Abstract
Vertebrate paired appendages, such as the pectoral fins in fish and the forelimbs in tetrapods, emerge at specific regions along the anterior-posterior axis of the body. Hox genes are considered prime candidates for determining the positioning of these paired appendages during development. Nevertheless, despite extensive phenotypic analyses of numerous single and compound Hox knockout mice, no genetic studies have identified substantial defects in limb positioning, leaving questions unresolved. In a previous study, we generated seven distinct hox cluster-deficient mutants in zebrafish. Here, we provide genetic evidence that zebrafish hoxba;hoxbb cluster mutants specifically exhibit a complete absence of pectoral fins, accompanied by the absence of tbx5a expression in pectoral fin buds. In these mutants, tbx5a expression in the pectoral fin field of the lateral plate mesoderm fails to be induced at an early stage, suggesting a lack of pectoral fin precursor cells. Furthermore, the competence to respond to retinoic acid is lost in hoxba;hoxbb cluster mutants, indicating that tbx5a expression cannot be induced in the pectoral fin bud. We also identify hoxb4a, hoxb5a, and hoxb5b as pivotal genes underlying this process. Although the frameshift mutations in these hox genes do not recapitulate the absence of pectoral fins, we demonstrate that deletion mutants at these genomic loci show the absence of pectoral fins, albeit with low penetrance. Our results suggest that the positioning of zebrafish pectoral fins is cooperatively determined by hoxb4a, hoxb5a, and hoxb5b within hoxba and hoxbb clusters, which induce tbx5a expression in the restricted pectoral fin field. Our findings also provide insights into the acquisition of paired appendages in vertebrates.
Competing Interest Statement
The authors have declared no competing interest.