Why some frogs can survive killer fungal disease | Science News for Students

Why some frogs can survive killer fungal disease

Studying how frogs evolve natural resistance to diseases could help protect their declining species
Jun 6, 2016 — 7:00 am EST
Leopard frog

Lowland leopard frogs make their home in Arizona. Some have evolved immune defenses against a killer fungus that infects their skin. 

William R. Radke/USFWS

Anna Savage hikes through a Florida swamp looking for frogs. She spots a shiny black critter hopping in the swamp grass. It’s a tiny cricket frog, no bigger than a bug. She’s hunting a killer infection — and the few frogs able to survive it.

Learning what makes these frogs special could hold a key to managing the fungal scourge.

Savage is a frog biologist at the University of Central Florida in Orlando. She catches amphibians like this tiny cricket frog to sample their skin cells. She wants to know which ones host a killer fungus. Huge numbers of frogs have been dying from a skin disease that it causes. But the fungus doesn’t kill off every infected frog. Savage is trying to figure out why. She thinks it might have something to do with the animals’ genes.  

Amphibians include frogs, toads, salamanders and newts. And overall, notes Savage, “Amphibians are in trouble.” A recent study showed that populations have been shrinking for more than four in every 10 of the world’s amphibian species. Nearly one-third are threatened with extinction. And that could be bad for the environment. These animals are an important part of the food chain. They eat pesky insects, like flies and mosquitoes. Amphibians, in turn, are an important food source for many birds, reptiles and mammals.

Global loss of wetlands and other critical amphibian habitat may be causing some populations to shrink. A frog disease is causing major declines too. Its source is a single-celled fungus known as chytrid (KIH-trid). Fungal diseases are known as mycoses (My-KO-sees). So this one is called chytridiomycosis (Kih-TRID-ee-oh-my-KO-sis).

In the past few decades, this infection has been turning up all over the world. The fungus lives in wetlands and ponds. No one knows exactly where chytrid first came from. But once it infects their skin, many frogs now sicken and die.

The big question has been why some frogs have been spared. Savage now thinks she has an answer: their immune system.

Helpful mutations guard some frogs

The immune system is the body’s natural defense system against sickness and injury. Savage has been studying how this system differs in frogs that chytrid kills and those that survive. This work started a few years back while she was a PhD student at Cornell University in Ithaca, N.Y. Her first clues came from  a group of lowland leopard frogs that she studied in Arizona. The fungus had infected these animals.

Genes mutate, or change, all of the time. These can lead to variations in immune system genes. The type of changes that helped leopard frogs survive the fungus were becoming more common in some populations, Savage discovered.

She and Kelly R. Zamudio of Cornell University have just shared these new data. They appear in the March 23 in Proceedings of the Royal Society B.

Frogs with genes that allow them to survive the disease will pass those genes on to their offspring, Savage explains. These offspring will also survive the fungus. Eventually, more and more frogs will be the survivors with those genes. This process of genetic change within a population is called natural selection. It’s a driving factor in the evolution of species.

The evolution of major changes within species can take thousands or even millions of years. But when there’s a very strong selective pressure, like the chytrid disease, a species may not have the luxury of millions of years. Frog species that don’t have the right genes or can’t adapt will get wiped out — immediately.

Savage’s research now shows that in some groups of Arizona leopard frogs, the immune system has evolved fast — over a few decades — to deal with the killer fungus. Similar quick-change evolution has been seen in other species that also faced dire threats to their survival.

The findings are important because they show “that in a relatively short period of time, frogs in the wild that didn’t previously have immunity may be able to evolve natural defenses against chytrid,” says Jason Rohr. He’s a frog biologist at the University of South Florida in Tampa. He, too, studies the frog immune system, but was not involved with the new research.

It’s important to protect the places where frogs live, both Rohr and Savage argue. Natural selection won’t work if the population of a species already has shrunk to the point where the protective genes that allow animals to fight the disease are now gone.

Savage is now looking at Florida frogs. So far, no one has reported major frog die-offs from the chytrid fungus in that state. That doesn’t mean the disease isn’t there. Many frogs have traces of the fungus on their skin, she has found. If scientists better understand how frogs in the wild evolve natural defenses against this fungal disease, they might find ways to halt its spread, she says — and protect those frogs that don’t have immunity.

Power Words

(for more about Power Words, click here)

amphibians    A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.

biology    The study of living things. The scientists who study them are known as biologists.

cell    The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chytrid    A single-celled fungus that infects amphibians. It is thought to be one factor behind a growing global decline in amphibian populations.

chytridiomycosis    An infectious disease in amphibians caused by the chytrid fungus.

conserve   To protect, as from loss or degradation.

evolution   A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

evolve    (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs). Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.

extinction   The permanent loss of a species, family or larger group of organisms.

food web    (also known as a food chain) The network of relationships among organisms sharing an ecosystem. Member organisms depend on others within this network as a source of food.

fungus    (plural: fungi) One of a group of single- or multiple-celled organisms that reproduce via spores and feed on living or decaying organic matter. Examples include mold, yeasts and mushrooms.

gene    (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

habitat   The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

immune system   The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

immunity    The ability of an organism to resist a particular infection or poison by producing and releasing special protective cells.

infection   A disease that can spread from one organism to another.

mammal    A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

mutation     (v. mutate) Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.

natural selection   This is guiding concept underlying evolution, or natural adaptation. It holds that natural mutations within a population of organisms will create some new forms that are better adapted to their environment. That adaptation makes them more likely to survive and reproduce. Over time, these survivors may come to dominate the original population. If their adaptive changes are significant enough, those survivors may also constitute a new species.

PhD    (also known as a doctorate) A type of advanced degree offered by universities — typically after five or six years of study — for work that creates new knowledge. People qualify to begin this type of graduate study only after having first completed a college degree (a program that typically takes four years of study).

reptile   Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.

scourge    A person or other organism that inflicts pain on those around it or threatens their success and wellbeing.

swamp    A type of low-lying wetland where water collects. It tends to sustain more trees and woody species than will a marsh.

wetland    As the name implies, this is a low-lying area of land either soaked or covered with water much of the year. It hosts plants and animals adapted to live in, on or near water.

NGSS: 

  • MS-LS1-5
  • MS-LS2-2
  • MS-LS3-1
  • HS-LS2-2

Citation

A.E. Savage and K.R. Zamudio. Adaptive tolerance to a pathogenic fungus drives major histocompatibility complex evolution in natural amphibian populations. Proceedings of the Royal Society B. Vol. 283, March 23, 2016. doi: 10.1098/rspb.2015.3115.

Further Reading

S. Ornes. Fungi as carbon keepersScience News for Students. April 11, 2013.

J. Raloff. Bat-killing fungus is a European importScience News. Vol. 181, May 19, 2012, p. 9.

S. Ornes. Bringing fish back up to sizeScience News for Students. March 24, 2009.

T. Saey. Hitting the redo button on evolutionScience News for Students. February 11, 2009.

T. Siegfried. The man who rocked biology to its coreScience News for Students. February 11, 2009.