Introduction

In the stunning world of Lepidopterans—commonly known as butterflies and moths—wing color patterns vary magnificently. Many species showcase striking contrasts between black and white, or dark and bright shades, primarily influenced by the presence or absence of melanin. These wing color variants provide textbook examples of natural selection and evolution in action. The iconic case of the British peppered moth (Biston betularia) illustrates this, as its melanic form surged in frequency due to the darker environment caused by industrial pollution in the late 1800s. Additionally, the mimetic radiation of Heliconius butterflies further exemplifies evolutionary adaptations tied to color patterns.

Despite a robust understanding of the ecological factors driving these color variations, the underlying genetic mechanisms have remained a puzzle—until recently.

How do butterflies and moths create dazzling wing colors?

Recent research over the last two decades has indicated that many melanic wing color variants are linked to a genomic region surrounding the protein-coding gene called "cortex," which was once thought to be the primary determinant of melanic coloration. However, a groundbreaking study led by Professor Antônia Monteiro and Dr. Shen Tian from the National University of Singapore (NUS), in collaboration with researchers from Japan and the United States, has overturned this assumption. Their findings, published in the journal *Science*, reveal that it is not the cortex gene that governs melanic coloration but rather a previously overlooked microRNA (miRNA) that serves as the true color switch.

Dr. Tian, the lead researcher, spoke candidly about his motivations: "Numerous studies raised doubts about the cortex theory, pushing me to explore other genomic elements, particularly miRNAs." Previously a PhD student and postdoctoral researcher in Monteiro’s lab, Dr. Tian is currently furthering his research at Duke University, USA.

The role of microRNAs in wing color

MicroRNAs, or miRNAs, are small RNA molecules that, unlike traditional genes, do not code for proteins. Instead, they play critical roles in regulating gene expression by inhibiting the target gene's activity. In their research, the team discovered a specific miRNA, mir-193, situated near the cortex gene. Through the use of CRISPR-Cas9 gene editing technology, they disrupted mir-193 in three distinct butterfly lineages: the African squinting bush brown butterfly (Bicyclus anynana), the Indian cabbage white butterfly (Pieris canidia), and the common Mormon butterfly (Papilio polytes). The outcome was striking—disrupting mir-193 completely removed melanic colors from their wings, while altering the cortex and nearby protein-coding genes did not produce any notable changes.

Further analysis revealed that mir-193 is derived from a long non-coding RNA known as ivory, directly inhibiting several pigmentation-related genes. Surprisingly, this evolutionary conserved miRNA also proved influential in Drosophila (fruit flies), suggesting a fundamental, broader role across various taxa well beyond Lepidoptera.

Significance of the findings

Professor Monteiro emphasized the significance of their findings, stating, "Our research challenges the traditional focus on cortex and highlights how small, non-protein-coding RNAs can control the diversity of melanic color variations in nature." They cautioned against underestimating the role of non-coding RNAs like miRNAs in genetic studies, noting that overlooking these could lead to erroneous conclusions about genotype-phenotype relationships.

Dr. Tian further urged caution regarding the underexplored role of non-coding RNAs in phenotypic diversification, asserting that this study should ignite further research into the impact of miRNAs and similar molecules on the evolutionary and developmental biology of organisms.

Conclusion

In summary, through innovative research methods and collaborative efforts, scientists are unraveling the complexities of butterfly wing coloration. The discoveries surrounding mir-193 not only redefine our understanding of lepidopteran biology but also underscore the importance of minute genetic factors in the grand narrative of evolution. Prepare to see butterflies in a whole new light!