Genetic dissection of kit-dependent melanocyte development in zebrafish, Danio rerio

Vertebrate melanocyte development serves as a classic model for the study of cell migration and survival. However, the findings that adult melanocytes in mice and fish, such as those that re-establish the melanocyte stripes of the zebrafish (Danio rerio) during fin regeneration, arise from stem cells present new opportunities to study stem cell biology. Using melanocyte regeneration in the zebrafish fin as model to investigate the mechanisms underlying melanocyte stem cell development, I showed that zebrafish mutant for the kit receptor tyrosine kinase (sparse) are defective for melanocyte regeneration. My studies also revealed a secondary class of regeneration melanocytes that regulates its activity in the absence of kit function to reconstitute the stripes during late stages of fin regeneration. Whether adult phenotypes in kit mutants such as defects in melanocyte regeneration are the consequence of embryonic roles in melanocyte stem cell establishment, or instead reflect roles for kit during adult stages was unclear. To distinguish between these possibilities, I generated a temperature-sensitive allele of kit and used it to show that kit is required during fin regeneration to promote the population of the regenerate by melanoblasts, rather than during embryonic stages to establish melanocyte stem cells. My work further revealed that regeneration melanocytes transiently require kit for their survival following overt differentiation. During embryonic development, kit mutant melanocytes in zebrafish, or their precursors in mammals, are unable to migrate and subsequently undergo programmed cell death. However, the specific requirements for kit in embryonic melanocyte migration and survival remained unresolved. I used a temperature-sensitive kit allele to show that kit is required for melanocyte migration during early stages of larval development. Further, I showed that melanocytes developing in the absence kit function progressively commit to cell death, and that initial migration of melanocytes is not sufficient for subsequent survival following removal of kit function. In support of independent roles for kit in melanocyte migration and survival, I identified alleles of kit that disrupt either migration or survival. The lesions encoded by these alleles reveal relationships between the structure of the kit gene and its separable developmental roles.