Coordinated adaptive changes in insulin, insulin receptor, and inceptor genes in hystricognath rodents

In the human body, a hormone called insulin acts as a master key, unlocking cells to allow sugar from the blood to enter and be used for energy. This system is so vital that it is usually kept nearly identical across almost all mammals, from mice to whales. The hormone itself, the lock it fits into on the cell surface, and the internal machinery that manages the key are all tightly conserved, meaning they rarely change over millions of years. When this system works correctly, blood sugar stays stable. When it fails, the result is diabetes, a condition where sugar builds up to dangerous levels. Because this biological machinery is so fundamental, scientists generally expect it to remain unchanged. However, nature sometimes takes a different path, and one group of rodents has evolved a version of this system that is radically different from anything seen in other mammals.

These rodents, known as hystricognaths, include familiar animals like guinea pigs, capybaras, and degus, as well as less common relatives like the naked mole-rat. For decades, scientists have known that these animals produce a strange version of insulin. Unlike the insulin found in humans or mice, which forms neat clusters for storage, the insulin in these rodents is structurally unique and significantly weaker. It is so weak that it has only a tiny fraction of the power to lower blood sugar compared to standard mammalian insulin. Yet, these animals do not suffer from diabetes; they maintain healthy blood sugar levels. This survival implies that the rest of their system must have changed to compensate for the weak hormone. A new study by researchers in Uruguay and Chile has now mapped out exactly how this entire system was rewired, revealing a step-by-step evolutionary story where the hormone, its receptor, and its regulators all changed together to keep the animals alive.

The researchers began by looking at the genetic code of these rodents to see how their insulin genes had changed. They found clear signs that the insulin gene had been under intense pressure to evolve, with specific changes accumulating in the parts of the molecule that are supposed to lock onto the cell receptor. These changes explain why the hormone is weak and why it cannot form the storage clusters seen in other animals. But the team realized that a weak hormone alone would not be enough to keep blood sugar stable. They needed to find out if the "locks" on the cells and the "managers" that control the system had also changed.

To answer this, the scientists examined the genes for the insulin receptor, the protein on the cell surface that catches the hormone, and a newly discovered protein called Inceptor, which helps regulate how much insulin the body sees. They found that the gene for Inceptor had also evolved rapidly, suggesting it plays a major role in this new system. The story for the insulin receptor, however, was more complex. While the gene itself showed some signs of change, the most dramatic shift was not in the genetic code of the protein itself, but in how the gene was read. In most mammals, the insulin receptor gene can be read in two ways: one version is used during embryonic development, and a different version, which includes a specific extra section of code, is used in adults to manage blood sugar. The researchers discovered that in the hystricognath lineage, the switch to the adult version had been broken.

The study traced this breakdown to a specific family of rodents called Caviidae, which includes guinea pigs and capybaras. In these animals, the genetic instructions for the adult version of the receptor had been silenced. In the capybara, the gene was still present but was being read in a way that skipped the adult section entirely, leaving the animal with only the embryonic version of the receptor in its liver. In the guinea pig, the situation was even more extreme. The genetic code for that specific section had been so damaged that it was effectively dead, a process scientists call pseudogenization. A single missing letter in the DNA code meant the cellular machinery could no longer even see the instructions to build the adult receptor. As a result, guinea pigs rely completely on the embryonic version of the receptor to manage their blood sugar, a state that is usually temporary in other mammals.

This loss of the adult receptor created a new problem: the embryonic receptor is not as good at handling blood sugar, and the weak insulin hormone was still circulating. To solve this, the animals evolved a second layer of compensation. The researchers found that guinea pigs had started producing insulin directly in their liver, an organ that normally only receives insulin from the pancreas. This local production likely helps flood the liver with enough hormone to overcome the weak signal. But flooding the liver with insulin could be toxic, so the animals also massively increased the production of the Inceptor protein. This protein acts as a brake, helping to remove excess insulin and prevent the cells from being overwhelmed. The capybara took a slightly different path; it did not produce insulin in the liver, but it still lost the adult receptor, relying on the embryonic version while keeping its insulin production strictly in the pancreas.

The researchers pieced together the history of these changes by looking at the DNA of many different species in the group. They found that the loss of the adult receptor was an ancient event that happened before the guinea pig and capybara split apart. Over time, the genetic damage to the receptor gene worsened. In the capybara, the gene was still there but silenced by complex chemical signals that told the cell to skip it. In the guinea pig and its wild relatives, the gene had suffered a physical break that made it impossible to read. This step-by-step degradation shows that the animals did not just randomly mutate; they followed a clear evolutionary path where the system was dismantled and rebuilt piece by piece.

The study also looked at the physical shape of the proteins involved. Even in the animals that still kept the adult receptor, the shape of the lock had changed to match the weak key. The researchers used computer models to show that the surface of the receptor had been altered to neutralize electrical charges, allowing it to bind to the strange, weak insulin of these rodents. This structural remodeling happened alongside the genetic changes, ensuring that the system worked as a whole.

This research redefines how we understand the evolution of blood sugar control. It shows that when a critical hormone changes, the entire system can be reorganized in surprising ways. The guinea pig, for example, has evolved a system where the liver makes its own insulin and uses a different type of receptor, a combination that would be fatal in most other mammals. The capybara and the degu show intermediate steps in this process, proving that evolution can take different routes to the same goal. The findings suggest that the ability to survive with a broken or altered system is not just a matter of luck, but the result of coordinated changes across multiple genes. By studying these rodents, scientists gain a clearer picture of how flexible the human body's metabolic systems might be, and how different combinations of genetic changes can lead to the same physiological outcome. The work highlights that evolution is not just about making things stronger or faster, but about finding new ways to make a broken system work again.