Abstract
Sex-linked barring is a fascinating plumage pattern in chickens recently shown to be associated with two non-coding and two missense mutations affecting the ARF transcript at the CDKN2A tumor suppressor locus. It however remained a mystery whether all four mutations are indeed causative and how they contribute to the barring phenotype. Here, we show that Sex-linked barring is genetically heterogeneous, and that the mutations form three functionally different variant alleles. The B0 allele carries only the two non-coding changes and is associated with the most dilute barring pattern, whereas the B1 and B2 alleles carry both the two non-coding changes and one each of the two missense mutations causing the Sex-linked barring and Sex-linked dilution phenotypes, respectively. The data are consistent with evolution of alleles where the non-coding changes occurred first followed by the two missense mutations that resulted in a phenotype more appealing to humans. We show that one or both of the non-coding changes are cis-regulatory mutations causing a higher CDKN2A expression, whereas the missense mutations reduce the ability of ARF to interact with MDM2. Caspase assays for all genotypes revealed no apoptotic events and our results are consistent with a recent study indicating that the loss of melanocyte progenitors in Sex-linked barring in chicken is caused by premature differentiation and not apoptosis. Our results show that CDKN2A is a major locus driving the differentiation of avian melanocytes in a temporal and spatial manner.
Highlights
Birds show an astonishing variety of plumage coloration and pattern, both across the body as well as on individual feathers
Sex-linked barring is a striping pattern present on individual feathers in domestic chickens, which can be utilized as a model species to gain an understanding of the underlying ‘mode of action’ of biological pattern formation
Our findings suggest that regulatory mutations in the tumor suppressor gene CDKN2A first resulted in a primitive barring pattern and that two missense mutations in the same gene occurred later and independently, causing the more distinct barring pattern of extant chicken breeds
Summary
Birds show an astonishing variety of plumage coloration and pattern, both across the body as well as on individual feathers. As it is more challenging to understand how color patterns are generated than to explain reductions or absence of pigmentation, barring is one of the most interesting feather patterns in chickens yet to be understood. There are two different barring patterns in chicken, Autosomal and Sex-linked barring. Both barring patterns are characterized by alternating bars of two different colors on individual feathers. Whereas chickens said to carry Autosomal barring, exhibit a black bar on a white or red background, feathers of Sex-linked barred chickens are characterized by a fully white bar on a red or black background (Fig 1A) [1]. Other characteristics of Sex-linked barring are the dilution of dermal pigment in the shanks and beak as well as a white spot on the head present at hatch (S1 Fig), which can be utilized for sex determination [2]
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