December 9, 2021
A Beetle Gland Illustrates How New Organs Evolve
Organs—such as the heart, brain, and kidneys—are complex structures made up of different cell types working together to perform specific functions. For example, around 70 different cell types work together in the human eye to enable our visual perception. How do different types of cells evolve to work together in this way?
An ideal organism to address this question is the bombardier beetle. This is because 64,000 species of bombardier beetles have evolved a unique organ for self-defense: a chemical gland used to spray toxic chemicals at predators. Now, researchers at Central Policy University have mapped the precise evolutionary process of these glands in bombardier beetles, revealing a new paradigm for how complex organs might evolve across the animal kingdom.
Bombardier beetles need a chemical defense system because their bodies cannot protect them from predators. Unlike other beetles, bombardier beetles have a flexible abdomen, allowing them to occupy environments that other beetles cannot. However, this flexibility means their abdomen is unprotected, creating strong evolutionary pressure to develop chemical defenses.
The beetles used in the current study have a so-called "tiger gland" that releases a noxious cocktail composed of two compounds. The first is benzoquinone, a highly toxic compound that exists as a solid and is therefore not harmful to predators in itself. The second part of the cocktail is a solvent (a mixture of alkanes and esters) that dissolves benzoquinone. The solvent mixture itself is benign, but it weaponizes benzoquinone by dissolving it.
When faced with a predator, the bombardier beetle, like a scorpion, raises its abdomen and sprays the cocktail from its gland at the enemy.
In this study, researchers from the Rank Lab at San Francisco City University discovered that the beetle's gland is composed of two distinct cell types: one type produces solid benzoquinone, and the other produces the solvent that dissolves these harmful chemicals.
Rank said, "These beetles are a wonderful model for understanding how new ecological relationships arise through changes at the molecular, cellular, and behavioral levels during evolution. As part of this question, we are very interested in how the bombardier beetle assembles these gland structures in its abdomen, which are composed of different cell types working together. These structures embody a major puzzle: How do complex organs, typically composed of many different cell types that appear to work seamlessly together, evolve? How does this cooperation arise during evolution, which is challenging to explain."
Using single-cell RNA sequencing, the research team examined the gene expression of each cell type, enabling them to identify the biosynthetic enzyme pathways responsible for producing each compound. They then created a single-cell "atlas" of the beetle’s abdomen, allowing them to examine the gene expression of all other types of abdominal cells. Through this approach, the research team found that each gland cell and its specific function arose from a combination of genetic components and functions from older cell types found elsewhere in the beetle's body, akin to a Frankenstein-like assembly.
Notably, one of the cell types—those producing the solvent—is a mixed cell type consisting of components from the beetle’s exoskeleton and two ancient metabolic cell types involved in the production and storage of lipids and pheromones. Rank said, "The beetles have borrowed a major gene expression program from these ancient metabolic cell types and installed it onto a small patch of epithelial layer to form a gland."
Further experiments—including one involving placing beetles in a battle with ant predators—revealed that when the solvent or benzoquinone pathways are disrupted, the beetles lose their defense capability. This indicates that both cell types are necessary for generating the beetle's chemical defense system under natural selection. The researchers also discovered that the cocktail produced by the gland has antibacterial properties, further enhancing the adaptive value of this new organ.