Regulatory and disruptive variants in the CLCN2 gene are associated with modified skin color pattern phenotypes in the corn snake

Snakes exhibit a broad variety of adaptive colors and color patterns, generated by the spatial arrangement of chromatophores, but little is known of the mechanisms responsible for these spectacular traits. Here, we investigate a mono-locus trait with two recessive alleles, motley and stripe, that both cause pattern aberrations in the corn snake. We use mapping-by-sequencing to identify the genomic interval where the causal mutations reside. With our differential gene expression analyses, we find that CLCN2 (Chloride Voltage-Gated Channel 2), a gene within the genomic interval, is significantly downregulated in Motley embryonic skin. Furthermore, we identify the stripe allele as the insertion of an LTR-retrotransposon in CLCN2, resulting in a disruptive mutation of the protein. We confirm the involvement of CLCN2 in color pattern formation by producing knock-out snakes that present a phenotype similar to Stripe. In humans and mice, disruption of CLCN2 results in leukoencephalopathy, as well as retinal and testes degeneration. Our single-cell transcriptomic analyses in snakes reveal that CLCN2 is indeed expressed in chromatophores during embryogenesis and in the adult brain, but the behavior and fertility of Motley and Stripe corn snakes are not impacted. Our genomic, transcriptomic, and functional analyses identify a plasma membrane anion channel to be involved in color pattern development in snakes and show that an active LTR-retrotransposon might be a key driver of trait diversification in corn snakes.