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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  embryos. In  T. transversa , this  engrailed  domain is abutted by a stripe of  wnt1  expression in a pattern similar to the parasegment boundaries of insects—except for the expression of  hedgehog , which is restricted to endodermal tissues of the brachiopod embryos. We found that  pax6  and  pax2/5/8 , 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, non-segmental domain of  engrailed  expression during early embryogenesis.

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