Expression patterns of hh (A-C), ptc (D-F) and Gli (G-I) orthologues in Platynereis late trochophores (48 hpf), early three-segmented larvae (72 hpf) and during posterior growth. All views are ventral except I, lateral right (Lat). Anterior or apical is up. Black dashed line: prototroch of larvae. The stomodeum is delimitated by a red dashed circle. Larval segment anlagen are numbered 1-3. Posterior growth panels (post. gr., C, F, I) show patterns in posterior regenerates one week after caudal amputation, in front of the thin segment addition zone (yellow dashed line). Scale bars represent 50 μm. py: pygidium.

Effects of mid-trochophore stage cyclopamine treatments on the development of Platynereis larvae. (A-D) 3-day larvae stained with an anti-acetylated tubulin antibody revealing the ciliary belts (trochae) (A, B) and the axonal scaffold of the nervous system (C, D). (A, B) are mid-body frontal confocal sections of Hoechst 33342 stained larvae whereas (C, D) are ventral frontal confocal sections. (E-L) Effects of cyclopamine treatment on the expressions of segment polarity–like genes and Pdu-Cdx, a pygidial ectoderm marker. The first and second columns show expression in untreated larvae and in the majority of treated larvae, respectively. Pie graphs show the frequencies of abnormal expressions with normal in blue, abnormal in red and the maternal batch variability in violet where available. Small fractions of abnormal expressions in the untreated control reflect both a small proportion of spontaneous abnormal development and the probe-dependent imperfect sensitiveness of the WMISH protocol. Prototrochs: dashed black line. Stomodeum: white dashed circle and “s”. p: prototroch. 1, 2, 3: larval segments in A, B, segmental trochae in C, D. Yellow arrows: segmental grooves. Red asteriks: main lateral nerve roots. n: protonephridium. h: head lobes. Red arrows, blue asterisks and green asterisks indicate persistent mesodermal, stomodeal or pygidial/proctodeal expressions, respectively.

(A) Comparison of the antero-posterior extension of striped expression patterns of proven and presumed segment polarity orthologous genes in Platynereis and Drosophila. Segment boundaries and limb anlagen positions are represented with a schematic body wall section in black. Orthologous genes are color-coded whereas segment polarity-like genes specific to Platynereis are represented by gray rectangles. The morphogenetic actions of the diffusible Hh proteins are represented with blue arrows. A white arrow with question mark indicates that Hh action on the anterior parts of annelid segments is not proven in this study. Unlike in fly, Pdu-ptc is expressed in en/hh positive cells and Pdu-Gli expression does not abut en/hh positive cells in Platynereis. (B) Comparison of expression patterns of some of these segment polarity-like genes in the early stages of appendage formation. The homology of annelid segments with arthropod parasegments implies that annelid parapodia do not correspond in embryonic position with the arthropod appendages, suggesting that these appendages do not share a common origin in the protostome ancestor. PSB: parasegmental boundary.