Agouti and BMP signaling drive a naturally occurring fate conversion of melanophores to leucophores in zebrafish

Abstract

The often-prominent pigment patterns of vertebrates are varied in form and function and depend on several types of pigment cells derived from embryonic neural crest or latent stem cells of neural crest origin. These cells and the patterns they produce have been useful for uncovering features of differentiation and morphogenesis that underlie adult phenotypes, and they offer opportunities to discover how patterns and the cell types themselves have diversified. In zebrafish, a body pattern of stripes arises by self organizing interactions among three types of pigment cells. Yet these fish also exhibit white ornamentation on their fins that depends on the transdifferentiation of black melanophores to white cells, "melanoleucophores." To identify mechanisms underlying this conversion we used ultrastructural, transcriptomic, mutational and other approaches. We show that melanophore- melanoleucophore transition depends on regional BMP signals transduced through non-canonical receptors (Rgmb-Neo1a-Lrig2) as well as BMP-dependent signaling by Agouti genes, asip1 and asip2b. These signals lead to expression of transcription factor genes including foxd3 and runx3 that are necessary to induce loss of melanin by an autophagy-like process, curtail new melanin production, and deploy a pathway for accumulating guanine crystals that, together, confer a white phenotype. These analyses uncover an important role for positional information in specifying ornamentation in zebrafish and show how tissue environmental cues and a novel gene regulatory program have allowed terminal addition of a distinct phenotype to a pre-existing cell type. SignificanceFish often have striking color patterns with important functions in behavior. In zebrafish, the familiar striped pattern forms through self-organizing interactions between pigment cells, yet the white highlights on their fins arise differently--through the transformation of black pigment cells into white ones. This study reveals how this dramatic cell transformation happens: signals from the surrounding tissue, specifically BMP and Agouti proteins, instruct black cells to change their fate. These signals trigger expression of specific genes that cause the cells to break down their black pigment while acquiring white, crystal-like structures. This work shows how local signals in tissues can drive the development of ornamental features and provides insights into how new cell types evolve.