Within-colony color variation in a chalice coral is associated with region-specific pigment expression and higher photoconvertible pigment transcripts

Coral reefs are often celebrated for their kaleidoscope of colors, but a closer look at a single coral colony reveals an even more surprising secret: one individual can wear many different coats. A single chalice coral, which is a clonal organism made of thousands of genetically identical polyps, might display a bright green body, a vivid orange mouth area, and a gradual shift between the two. Since every part of that colony shares the exact same DNA, the difference in color cannot be caused by different genes. Instead, the variation must come from how those genes are used. In the world of biology, this is known as regulation; it is the process of turning specific genetic instructions up or down to create different outcomes from the same blueprint. Understanding how a single genetic code can produce such a diverse palette is crucial for scientists trying to grasp how corals adapt to their environments and how their complex relationships with microscopic algae function.

To uncover the molecular machinery behind this phenomenon, researchers focused on a specific green-and-orange chalice coral colony. They treated the coral not as a single uniform block, but as a collection of distinct neighborhoods. The team carefully collected tissue samples from three specific zones: the main green body, the orange oral region, and a transition zone where the colors blend. Because coral lives in a tight partnership with microscopic algae called dinoflagellates, the researchers had to separate the genetic messages coming from the coral animal itself from those coming from its algal partners. They built a complete genetic library, known as a transcriptome, from the RNA in each sample, which acts as a snapshot of which genes were active at that moment. By sorting these messages, they could look specifically at the coral's own instructions without the noise of the algae's activity.

The investigation centered on a family of genes that produce fluorescent proteins, the molecules responsible for the coral's glow and pigment. The researchers found that the different colors were not random but were driven by specific sets of instructions being switched on in different parts of the colony. The orange sections were not just a different shade; they were actively producing two distinct types of proteins that the green sections were not making in the same way. One set of proteins created the orange color directly, while another set produced green-to-red photoconvertible proteins. These special proteins start out green but can change to red when exposed to light. The study suggests that the orange color in the coral's mouth area is partly the result of these proteins undergoing a chemical change triggered by light, a process called photoconversion. This means the coral is not just painting itself with static colors but is using light to actively shift its own pigments.

While the findings offer a clear picture of the genetic activity driving these colors, the researchers remain cautious about declaring the mechanism fully solved. The study demonstrates a strong link between the presence of specific pigment transcripts and the visible color zones, but it is a pilot investigation. The authors note that further work is needed to confirm with direct light measurements that photoconversion is indeed the cause of the color shift, and to see if this pattern holds true across many different colonies rather than just the one examined. For now, the work provides a solid foundation, showing that the stunning color variations within a single coral are the result of precise, region-specific genetic regulation, where the coral turns on different pigment recipes to create a living mosaic.