Abstract

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. I investigated 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. I found that a stripe of engrailed expression demarcates the brachiopod embryonic head/trunk boundary, and that at least in one species, this engrailed stripe is abutted by the expression of wnt1, in a pattern similar to the parasegmental boundaries of insects. Thus, the adjacent expression of engrailed/wnt1 is not exclusive of arthropod and annelid segments, but also occurs in a non-segmental boundary of larval brachiopods. When broadening the analysis to all bilaterians, I found that in several lineages, including the segmented arthropods and annelids, engrailed is first expressed at the head/trunk distinction during embryogenesis, much like in brachiopods. This suggests that bilaterians share a non-segmental domain of engrailed expression during early embryogenesis, and that such pattern might be ancestral to the group. In a comparative phylogenetic context, these findings support that engrailed has an ancient axial patterning role—perhaps related to the establishment of the head/trunk boundary—and was secondarily recruited to the segmental boundaries of arthropods and annelids.