The Significance of Genetics in Insect Coloration and Evolution

From vibrant butterfly wings to subtle fruit fly patterns, insect coloration isn't just nature’s canvas—it’s evolution at play. Delving into genetics reveals that tiny regulatory tweaks in gene expression can trigger massive shifts, influencing everything from camouflage and mimicry to courtship behaviors. By understanding these molecular mechanisms, scientists unravel not only how insects communicate visually but also how evolution endlessly crafts complexity and diversity from genetic simplicity.

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Heliconius Melpomene, the Postman Butterfly - a likely hybrid between melpomene subspecies malleti and plesseni (by Miroslav Srb, licensed for educational use via Adobe Stock).

The most captivating aspect of insects, looking past their naturally uncharismatic morphologies, is the diversity of pigmentation and patterning they can display; this field of study within insect biology spans over a breadth of sub-disciplines, including development, biochemistry, biophysics, genetics, evolution, and ecology. Its significance as an evolutionary trait is undeniable, playing roles in intra- and interspecific communication patterns, mimicry and camouflage, as well as physiological and biochemical processes [1]. While the functional importance behind the evolution of coloration is well characterized, what we often don’t understand is the basis of coloration on a molecular level. Insects, as a class, boast an astounding diversity of color patterns, ranging from 300 to 700 nm wavelengths—reaching outside the spectrum of visible light for mammals [2].

Modern research has only scratched the surface in uncovering the genes responsible for generating this variability between insect species; beginning to understand the developmental role that a gene may play tells us only a fraction of the picture, as these functions are not set in evolutionary “stone.” For example, yellow, a gene known to produce melanin pigment, is ubiquitously expressed throughout the wings and wing veins of fruit flies to produce patterns necessary for communication [3]. While various homologues—genes inherited from common ancestry—of yellow are known to code for similar functions, slight tuning of its spatial and temporal expression can give rise to not only an incredible number of novel phenotypes and patterns but completely new functions as well. How a particular protein or RNA interacts with its surrounding cellular environment is of utmost importance in determining its function [4]. To put this into perspective, yellow can take on an additional role, as it is necessary for male courtship behavior in several fruit fly species. Behavioral assays have determined that the presence of these melanic wing spots is an agent of sexual selection—tying yellow to not only producing secondary sexual characteristics from a morphological standpoint but doubling down on sexual selection through mediating physiological and neuronal processes, ultimately affecting behavior to increase direct fitness [5]. Interestingly, this goes both ways; the yellow orthologue (the same yellow gene passed down from common ancestry, granted with some small changes between species) in Bicyclus butterflies acts to repress male courtship behavior, displaying the volatility of small evolutionary tuning of gene expression. Here we see selection acting two ways on a single gene; it’s not uncommon that we find genes responsible for producing a visual signal also encode either positive or negative behavioral preference for such signals [6]. Visual signals are essential for communication, and with an expanded toolkit of diverse chemical pigments, insects are the most interesting system to study these visual signals.

Despite the wide range of possibilities, nature finds ways to reuse especially useful patterns and colors for certain signals or signaling systems. To give an example, we can look to the evolution of mimicry, a strategy used often by various insect species. Mimicry describes how those of different species begin to evolve similar, or ‘mimicked,’ phenotypes as a way to amplify a signal—as some are especially valuable to many insect species. Aposematic coloration, a warning signal to predators to alert them of toxicity (or just of bad taste), holds much value as a phenotype to mimic. Many species take up similar signals like these, not because they think it's pretty, but because an evolutionary force selecting for these traits pushes species towards congruence with these patterns and colors—all driven by maximizing fitness, the success rate at which genes are passed onto the next generation.

 Limenitis archippus (Viceroy, Left) Danaus plexippus (Monarch, Right)

What has been more elusive to our understanding, however, isn’t why these phenomena evolve—but how. Recent research suggests that certain natural instances of pattern mimicry can be explained by a single master regulator gene; optix, a homeobox (a family of proteins, all of which include a characteristic ‘homeobox’ domain) transcription factor, is responsible for the phenotypic expression of red patterning in Heliconius butterflies [7]. What makes this case stand out is the repeated evolution of optix in multiple different species to give rise to a seemingly infinite number of patterns and variations.

Heliconius erato amazonia (Red Postman, Left) and Heliconius melpomene melpomene  (Postman, Right)

This isn’t as simple as directly mutating the gene sequence repeatedly—no, that’s much too random and deleterious to consistently act as a driving force for evolution. The most common form of molecular and developmental change in evolution comes from regulatory tweaks. Similar to yellow, nature can twist parameters on where, when, and how these genes are expressed, all through changes in cis-regulatory expression. Cis-regulatory elements include enhancer and repressor regions in DNA—where important transcription factors bind in order to activate or hinder gene expression (the process of turning DNA sequences into RNA, and ultimately [but not always] protein). As mentioned, these cis-regulatory elements truly control every advanced direction for a gene during development [8]. Variations in DNA sequence, transcription factor or cofactor accessibility, and epigenetic modifications are all examples of factors that influence enhancer or repressor activity. These changes, in turn, regulate the expression of genes like optix, determining its activation in specific scale types or regions of the butterfly wing. Similar to yellow, evolution has a way of shifting not just parameters, but function, as a result. While the optix gene sequence remains the same in other butterfly & moth species, it has been shown to be absent in the production of red pigment. Understanding the mechanisms behind the gain and loss of regulatory specifications is a compelling way to study evolution, and researchers are only beginning to pull back the film in this field.

Sources:

[1] O’Hanlon, et al. 'Visual communication', Insect Behavior: From Mechanisms to Ecological and Evolutionary Consequences Oxford Academic, 2018

[2] Stella D., Kleisner K. Visible beyond Violet: How Butterflies Manage Ultraviolet. Insects, 2022 

[3] Gompel N., Prud'homme B., Wittkopp P.J., Kassner V.A., Carroll S.B. Chance caught on the wing: cis-regulatory evolution and the origin of pigment patterns in Drosophila. Nature, 2005

[4] Wang M., et al. Intracellular environment can change protein conformational dynamics in cells through weak interactions. Sci Adv., 2023

[5] Hegde SN, Chethan BK, Krishna MS. Mating success of males with and without wing patch in Drosophila biarmipes. Indian J Exp Biol. 2005

[6] Jiggins, C. D., et al. Reproductive isolation caused by colour pattern mimicry. Nature, 2001

[7] Robert D. Reed et al., optix Drives the Repeated Convergent Evolution of Butterfly Wing Pattern Mimicry. Science, 2011

[8] Katya L Mack, Tyler A Square, Bin Zhao, Craig T Miller, Hunter B Fraser, Evolution of Spatial and Temporal cis-Regulatory Divergence in Sticklebacks, Molecular Biology and Evolution, 2023