A deadly fungus has driven entire frog populations to the brink of extinction, but some groups bounce back. Scientists have now found that the survivors start building their immune defenses while they are still tadpoles, long before the fungus can strike.
Researchers from University College London, ZSL, and Imperial College London studied common midwife toads in the Pyrenees mountains of France and Spain. The chytrid fungus Batrachochytrium dendrobatidis, known as Bd, has caused catastrophic declines in amphibians worldwide. It attacks the skin of adult frogs and toads, disrupting their ability to regulate water and minerals. Tadpoles are safe because their skin lacks the keratin that Bd feeds on. But once they metamorphose into adults, they become vulnerable and often die in large numbers.
Four Lakes, Two Very Different Outcomes
The team focused on toad populations living around four lakes in the Pyrenees. All four had experienced severe Bd outbreaks. At one lake, the toad population was still collapsing and had nearly disappeared. At the other three lakes, however, the populations had rebounded even though the fungus remained present in the environment. The question was why.
Tadpoles That Grow Up Ready to Fight
The researchers examined antimicrobial peptides, natural chemicals released from amphibian skin that form a key part of their immune system. They discovered that toads from the recovering populations produced these protective peptides much earlier, while they were still tadpoles. By the time they reached adulthood and became susceptible to Bd, their immune defenses were already well established. Toads from the struggling population produced far fewer of these peptides during the tadpole stage, leaving them unprepared once they matured.
Lead author Dr. Phillip Jervis said the study shows that species which have declined heavily from this disease can still recover. They have the tools to fight off infection, but it depends on timing. The disease kills toads and frogs as they turn from tadpoles to adults. Getting mature immunity at the tadpole stage helps these toads survive and the population to continue.
The findings were published in the journal Nature Chemical Biology. The next step for researchers is to investigate what factors prevent these immune systems from maturing early, whether genetic or environmental. The study also uncovered a vast collection of previously unknown antimicrobial peptides that could someday inspire new drugs to fight infections in humans.
