Metameric and posterior expression of Wnt genes in protostomes. Expression of Wnt genes is illustrated with respect to the parasegmental and segmental boundaries of arthropods and annelids respectively, and the SAZ/Pg (boundaries are represented by dashed vertical lines). Anterior is to the left. Expression in the ventral part of the segment and SAZ/Pg is shown as filled boxes and expression in the dorsal represented by hatched boxes. Note that the metameric dorsal expression of Gm-Wnt7 and Gm-Wnt16 is restricted to presumptive heart tissue. SAZ, segment addition zone; Pg, pygidium; NA, not applicable; Dm, Drosophila melanogaster; At, Achaearanea tepidariorum; Cs, Cupiennius salei, Gm, Glomeris marginata; Pd, Platynereis dumerilii; Tc, Tribolium castaneum.