| In a screen for genes differentially expressed during fin regeneration in adult zebrafish, two novel genes, 2-H06 and 2-F11, belonging to the same family coding for secreted proteins with unknown function, were identified. Besides their expression during fin regeneration, they are specifically expressed in the embryonic fins of zebrafish. During development, their expression is restricted to the developing fin folds of both paired and unpaired fins. Searches of the current genomic databases did not reveal any orthologs of 2-H06 or 2-F11 genes in tetrapods, plants and invertebrates species. However, orthologs of the genes are found in other teleost fish like the puffer fish Takifugu rubripes (fugu), Tetraodon nigroviridis and the Medaka fish (Oryzias latipes), and in the cartilaginous fish, elephant shark (Callorhinchus milii). In zebrafish embryos, around hatching time, the first component of the fin exoskeleton is seen. It is made of two layers of collagenous fibrils called actinotrichia. These fibrils are only present in fish fins and their appearance correlates with the onset of the developmental divergence between fish paired fin buds and higher vertebrate limb buds. Functional analysis was performed using the gene "knock-down" approach by injection of morpholino oligonucleotides in 1-cell-stage zebrafish embryos. It revealed no significant defects when the morpholino of each gene was injected individually but co-injection of both of the morpholinos induced a complete absence of the actinotrichia, which led to the fin fold losing its structural integrity. Moreover, rescue analysis performed by co-injecting single stranded mRNA (2-H06 or 2-F11) with the two morpholinos in embryos confirmed that the absence of actinotrichia was a defect specific to the knockdown of the 2-H06 and 2-F11 genes. The results of this study suggest that these two genes encode structural proteins of the actinotrichia and loss of this gene family in species other than fish may be one of the causes or consequences of the evolutionary transition from fins to limbs. |
