Selfish genes are rogue genetic elements. Unlike common genes that are selected because they confer an advantage to the organism, selfish genes propagate in nature by bending inheritance in their favor. One particularly ruthless type, called toxin–antidote (TA) elements, poisons the mother’s eggs so that only those embryos that inherit the same element—and can produce the antidote—survive. 

In a new paper in Nature Ecology & Evolution, researchers in Alejandro Burga’s lab at IMBA have traced the evolutionary origins of a family of toxin–antidote elements in the nematode Caenorhabditis tropicalis. Their work sheds light on how “selfishness” can evolve from normal cellular genes—solving a long-standing chicken-and-egg problem in evolutionary biology. We sat down with Alejandro Burga and first author Polina Tikanova to discuss the implications of their findings. 

What exactly are toxin–antidote elements, and why are they interesting? 

Alejandro Burga: When we talk about selfish genetic elements, most people think of transposons, DNA “parasites” that rely on copy-pasting themselves across the genome to survive. But toxin–antidote elements are different. They’re like a genetic blackmail system: a mother carrying the TA element loads a toxin into all her eggs, and only those embryos that inherit the matching antidote survive. It’s a brutal yet efficient way for a genetic element to ensure it’s passed on. 

Polina Tikanova: A mother “poisoning” her offspring sounds counterintuitive, but the logic is genetic, not maternal. Of course, genes do not have goals nor intentions, but, to put it simply, the TA element doesn’t care about the worm; it only cares about perpetuating itself. Or, as an evolutionary biologist would say, the existence of these elements is simply the natural consequence of the fact that the laws of inheritance can be bent. That’s what makes these elements so fascinating: they’ve found ingenious ways to distort inheritance in their favor, and yet, there is ample evidence they keep evolving independently across many species of animals, fungi and plants. 

Why was their origin such a puzzle? 

Burga: The big question has always been: how could such a system evolve in the first place? The toxin is lethal without its antidote, and the antidote is useless without the toxin. It’s a chicken-and-egg problem. If one appears without the other, it should be immediately eliminated by natural selection. So, how do you get both at once? 

So, what did you find in C. tropicalis? 

Tikanova: We discovered three toxin–antidote pairs that all trace back to the same ancestral gene: the fars-3 gene, which encodes part of the phenylalanyl tRNA synthetase—an enzyme essential for life in every organism. These toxins arose through gene duplication, meaning that a harmless copy of fars-3 was freed from its original role and started evolving new properties, which turned out to be toxic for the individual. 

Burga: What’s remarkable is that the antidotes to these different toxins also share a common origin. They’re all derived from F-box proteins, a family of proteins that normally tag other proteins for degradation. So, the antidote works by binding to the toxin and sending it to the cell’s waste bin. 

The key insight came when we realized that the simplest way to explain these observations is that, long before any of these proteins became toxic, the ancestral F-box protein that gave rise to the antidotes was already capable of interacting with the tRNA synthetase. That pre-existing link meant that when the gene duplicated and mutated, the antidote was already there, ready to neutralize any harmful versions that might arise. 

That’s a clever solution to the chicken-and-egg problem. 

Burga: Exactly. In evolution, we call it pre-suppression. Because the antidote’s ancestor was already able to bind the enzyme, it could buffer the damage from new, slightly harmful mutations in duplicated copies. Those “proto toxins” weren’t eliminated, which gave them the evolutionary space to evolve into true selfish elements. 

Tikanova: We believe this phenomenon is an unintended side effect of the role F-box proteins play in nematodes, which could be related to recognizing and eliminating proteins from pathogens. And, like our own immune system, this could lead to mistaking “self” for “non-self.” Nematodes, or round worms, have thousands of F-box proteins that evolve very quickly, and sometimes they might end up targeting the organism’s own proteins. In this case, that misrecognition set the stage for TAs to emerge. 

Burga: Another interesting aspect is that, for a TA element to evolve, the genes encoding the toxin and the antidote need to be very close to each other within the genome, so they can be inherited together. It’s this combination of pre-existing interaction and genetic proximity that sets the conditions for a TA to emerge, as we could show in our model system. 

How does this change how we think about evolution? 

Burga: We often think that evolution is only driven by positive selection, choosing those genetic sequences that encode useful traits for the individual’s survival and reproduction. But our findings suggest that, sometimes, complexity arises through neutral or even accidental steps—what we call constructive neutral evolution. Here, an initially harmless interaction between two proteins allowed a complex interdependent relationship to evolve later. And chances are, similar forces were at work hundreds of millions of years ago, shaping the core molecular machines inside our very own cells. 

Tikanova: It also shows that the same mechanisms that protect the genome—like protein degradation or immune surveillance—can give rise to new selfish genetic elements that work against the individual or its progeny. 

What’s next for your research? 

Tikanova: We’re now investigating how exactly these toxins harm the embryo. We want to understand what genetic changes make proteins gain these toxic functions, what their molecular targets are, and how they affect embryonic development.  

Burga: And from an evolutionary perspective, other work suggests that similar mechanisms might operate in plants, insects, or even mammals. In fact, analogous selfish elements are also present in wild mice. However, since each one of those elements has evolved in a unique way, identifying them is challenging. Our research could lay the blueprint to understand how toxin-antidote elements arise, making it easier to find and study them in the future.