expression of segment polarity genes in brachiopods supports a nonsegmental ancestral role of engrailed for bilaterians the diverse and complex developmental mechanisms of segmentation have been more thoroughly studied in arthropods vertebrates and annelidsdistantly 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 brachiopodsmarine 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 embryos in t transversa this engrailed domain is abutted by a stripe of wnt1 expression in a pattern similar to the parasegment boundaries of insectsexcept for the expression of hedgehog which is restricted to endodermal tissues of the brachiopod embryos we found that pax6 and pax258 putative regulators of engrailed also demarcate the anterior boundary in the two species indicating these genes might be involved in the anterior patterning of brachiopod larvae in a comparative phylogenetic context these findings suggest that bilaterians might share an ancestral nonsegmental domain of engrailed expression during early embryogenesis