While the patterns and colors of lizard skin are fascinating, the mechanisms behind them are largely unknown.
A team from the University of Geneva (UNIGE) has focused on the leopard gecko, a popular lizard, to understand how the bands on juvenile skin transform into spots in adulthood. Their research reveals that this transition is orchestrated by interactions between different pigment cells, offering new insights into the biology of pigmentation in reptiles. These findings are published in the journal
Proceedings of the National Academy of Sciences (PNAS).
During the first months of its life, the leopard gecko displays banded patterns. As an adult, these bands give way to black spots, reminiscent of a leopard's coat.
© LANEVOL
The colors and patterns on the surface of reptile skin are astonishingly diverse, generated by interactions between three types of color cells, called chromatophores. Melanophores are responsible for black and brown colors; xanthophores for red/yellow coloration, and iridophores, filled with light-reflecting crystals, produce metallic colors. Depending on the distribution of these chromatophores in the skin, different color patterns emerge. For most species, the color pattern is defined during embryonic development and remains stable throughout the animal's life.
Transition to adulthood
However, there are exceptions. This is the case for the leopard gecko (
Eublepharis macularius), well-known among terrarium enthusiasts and native to India and Pakistan. This lizard, about eight inches (20 cm) as an adult, sports a white band on its nape and several yellow and black transverse bands on its back during the first few months of life. Once it reaches adulthood, its light and uniform skin is covered with black spots reminiscent of a leopard's coat. Athanasia Tzika's group, researcher at the Department of Genetics and Evolution of the Faculty of Sciences at UNIGE, has investigated the phenomena that allow the transition from bands to spots in the leopard gecko to better understand the formation of color patterns in reptiles.
The biologists analyzed the distribution of color cells in lizard skin at different life stages. "During our observations of skin samples, we found that while all three types of chromatophores are present in juveniles, the iridophores disappear after a few months, coinciding with the transition from bands to spots," explains Asier Ullate-Agote, former PhD student at the Department of Genetics and Evolution of the Faculty of Sciences at UNIGE, and first author of the study.
The leopard gecko also has the ability to regenerate its tail if it is cut off due to physical aggression by a predator. Regardless of the age of the lizard, this new tail always grows back covered with spots, never bands. Scientists have analyzed the skin of these regrown tails and observed that they lack iridophores. These observations again correlate the presence of spots with the absence of iridescent chromatophores.
A mutant gecko to confirm these results
"These observations led us to analyze the skin of a particular animal that sometimes appears naturally in our breeding facilities: the 'Mack Super Snow' leopard gecko. This animal, carrying a spontaneous mutation, is born with black skin without bands but displays a spotted skin as an adult, just like other non-mutant lizards. We found that the skin of juveniles shows the presence of only melanophores, but no iridophores or xanthophores," describes Athanasia Tzika.
Genetic analyses of these individuals allowed the identification of a mutation in the PAX7 gene. This gene is normally expressed in iridophores and xanthophores, suggesting that it plays a key role in the differentiation of these two chromatophores, whose presence is not necessary for the formation of spots.
"We thus believe that it is the interactions between melanophores and iridophores that give rise to the banded pattern in juveniles, and that melanophores can form spots without the help of other chromatophores. Our study provides a better understanding of the regulatory mechanisms that govern skin pigmentation in leopard geckos and, more generally, the formation of color patterns in reptiles," concludes Athanasia Tzika.