(A) Schematic of mef2d exonic structure, mutant allele used in this study, and regions encoding proposed functional domains are annotated. (B) Zebrafish heterozygous for both mef2ca b1086 and mef2d were pairwise intercrossed. 6 days post fertilization (dpf) larvae were stained with Alcian blue and Alizarin red to label cartilage and bone. Stained larvae were genotyped, flat mounted, and imaged. The following craniofacial skeletal elements are indicated in a wild-type individual: opercle bone (op), branchiostegal ray (br), Meckel’s (mc), ceratohyal (ch), symplectic (sy) cartilages, interhyal (ih), and jaw (jj) joints. Indicated phenotypes associated with mef2ca mutants include: ectopic bone (arrowheads), interhyal and jaw-joint fusions (^), dysmorphic ch (arrows), reduced mc (double arrowhead), and a shortened sy (red arrows). Dashed outline indicates symplectic cartilage. Scale bar: 50 μm (C) The penetrance of mef2ca mutant-associated phenotypes observed in 6 dpf larvae is indicated. Asterisk denotes significant difference in penetrance between the indicated genotypes by Fishers exact test. (D) Symplectic cartilage length was measured from 6 dpf larvae from the indicated genotypes, and asterisk indicates significant differences in symplectic length (*≤0.05, **≤0.01, and ***≤0.001). (E) Table listing F-test values testing for significant differences in variation between genotypes. For box and whisker plots, the box extends from the 25th to 75th percentiles. The line in the middle of the box is plotted at the median, and the bars are minimum and maximum values. N’s for all analyses are indicated in C.