Lab Projects

Color Patterns
Evolution of Mammalian Coloring
Melanocortin Signaling

Color Patterns

Camouflage, species recognition, and morphologic diversity are all effects of mammalian fur patterns. These patterns arise from the dynamic regulation of pigment-type switching, a phenomenon in which melanocytes choose between synthesizing eumelanin (black or brown pigment) or pheomelanin (red or yellow pigment), depending on the phase of the hair growth cycle and position on the body.

The molecular basis and evolutionary variation of periodic mammalian color patterns have been difficult to investigate from genetic crosses of model organisms. Domestic cats (Felis catus) exhibit heritable variation of tabby markings – mackerel versus blotched – that provide an opportunity for such genetic analysis. Periodic color patterns in other felids may represent the same process; for example, dark tabby markings in domestic cats may be homologous to black stripes or spots in tigers or cheetahs, respectively.

Further studies in the Barsh Lab of color pattern in domestic-wild cat hybrids offer the opportunity to study these complex color markings and add to our knowledge of how felids acquire their color patterns.

Evolution of Mammalian Coloring 

Dun is a wild-type coat color in horses characterized by pigment dilution with a striking pattern of dark areas termed primitive markings. The Barsh Lab has shown that the dilution in the dun phenotype is due to radially asymmetric deposition of pigment in the growing hair caused by localized expression of the T-box 3 (TBX3) transcription factor in hair follicles, which in turn determines the distribution of hair follicle melanocytes.

Most domestic horses have non-dun coat color, a more intensely pigmented phenotype caused by regulatory mutations impairing TBX3 expression in the hair follicle, resulting in a more circumferential distribution of melanocytes and pigment granules in individual hairs. We identified two different alleles (non-dun1 and non-dun2) causing non-dun color. non-dun2 is a recently derived allele, whereas the Dun and non-dun1 alleles are found in ancient horse DNA, demonstrating that this polymorphism predates horse domestication. These findings uncover a new developmental role for T-box genes and new aspects of hair follicle biology and pigmentation.

In addition to horse coat color, we are exploring mechanisms of zebra coat color using what we’ve already shown regarding pigment in equine hairs.

Melanocortin signaling

The spectrum of color and diversity of patterns in mammals arises from variation in the quantity, quality and regional distribution of two types of pigment – black eumelanin and yellow pheomelanin. Switching between eumelanin and pheomelanin production – a process commonly known as pigment “type-switching” – is controlled primarily by the melanocortin system, in which a family of G protein-coupled receptors has been implicated not only in pigmentation but also in cortisol production, body weight regulation and exocrine gland secretion.

Two “classical” coat color mutations in non-model organisms, Orange in the domestic cat and Sex-linked yellow (Sly) in the Syrian hamster are especially interesting because they are X-linked, yet mimic a loss-of-function mutation in the autosomal Melanocortin 1 Receptor (Mc1r). These apparent exceptions to Ohno’s law – conservation of gene content on the X chromosome across all eutherian species – offer the potential for new insight into a critical paracrine signaling pathway used by the neuroendocrine, adrenal, exocrine, and pigmentary systems.

In the Barsh Lab, we employ experimental genetic studies in cultured cells and in transgenic animals to confirm the causal role of the Orange deletion and to investigate the molecular pathophysiology. We use genome editing technology to delete the homologous region in mouse and human melanocytes and evaluate the consequences on gene expression and cellular phenotype. We also manipulate expression of Arhgap36 in cultured cells and in transgenic mice to determine if and how Arhgap36 intersects with melanocortin receptor signaling.

At the same time, the lab is directing our efforts toward molecular identification and characterization of the hamster Sly mutation. Sly is similar to Orange with regard to its epistatic and developmental interactions, and preliminary digital gene expression profiling data suggests that the effects of Sly also impinge on melanocortin receptor signaling. We use deep RNA-Seq data of mutant and non-mutant tissues to develop an assembled transcriptome, identify single nucleotide variants (SNVs) associated with the mutation, and use those SNVs to refine the region to a critical interval suitable for targeted resequencing by clone-adapted capture. Bioinformatic analysis of transcriptome and genome sequencing data will reveal potential regulatory and/or coding alterations that cause Sex-linked yellow.