Expression patterns of seven Wnt genes during posterior segment addition in the annelid Platynereis. All panels show series of segments produced 8 days after caudal amputation and regeneration; vent, dors, lat: ventral, dorsal and lateral views respectively. Sag and front: sagittal and frontal sections (optical or tissue) respectively. The SAZ is highlighted with a yellow dashed line in all micrographs. The pygidium is located below or right of the SAZ line, depending on the view. In (g), (j), (l), (n), arrows indicate the position of segmental grooves. (a), (b) Expression of Wnt1 in the posterior part of forming segments and parapodia (a), in the hindgut (hollow arrowheads) and in the ectoderm of the pygidium (black arrowhead). (c), (d) Expression of Wnt10 in the posterior part of forming segments and parapodia, as well as in the hindgut (hollow arrowheads). (e)-(g) Expression of Wnt11 in the posterior part of forming segments and parapodia, in a pair of cells of the ganglia of the ventral nerve cord (black arrowhead) and in the ectoderm at the base of the pygidial cirri (hollow arrowheads). (g)-(k) Expression of Wnt16 in the posterior part of forming segments but not in parapodia. (k) Expression of Wnt16 in the mesoderm of the pygidium (red arrowhead) but not in the ectoderm (black arrowheads). (l) Expression of Wnt4 in the ventral midline of forming segments (black arrow), in the ventral part of the SAZ and pygidium (hollow arrowhead) and in the anterior part of forming segments. (m)-(o) Expression of Wnt5 in stripes in the anterior part of forming segments and in a complex pattern in the forming parapodia. (n) Is a close up view of (m) at the level of the ventral ectoderm, showing the location of Wnt5 stripes posterior to the segmental grooves. (o) Shows the weaker dorsal stripes of Wnt5 that do not reach the dorsal midline, unlike Wnt10, Wnt11 and Wnt16. Black arrowheads show the forming parapodia. (p) Expression of Wnt7 in the hindgut (hollow black arrowhead) and broadly in the segmental mesoderm (red arrowheads). Patterns are recapitulated schematically in the lower part of the figure. All schemes are ventral views. A brown dashed line represent the SAZ. For the purpose of clarity, the expression of Wnt5 and Wnt7 in the mesoderm of forming segments has been omitted.