Wnt family genes in the cephalochordate amphioxus: Structure, phylogenetic analysis, and developmental expression

Amphioxus, as the closest living invertebrate relative to vertebrates, is the best available stand-in for the proximate invertebrate ancestor of vertebrates. Amphioxus is vertebrate-like but much simpler, both in its body plan and genome, which has not undergone the extensive gene duplications that occurred early during vertebrate evolution. My research focused on the diversification of the Wnt gene family in the course of vertebrate evolution. My goals were to: (1) isolate as many Wnt genes as possible from amphioxus, (2) understand their phylogenetic relationship within the Wnt gene family, and (3) compare developmental expression of Wnt genes between amphioxus and other animals. I obtained full-length clones of AmphiWnt1, 3, 4, 5, 7, 8, and 11, most of AmphiWnt6, and a small fragment of AmphiWnt10, the largest number of Wnt genes currently known for any invertebrate. Developmental expression patterns indicate that, unlike the related wingless gene of Drosophila, none of the amphioxus Wnt genes interacts with engrailed to pre-pattern the rostrocaudal axis into segments. In contrast, the exclusively posterior expression of AmphiWnt1 appears comparable to Drosophila wingless expression in the invaginating hindgut primordium. AmphiWnt1 has no counterpart to the neural expression of vertebrate Wnt1, which evidently represents a vertebrate innovation. In spite of such differences between amphioxus and vertebrates (especially in the Wnt4 subfamily), most of the developmental expression patterns indicate that amphioxus and vertebrate Wnt genes play many conserved roles in neurogenesis, myogenesis, tail outgrowth, and patterning of the anteroposterior and dorsoventral body axes. At the posterior end of the amphioxus larva, overlapping expression of Wnt genes is especially conspicuous and suggests that a combinatorial code of gene transcription patterns the tail bud into a mosaic of sub-regions. Eight of the nine amphioxus Wnt sequences are included in a phylogenetic analysis indicating that the Wnt3 subfamily is basal for animals and that the canonical and non-canonical Wnt signaling pathways are, respectively, primitive and derived.