Canonical Wnt/β-Catenin Signaling Promotes Pelvic Fin Bud Initiation in Zebrafish

In vertebrates, anterior (forelimbs and pectoral fins) and posterior (hindlimbs and pelvic fins) paired appendages arise from specific regions of the lateral plate mesoderm. In zebrafish, pectoral fin development illustrates a tightly coupled process in which fin field regionalization and subsequent bud initiation occur in close succession. Regionalization is governed by TBX5, which also promotes bud outgrowth. Canonical Wnt/β-catenin signaling is involved in these processes. In contrast, pelvic fin development is temporally dissociated: the presumptive pelvic fin field is regionalized during early embryogenesis, whereas fin bud initiation does not occur until metamorphosis, approximately 3 weeks later. This separation provides a unique opportunity to examine the distinct roles of Wnt/β-catenin signaling in early field regionalization and later bud initiation. Here, we show that canonical Wnt/β-catenin signaling is inactive during early pelvic fin field regionalization but is essential for fin bud initiation. Wnt/β-catenin activity, represented as GFP signals in Tg(Tcf/Lef-miniP:dGFP), was undetectable in the pelvic region during early embryogenesis but became evident in both the epithelium and mesenchyme of the pelvic fin bud during metamorphosis. Notably, expression of wnt8a, which activates this pathway in the early intermediate mesoderm, was not detected in the pelvic fin region at the time of bud formation. Pharmacological inhibition of Wnt/β-catenin signaling at metamorphic stages impaired pelvic fin bud outgrowth. These findings suggest that canonical Wnt/β-catenin signaling may not be involved in the early regionalization of the pelvic fin field but is essential for the later initiation of pelvic fin bud formation in zebrafish.