Comparison of Hox gene expression patterns across annelids. Generalized diagram comparing Hox gene expression along the main body axis of Alitta virens with the Hox expression in Capitella, Chaetopterus, and Helobdella. Body axes are shown as boxed diagrams next to the species names and include segments and tagmata for each species. Black vertical lines mark the boundary between larval and postlarval segments for Alitta virens, red vertical lines mark tagmata borders for other species. For Alitta and Capitella, expression is shown at late larval stages and for juvenile animals. Solid color bars indicate uniform expression level, while gradient color bars indicate gradient expression. Larval Nvi-Hox gene expression patterns have stable colinear expression boundaries like other polychaetes Hox genes. During postlarval development, most Nvi-Hox genes have gradient expression patterns. During worm growth, anterior boundaries of Nvi-Hox7, Nvi-Lox4, Nvi-Lox2, and Nvi-Post2 shift from the fourth (first postlarval) segment toward the posterior end, while the posterior boundaries of Nvi-Hox1 and Nvi-Hox5 move toward the anterior end. CapI-Hox genes have stable expression boundaries corresponding to the morphological tagmata boundaries, but expression of many Hox genes is not uniform and forms a gradient. Abbreviations: GZ, growth zone; Pe, peristome; Pr, prostomium. Taxon-specific abbreviations: Capitella: A, abdominal segments; T, thoracic segments; Chaetopterus: A, B, C, segments of the body regions A, B, and C, respectively; Helobdella: C, caudal segments; M, medial segments; R, rostral segments. See text for details.