Temporal skin factors influencing the development of melanoblasts in piebald mice

Mutational analysis of endothelin receptor b1 (rose) during neural crest and pigment pattern development in the zebrafish Danio rerio

Pigment patterns of fishes are a tractable system for studying the genetic and cellular bases for postembryonic phenotypes. In the zebrafish Danio rerio, neural crest-derived pigment cells generate different pigment patterns during different phases of the life cycle. Whereas early larvae exhibit simple stripes of melanocytes and silver iridophores in a background of yellow xanthophores, this pigment pattern is transformed at metamorphosis into that of the adult, comprising a series of dark melanocyte and iridophore stripes, alternating with light stripes of iridophores and xanthophores. Although several genes have been identified in D. rerio that contribute to the development of both early larval and adult pigment patterns, comparatively little is known about genes that are essential for pattern formation during just one or the other life cycle phase. In this study, we identify the gene responsible for the rose mutant phenotype in D. rerio. rose mutants have wild-type early larval pigment patterns, but fail to develop normal numbers of melanocytes and iridophores during pigment pattern metamorphosis and exhibit a disrupted pattern of these cells. We show that rose corresponds to endothelin receptor b1 (ednrb1), an orthologue of amniote Ednrb genes that have long been studied for their roles in neural crest and pigment cell development. Furthermore, we demonstrate that D. rerio ednrb1 is expressed both during pigment pattern metamorphosis and during embryogenesis, and cells of melanocyte, iridophore, and xanthophore lineages all express this gene. These analyses suggest a phylogenetic conservation of roles for Ednrb signaling in the development of amniote and teleost pigment cell precursors. As murine Ednrb is essential for the development of all neural crest derived melanocytes, and D. rerio ednrb1 is required only by a subset of adult melanocytes and iridophores, these analyses also reveal variation among vertebrates in the cellular requirements for Ednrb signaling, and suggest alternative models for the cellular and genetic bases of pigment pattern metamorphosis in D. rerio.

Quantitative trait loci that modify the severity of spotting in piebald mice

Mice homozygous for the recessive mutation piebald (s) exhibit a white-spotted coat caused by the defective development of neural crest-derived melanocytes. The severity of white spotting varies greatly, depending on the genetic background on which s is expressed. A backcross between two inbred strains of s/s mice that exhibit large differences in the degree of spotting was used to identify six genetic modifiers of piebald spotting on chromosomes 2, 5, 7, 8, 10, and 13. The loci differed in their spatial contribution to spotting on the dorsal versus ventral surfaces of mice; nonadditive interactions were observed between loci on chromosomes 2 and 5. This study underscores the power of using genetic analyses to identify and analyze loci involved in modifying the severity of phenotypic traits in mice.

Quail melanoblast migration in two breeds of fowl and in their hybrids: evidence for a dominant genic control of the mesodermal pigment cell pattern through the tissue environment

In the Silkie fowl large numbers of melanocytes invade most internal tissues and organs. The factors involved in this internal pigment cell pattern were studied by grafting quail neural tube segments into White Leghorn, White Silkie, and F1 hybrids (White Silkie male X White Leghorn female). Sections of quail neural tube five somites long, excised at the level of the last formed somites, were grafted isotopically and ischoronically. Various tissues and organs (mesenteries, muscles, testis, ovary, mesonephros, metanephros, and adrenals) excised from the internal region corresponding to the peripheral transverse strip of quail melanocytes, were studied after staining by the Feulgen-Rossenbeck technique. Despite some variations in pigment cell density, Silkie and hybrid grafted embryos exhibited an extensive quail internal pigmentation similar to the melanocyte distribution in the Silkie breed. In white Leghorn host embryos, the internal pigmentation remained limited. These results show the part played by tissular factors in the expression of the Silkie pigment phenotype and that this genetic tissular character is dominant. On the contrary, White Leghorn embryos, grafted with Silkie neural tube segments, never exhibited any internal pigmentation; the melanocytes deriving from the grafted Silkie neural tube were only localized at the dermoepidermal level. Thus, the migrating and/or differentiating capabilities of the Silkie premelanoblasts are different from those of quail premelanoblasts. The sex-linked inhibitor of the White Leghorn tissue interferes at the level of the pigment cells of chickens but not of quails.

Site of gene action of the white allele (Miwh) of the microphthalmia locus: a dermal-epidermal recombination study

The white allele Miwh of the microphthalmia locus, when homozygous, causes a complete absence of neural crest-derived melanocytes in skin and internal organs. The site(s) of gene action of Miwh on melanoblast differentiation were examined by making dermal-epidermal recombinant grafts with skin from 13-14-day embryos of the following genotypes: normal (+/+), heterozygous white (Miwh/+), homozygous white (Miwh/Miwh), and dominant spotting (W/W). Based on results from these grafts, the following conclusions were reached: Miwh/Miwh epidermis is as effective as W/W epidermis in supporting the differentiation of +/+ follicular melanocytes. Miwh/Miwh dermis is completely permissive to in situ differentiation of +/+ follicular melanocytes. Thirteen-fourteen-day embryonic Miwh/Miwh skin, both epidermis and dermis, alters of blocks the differentiation of +/+ melanoblasts into dermal melanocytes. By 13 days of development, melanoblasts from both Miwh/+ and Miwh/Miwh skin, even when presented with a permissive W/W environment, are irreversibly redirected into abnormal developmental pathways resulting in either a reduction in follicular pigmentation (Miwh/+) or a complete absence of follicular melanocytes (Miwh/Miwh).