Fig. 6

Zebrafish alx1 mutants (blue) show reduced detectable expression of alx1 but increased expression of alx3, alx4a compared to wild‐type controls (green). alx4b expression levels are similar between wild‐type and alx1^−/− lines. Data are represented as the mean of all pooled embryos from three different clutches. The RT–qPCR relative expression values were normalized to elfa and 18S expression using the ΔΔCT method. Data from each clutch were pooled, and the mathematical mean was calculated. SEM was used to determine the standard error. To test statistical significance, an ANOVA test was performed. A P‐value < 0.05 was considered to be statistically significant. Statistical significance denoted by *; P < 0.0001 between WT zebrafish and alx1^−/− at all measured time points; at 10 ss, 24 hpf and 36 hpf for alx3; and at 24 hpf and 36 hpf for alx4a. Refer to Table EV2 for P‐values.

Dissected flatmount of zebrafish embryos injected with Alx1DN, after Alcian blue staining. The embryos developed an absence of the frontonasal‐derived median portion of the anterior neurocranium (ANC) and a profound hypoplasia of Meckel's and ventral cartilages. In the most severely affected zebrafish, a nearly abrogated ANC was observed. Scale bar: 200 μm.

Lineage tracing experiments in control and Alx1DN mutant embryos revealed aberrant migration of anterior cranial NCC when alx1 is disrupted. In the control animal, the anterior cranial NCC always migrate to contribute to the median portion of the ANC. In contrast, the anterior cranial NCC labeled in the Alx1DN animals fail to migrate to the median ANC, where the ANC structure is narrower and the labeled cranial NCC are found in an anterior and lateral ectopic location (white asterisks). Scale bar: 250 μm.