fgfr1a, fgfr1b, and fgfr2 function redundantly to regulate pectoral fin development. fgfr1a, fgfr1b, and fgfr2 function redundantly to regulate pectoral fin development. (A) Model of pectoral fin bud development during pectoral fin bud Induction (top) and Outgrowth (bottom). Underlines denote genes assayed in (C–H). Arrows denote an epistatic (but not direct) link between molecules. Asterisk signifies that Fgf8 has not yet been shown to play this role in zebrafish, but is hypothesized from forelimb work in chick and mouse. (B) Stacked column chart depicting the average number of pectoral fins per animal at 5 dpf, according to genotype. Sample size for each genotype is listed at the top of each bar. Representative images of larvae with 2, 1, or 0 pectoral fins to the right: dorsal views, anterior to the left, with arrowheads denoting pectoral fins where present. (C–H) Fin bud marker analysis of fgfr double and triple mutant embryos at the 18-somite stage (tbx5a, C), 24 hpf (fgf24, D; fgf10a, E), and 44 hpf (fgf24, F; fgf8a, G; dlx2a, H). Whole mount in situ hybridization was performed, embryos were scored for expression, and genotypes were determined post hoc. In each panel, the percentage of embryos expressing particular levels of each marker gene is represented in a stacked column chart on the left, and representative images of those expression levels are shown for each marker to the right (dorsal views, anterior up; developing fin buds are seen as two spots on either side of the embryo, denoted by arrowheads). Bars: in (B), 200 μm; in (C), 50 μm for (C–H). Dena M. Leerberg et al. Genetics 2019;212:1301-1319 Copyright © 2019 by the Genetics Society of America