Temperature-sensitive kit allele facilitates the selective removal or restoration of kit function. Lateral views of 2.5 days postfertilization (dpf) wild-type (A) and kit^j1e99 larvae reared continuously at the permissive temperature of 25°C (C) show melanocytes that have migrated into ventral (black arrowheads) and rostral (open arrowheads) positions. In contrast, melanocytes in 2.5-dpf kit^b5 null mutants (B) and kit^j1e99 mutants reared continuously at the restrictive temperature of 33°C (D) are largely restricted to the dorsal aspect of the neural tube (black arrowheads) and the region immediately posterior to the otic vesicle (open arrowheads). Dorsal view of 12-dpf kit^j1e99 larvae reared continuously at 25°C (E) shows that melanocytes persist through late larval development, similar to 12-dpf wild-type larvae (not shown). In contrast, 12-dpf kit^j1e99 larvae reared continuously at 33°C (F) are almost entirely devoid of all melanocytes, similar to 12 dpf kit^b5 larvae (not shown; [Parichy et al., 1999]. Animals in (E) and (F) were treated with epinephrine to contract melanosomes and reveal individual melanocytes. Scale bars: 500 μm (A–D), 100 μm (E, F).

New kit alleles separate kit′s developmental roles in melanocyte migration and survival. Melanocytes in kit^j1e60 mutants largely fail to migrate to peripheral locations through 2.5 dpf (A), yet persist through 12 dpf (B) along the dorsal aspect of the neural tube (black arrowheads). In contrast, melanocytes in kit^j1e78 mutants at 2.5 dpf (C) successfully migrate to peripheral locations (black arrowheads), but subsequently disappear from the animal by 12 dpf (D). Scale bars: 1 mm (A, C); 1 mm (B, D).