In many animals, skin colors and patterns play an essential role, whether for camouflage, communication, or thermoregulation. In corn snakes, some lineages display red, yellow, or pink hues, and their dorsal spots may blend or form stripes.
What are the genetic and cellular mechanisms that determine these colorful patterns? A team from the University of Geneva (UNIGE) has discovered that a gene, CLCN2, is involved in these variations. This study, published in
Genome Biology, opens new perspectives on the evolution and genetics of animal coloration.
In corn snakes, some morphologies exhibit red, yellow, or pink spots, and their dorsal spots may merge or transform into stripes.
© LANEVOL
The colors and patterns of corn snakes (
Pantherophis guttatus) are linked to the arrangement and location of chromatophores, cells found in the integument of many animals that contain pigments or light-reflecting crystals. While these reptiles typically have a back dotted with red spots outlined in black on an orange background and a black-and-white checkerboard ventral pattern, they can also display a great diversity of other colors and patterns.
Among the frequently encountered variations is the "Motley" variant, whose dorsal spots are fused or interrupted, creating a more linear pattern. As for the "Stripe" variant, it features continuous longitudinal bands on the back. These two variations share the same characteristic of a uniform belly, lacking the typical checkerboard pattern.
A single gene responsible for different patterns
The team led by Athanasia Tzika and Michel Milinkovitch, respectively a teaching and research associate and professor at the Department of Genetics and Evolution in UNIGE's Faculty of Science, sought to characterize these mutations.
Following crossbreeding between Motley and Stripe snakes and genome sequencing of their offspring, the scientists identified that these two mutations involved only a single gene: CLCN2. This gene encodes a protein located at cell membranes, forming a channel for chloride ion transport across the membrane. The differential distribution of ions creates an electrical charge difference between the inside and outside of the cell, enabling cellular signal transmission.
In Motley snakes, it is not a mutation in the gene but a sharp decrease in its expression level. In Stripe snakes, however, a small piece of DNA—or transposon—is inserted into the CLCN2 gene, rendering the protein inactive.
"These results were very surprising because, in humans or mice, the CLCN2 channel is essential for neuron activity, and mutations in this gene are linked to severe pathologies, such as leukoencephalopathy, a disease affecting the brain's white matter," explain Sophie Montandon and Pierre Beaudier, researchers in the Milinkovitch/Tzika lab and co-first authors of the study. "We therefore developed genetic experiments in corn snakes to inactivate the CLCN2 gene. The resulting mutants displayed the Stripe form, confirming that this was indeed the gene we were looking for."
An unexpected player in pattern formation
To better understand the role of CLCN2, the scientists investigated in which organs and cells this gene is expressed in corn snakes. Transcriptomic analyses revealed that CLCN2 is expressed in the adult brain—as in mice and humans—but also in chromatophores during embryogenesis.
The researchers then focused on the appearance of color patterns during embryonic development. They observed that in mutants, chromatophores fail to aggregate properly to form characteristic spots: instead, they organize into stripes, visible in Stripe-type individuals.
"Our results show that a mutation in the CLCN2 gene in corn snakes does not cause brain or behavioral disorders. However, the protein plays an essential, and previously unknown, role in the development of color patterns," concludes Asier Ullate-Agote, co-first author of the study.
The next phase of the study will aim to better understand the role of the CLCN2 chloride channel in chromatophore membranes, particularly how it influences interactions between these pigment cells. The goal is to decipher the cellular mechanisms that enable the emergence of the spectacular diversity of color patterns observed in corn snakes—and other reptiles.