About the Blog Author
Dr. Benjamin Karin is an evolutionary biologist and herpetologist working at UC Berkeley as an NSF Postdoctoral Research Fellow based in Dr. Ian Wang’s Lab. He studies the processes that generate biodiversity and the evolutionary forces and genetic basis of unique traits. Ben works on interesting evolutionary systems both in California and in remote parts of Southeast Asia. Check out his Website Here.
How a California Skink Could Unlock the Secrets of Animal Coloration
Color in the animal kingdom isn’t just for show—it’s a powerful tool that plays major roles in survival and reproduction. From helping animals blend in or stand out, to attracting mates or confusing predators, color is one of the most striking and important traits in nature. Because it’s often easy to see and shaped by strong evolutionary pressures, color has become a key focus for scientists studying everything from genetics and development to evolution itself.
With support from the American Genetic Association, I’m exploring the genetic basis of coloration using a remarkable lizard native to California: Plestiodon gilberti, also known as Gilbert’s skink. This skink not only displays unique variation in vibrant coloration, but it also changes color throughout its life and across seasons—making it a perfect model to study how genetic pathways shape colorful traits in vertebrates.
Blue Tails and Red Heads
What makes P. gilberti so special? For one, its tail color varies from bright blue to red or even pink in juveniles (Richmond & Jockusch, 2007; Figure 1). These bright tails act as a decoy to lure predators away from the body. Even though this makes them more visible to predators, it actually lowers the chance of a lethal attack by allowing them to drop their tail and escape (Murali et al., 2018)!
But the color story doesn’t end there. As these lizards grow into adults, they lose the bright tail coloration altogether (Watson et al., 2019). And during the spring breeding season, adult males develop vivid red heads and throats, possibly signaling their health and mating quality (Vitt & Cooper Jr., 1985; Figure 2). These natural color transitions provide a rare opportunity to explore how genes control color at different life stages and under different selective pressures.
Pigments, Crystals, and Color
Animal colors come from two main sources: pigments (like reds, yellows, and browns) and structural colors (like blues), which result from microscopic structures that scatter light. Pigment genes are fairly well understood, but the genetic basis of structural colors—like hummingbird iridescence—is still only beginning to be understood (Price-Waldman & Stoddard, 2021).
Based on work in other closely-related lizards, we expect that blue tails are produced by intricately spaced-layers of guanine crystals (Kuriyama et al., 2020). And yes, Guanine is actually the same as the G base of the A,C,G,T code of DNA! On the other hand, red tails involve pigments like pteridines or carotenoids. By comparing gene activity in blue vs. red tails, we can uncover which genetic switches control these differences—and how mutations might flip those switches.
From the Genome to the Cell: Tools to Decode Color
To get to the bottom of these color differences in P. gilberti, I’m using three powerful tools: Whole Genome DNA Sequencing, Skin RNA Sequencing (RNAseq), and Skin Transmission Electron Microscopy (TEM). Whole genome sequencing allows us to understand the genes underlying red and blue tail coloration, RNAseq lets us see which genes are turned on or off in different color states, and TEM reveals the microscopic skin structures responsible for color—like the orientation of those guanine crystals.
Thanks to the California Conservation Genomics Project, we now have a high-quality, contiguous reference genome for Plestiodon gilberti. This reference acts like a genetic roadmap, guiding us to the specific regions of the genome that influence color. To uncover the genes responsible for red and blue tail coloration, I’ll add in genomes for over 100 additional skinks and align these genomes to the reference. By scanning for genomic regions that consistently differ between red-tailed and blue-tailed populations, I can pinpoint the genetic variants linked to each color. I hypothesize that just a small number of mutations—possibly even one—could shift development from producing structural blue coloration to pigment-based red.
In addition to studying color differences between populations, the AGA award will also allow me to investigate how coloration changes over time—both as individuals grow and as the seasons change. When juvenile tails fade to brown, is it because iridophores or pigments are being covered up by melanin? Or are entire pathways being switched off? My gene expression and TEM microscopy studies will help tease apart these possibilities.
Why It Matters
Understanding how color is genetically controlled can reveal deep insights into how evolution shapes complex traits. By identifying the key genes and pathways that drive color changes in this one skink species, we can begin to predict how different types of traits evolve in other organisms.
This research doesn’t just tell us about P. gilberti—it helps us understand how the complexity of nature comes about from the genetic instructions. This applies well beyond the vivid world of animal coloration, to human health and beyond.
References
Kuriyama T, Murakami A, Brandley M, Hasegawa M (2020) Blue, black, and stripes: Evolution and development of color production and pattern formation in lizards and snakes. Frontiers in Ecology and Evolution 8:1–14.
Murali G, Merilaita S, Kodandaramaiah U (2018) Grab my tail: Evolution of dazzle stripes and colourful tails in lizards. Journal of Evolutionary Biology 31:1675–1688.
Price-Waldman R, Stoddard MC (2021) Avian coloration genetics: Recent advances and emerging questions. Journal of Heredity 112:395–416.
Richmond JQ, Jockusch EL (2007) Body size evolution simultaneously creates and collapses species boundaries in a clade of scincid lizards. Proceedings of the Royal Society B: Biological Sciences 274:1701–1708.
Vitt LJ, Cooper Jr. WE (1985) The evolution of sexual dimorphism in the skink Eumeces laticeps: an example of sexual selection. Canadian Journal of Zoology 63:995–1002.
Watson CM, Degon Z, Krogman W, Cox CL (2019) Evolutionary and ecological forces underlying ontogenetic loss of decoy coloration. Biological Journal of the Linnean Society 128:138–148.