Expression of the engrailed Homologue in Larvae and Juveniles of the Annelid Alitta virens Characterizes the Formation of Segments from the Growth Zone

The evolutionary origin of segmentation remains a mystery. In arthropods, gene engrailed is recognized as one of the most important and conservative members of the segmentation developmental program. Orthologues of this gene have been identified in annelids, but their role is interpreted as contradictory, because their expression in some species precedes subdivision of the body into segments but it does not in others. The expression of engrailed in the nereid polychaete Alitta virens during metamorphosis and development of the first postlarval segments was studied herein. Our data support the possible involvement of this gene in the process of segment formation from the growth zone in A. virens . At the larval stages, engrailed is expressed in neuroectodermal cells, in the growth zone, and in metameric epidermal cell rows at the anterior boundary of each segment. Upon transition from the metatrochophore to the nectochaete stage, the circular expression domain in the growth zone expands and then resolves into two serial domains. Over time, the distance between these circular domains increases, indicating the growth of the first postlarval segment anlage. Formation of subsequent postlarval segments occurs in a similar way. Analyzing our results and literature data, we compared engrailed expression patterns in annelids and arthropods. Our work indicates an absence of conservation in patterning of sequentially developing segments from the growth zone in protostomes. We suggest that the anteroposterior axis elongation in A. virens occurs simultaneously with the specification of a new segment. These features differ from the known models of the growth zone and indicate the possibility that nereids have a specific mechanism of segmentation.