Fluctuating Environments Stabilize a Self-Destructive Survival Strategy in Yeast

A research group led by Tetsuhiro Hatakeyama from the Earth-Life Science Institute (ELSI) at Institute of Science Tokyo have revealed how a seemingly self-destructive survival strategy could persist in yeast for hundreds of millions of years.

By combining population-dynamics theory with comparative experimental observations, the team found that alternating periods of nutrient-rich conditions and starvation can stabilise this unusual behaviour over evolutionary timescales. The findings were published in the Journal of the Royal Society Interface.

The study addresses a long-standing puzzle in evolutionary biology. Budding yeast and fission yeast diverged approximately 300 to 600 million years ago, yet both retain the same "latecomer-killing" strategy and use the same autotoxins. Why such a costly behaviour has been conserved across such distant species has remained unexplained.

When glucose becomes scarce, yeast cells release toxins into their surroundings. Cells that have already adapted to these toxins survive, while genetically identical cells arriving later, before they have adapted, are killed. This phenomenon, known as latecomer killing, allows cells that have already adapted to a starvation environment to exclude late-arriving, toxin-sensitive competitors, even when those competitors are genetically identical clones.

At first glance, however, the strategy appears evolutionarily unstable. Both toxin production and resistance carry costs. Evolutionary theory predicts that "cheaters," cells that retain resistance but avoid the cost of producing toxins, should eventually outcompete toxin-producing cells. If so, the system should disappear over time.

To investigate this paradox, the researchers developed a population-dynamics model that simulated yeast populations under different environmental conditions.

The results revealed that neither constant nutrient-rich conditions nor continuous starvation could maintain the toxin-adaptation system. Instead, evolutionary stability emerged only when the environment alternated between long nutrient-rich periods and shorter episodes of starvation.

During nutrient-rich periods, cells with the toxin-adaptation system behave much like toxin-sensitive cells, while cheaters still pay the cost of maintaining resistance and are gradually removed from the population. When starvation returns, toxin-adaptation cells switch to producing toxins and resisting them, enabling them to eliminate toxin-sensitive cells. In this way, long nutrient-rich periods suppress cheaters, while shorter starvation episodes suppress sensitive cells, together preserving the strategy over long evolutionary timescales.

The surprising result was that neither constant abundance nor constant starvation could maintain this system. Only when the environment switched between the two did the toxin-adaptation strategy become evolutionarily stable. This shows that long-term environmental fluctuations can shape the survival of biological strategies that would otherwise disappear."

Tetsuhiro Hatakeyama, Earth-Life Science Institute (ELSI), Institute of Science Tokyo

The findings suggest that environmental fluctuations can play a much greater role in evolution than previously recognised. Rather than simply selecting the fittest strategy under a single condition, changing environments can preserve complex ecological strategies that become advantageous only through the alternation of different conditions.

The team also compared yeast species across the phylogenetic tree. Budding yeast and fission yeast, which diverged approximately 300 to 600 million years ago, both show latecomer killing and use the same autotoxins. In contrast, two yeast species more closely related to budding yeast did not show clear evidence of latecomer killing. This suggests that whether a species retains or loses the system may reflect its long-term ecological history rather than phylogenetic relatedness alone.

The theory predicts that similar toxin-adaptation systems should be found preferentially in microorganisms that inhabit environments which are generally nutrient-rich but occasionally experience starvation. Testing this prediction through environmental surveys and laboratory evolution experiments could reveal whether comparable strategies occur in bacteria and archaea.

More broadly, the work raises the possibility that these systems preserve clues about a species' long-term ecological history. Understanding how cooperation, competition and adaptation evolve under changing environmental conditions may also provide insights into major evolutionary transitions, including the emergence of multicellular life.