterians

The diverse and complex developmental mechanisms of segmentation have been more thoroughly studied in arthropods, vertebrates and annelids--distantly related animals considered to be segmented. Far less is known about the role of \"segmentation genes\" in organisms that lack a segmented body. Here we investigate the expression of the arthropod segment polarity genes engrailed, wnt1 and hedgehog in the development of brachiopods--marine invertebrates without a subdivided trunk but closely related to the segmented annelids. We found that a stripe of engrailed expression demarcates the ectodermal boundary that delimits the anterior region of Terebratalia transversa and Novocrania anomala embryo