Elaborate color patterns of individual chicken feathers may be formed by the agouti signaling protein.

Hair and feather pigmentation is mainly determined by the distribution of two kinds of melanin, eumelanin and pheomelanin, which produce brown to black and yellow to red colorations, respectively. The agouti signaling protein (ASIP) acts as an antagonist or an inverse agonist of the melanocortin 1 receptor (MC1R), a G protein-coupled receptor for α-melanocyte-stimulating hormone (α-MSH). This antagonism of the MC1R by ASIP on melanocytes initiates a switch of melanin synthesis from eumelanogenesis to pheomelanogenesis in mammals. In the present study, we isolated multiple ASIP mRNA variants generated by alternative splicing and promoters in chicken feather follicles. The mRNA variants showed a discrete tissue distribution. However, mRNAs were expressed predominantly in the feather pulp of follicles. Paralleling mRNA distribution, ASIP immunoreactivity was observed in feather pulp. Interestingly, ASIP was stained with pheomelanin but not eumelanin in pulp areas that face developing barbs. We suggest that the elaborate color pattern of individual feathers is formed in part by the antagonistic action of ASIP that is produced by multiple mRNA variants in chicken feather follicles.

Conserved distal promoter of the agouti signaling protein (ASIP) gene controls sexual dichromatism in chickens.

Brilliant plumage is typical of male birds, thus sexual plumage dichromatism is seen in many avian species; however, the molecular mechanism underlying this remains unclear. The agouti signaling protein (ASIP) is a paracrine factor that stimulates yellow/red pigment (pheomelanin) synthesis and inhibits black/brown pigment (eumelanin) synthesis in follicular melanocytes. In mammals, the distal promoter of the ASIP gene acts exclusively on the ventral side of the body to create a countershading pigmentation pattern by stimulating pheomelanin synthesis in the ventrum. Here, we examined the role of the distal ASIP promoter in controlling estrogen-dependent sexual dichromatism in chickens. Reverse-transcription polymerase chain reaction analyses revealed that ASIP class 1 mRNAs transcribed by the distal promoter were expressed exclusively on the ventral side of chicks and adult females displaying countershading. In showy adult males, the ASIP class 1 mRNAs were expressed in gold-colored ornamental feathers grown on the back. In the presence of estrogen, males molted into female-like plumage and ASIP class 1 mRNAs expression was altered to female patterns. These results suggest that the distal ASIP promoter produces countershading in chicks and adult females, similar to the ventral-specific ASIP promoter in mammals. In addition, the class 1 promoter plays an important role for creating sexual plumage dichromatism controlled by estrogen. This is the first evidence for a pigmentation gene having been modified in its expression during evolution to develop phenotypic diversity between individuals of different sexes.

Identification of a feather ß-keratin gene exclusively expressed in pennaceous barbule cells of contour feathers in chicken.

Feathers are elaborate skin appendages shared by birds and theropod dinosaurs that have hierarchical branching of the rachis, barbs, and barbules. Feather filaments consist of ß-keratins encoded by multiple genes, most of which are located in tandem arrays on chromosomes 2, 25, and 27 in chicken. The expansion of the genes is thought to have contributed to feather evolution; however, it is unclear how the individual genes are involved in feather formation. The aim of the present study was to identify feather keratin genes involved in the formation of barbules. Using a combination of microarray analysis, reverse-transcription polymerase chain reaction, and in situ hybridization, we found an uncharacterized keratin gene on chromosome 7 that was expressed specifically in barbule cells in regenerating chicken feathers. We have named the gene barbule specific keratin 1 (BlSK1). The BlSK1 gene structure was similar to the gene structure of previously characterized feather keratin genes, and consisted of a non-coding leader exon, an intron, and an exon with an open reading frame (ORF). The ORF was predicted to encode a 98 aa long protein, which shared 59% identity with feather keratin B. Orthologs of BlSK1 were found in the genomes of other avian species, including turkey, duck, zebra finch, and flycatcher, in regions that shared synteny with chromosome 7 of chicken. Interestingly, BlSK1 was expressed in feather follicles that generated pennaceous barbules but not in follicles that generated plumulaceous barbules. These results suggested that the composition of feather keratins probably varies depending on the structure of the feather filaments and, that individual feather keratin genes may be involved in building different portions and/or types of feathers in chicken.