A requirement for kit in embryonic zebrafish melanocyte differentiation is revealed by melanoblast delay

Abstract

Exploring differences in gene requirements between species can allow us to delineate basic developmental mechanisms, provide insight into patterns of evolution, and explain heterochronic differences in developmental processes. One example of differences in gene requirements between zebrafish and mammals is the requirement of the kit receptor tyrosine kinase in melanocyte development. kit is required for migration, survival and differentiation of all neural crest-derived melanocytes in mammals. In contrast, zebrafish kit is not required for differentiation of embryonic melanocytes during normal development. When melanoblast development in zebrafish embryos is delayed by injecting morpholinos targeted to the mitfa gene, we show that these delayed melanoblasts fail to differentiate in kit mutants. Thus, we show that there is a kit requirement for melanocyte differentiation in zebrafish when melanoblast development is delayed. Furthermore, we show that kit is not involved in maintaining melanocyte precursors through the developmental delay, but instead is required for differentiation of melanocytes after the block on their development is removed. Finally, we suggest there is a heterochronic shift in the onset of melanocyte differentiation between fish and mouse, and developmental delay of melanoblast development in zebrafish removes this heterochronic difference.

On the embryonic origin of adult melanophores: the role of ErbB and Kit signalling in establishing melanophore stem cells in zebrafish

“…Strikingly, in subsequent stages, larval melanophore pigmentation recovers and by 5 dpf the larval melanophore pattern is completely restored (Mellgren and Johnson, 2004) (Fig. 3A-C).…”

“…mitfa is expressed in melanoblasts at all stages of migration and differentiation (Lister et al, 1999). Interestingly, mitfa morphants, although completely deficient in early larval stripe formation, do recover and adopt normal larval and adult pigmentation (Mellgren and Johnson, 2004). This finding and evidence from temperature-sensitive mitfa alleles indicate that mitfa is not required for establishing or maintaining melanophore stem cells .…”

“…To investigate whether these newly formed melanophores originated from the putative DRG-associated stem cells, we depleted the directly differentiating melanophores with a mitfamorpholino. (Mellgren and Johnson, 2004) and traced melanophore renewal by 4D in vivo imaging in the Tg(mitfa:gfp) line.…”

“…As the NC is no longer present, these cells must arise from hypothetical NC-derived stem cells that are set aside in the embryo (Rawls et al, 2001;Parichy et al, 2003;Yang and Johnson, 2006;Budi et al, 2011). These cells are quiescent during embryonic development but start to proliferate during juvenile development (Quigley et al, 2004;Mellgren and Johnson, 2004;Hultman et al, 2009). …”

Evolution of danio pigment pattern development

“…Intriguingly, null alleles of mouse Kit and Ednrb completely lack melanocytes and have pleiotropic effects on other cell types (Besmer et al, 1993;Shin et al, 1999;Pla and Larue, 2003;Wehrle-Haller, 2003), whereas in D. rerio only embryonic and early appearing metamorphic melanophores require kit, only late appearing metamorphic melanophores require ednrb1, and even null alleles of these genes lack gross pleiotropic defects in other traits. These observations identify fertile ground for investigating how gene functions and pleiotropy itself evolve, particularly as simple gene duplication and partitioning of ancestral functions (Force et al, 1999) seem unlikely to explain genetic requirements for these particular loci (Mellgren and Johnson, 2004;Mellgren and Johnson, 2005). Double mutant analyses have revealed additional genes required by the kit-independent late metamorphic melanophores in D. rerio.…”