Independent size regulation of bones and appendages in zebrafish

Vertebrate appendages vary in length among species, despite a low divergence in bone configuration. In this study, we assessed whether the total sizes of anatomical features are constrained by the lengths of individual bones during development. We examined size control by expressing mutant genes related to allometric growth in zebrafish fins under cell type-specific promoters. Hyperactive potassium channel (kcnk5b ^W169L ) or dominant-negative gap junction protein connexin (Cx43 ^T154A ) expression in epidermal cells increased or decreased fin size, respectively, but did not influence the lengths of fin bone segments. Osteoblast expression of these mutant genes altered fin bone segment length but not total fin size. The combination of kcnk5b ^W169L in epidermal cells and Cx43 ^T154A in osteoblasts resulted in transgenic fish with large fins and short bone segments, and vice versa. These results clearly indicate that fin size and bone segment length are determined separately by independent regulatory systems, despite the use of the same genes. These findings shed new light on the evolution of allometric traits.