Frogs, Under Attack by a Lethal Pathogen
by Jackie Swift
Biodiversity is central to life on earth. Exactly how species evolve is one of the central questions of biology. What traits cause animals and plants to become differentiated? What mechanisms drive the process? What factors cause extinction?
“Evolutionary and ecological processes contribute to the creation of new species,” says Kelly R. Zamudio, Ecology and Evolutionary Biology at Cornell University. “You can think of extinction as the reverse of speciation. They’re really two sides of the same coin.”
Zamudio studies amphibians, especially frogs, combining field work and observation of behavior with genetics and genomics that allow her to glimpse the genetic processes underlying species traits. Recently she and her lab have turned that expertise to studying two virulent fungi of the genus Batrachochytrium, commonly called chytrids, that affect frogs and salamanders. The frog-killing fungus, Batrachochytrium dendrobatidis (Bd), has become a pandemic, spreading throughout the world, while the salamander-specific fungus Batrachochytrium salamandrivorans (B. sal) has only just begun to move out of Asia. Both fungi are devastating.
“They’re affecting huge numbers of species,” Zamudio says. “As far as we can tell, 501 species of frogs out of 8,000 total have been afflicted in some way by Bd — population declines, local extinctions, species extinctions. It’s a really serious biodiversity issue.”
The Evolution of Pathogen and Host — Chytrid Fungus and Frog
Different lineages of Bd have been evolving throughout the world, many for a long time, but the virulent panzootic lineage that has swept through the world has a recent origin. “I’m trying to understand the genetics of this lineage and how it acts,” Zamudio says. “When it goes into the skin of a frog, what genes is it up-regulating and down-regulating? How is it bypassing and evading the frog’s immune response? On the frog side, I want to figure out how frogs potentially are evolving resistance to this increased virulence. It’s a genomic view of this arms race between pathogen and host.”
With a collaboration of scientists from many institutions, Zamudio contributed to an assessment of genetic variability across 240 strains of chytrid collected from all over the world. “We found that Bd probably came out of Asia,” she says. “A lot of frogs in Asia are not susceptible because they evolved with this pathogen. But frogs in other parts of the world that didn’t evolve with it are highly susceptible because they’ve never encountered it before.”
The researchers compared gene expression between an endemic lineage of Bd specific to one location and a nonendemic one in the Americas. They wanted to determine what keeps one lineage confined to a place while the other can spread. “We found upregulation of various types of genes that probably have to do with pathogenicity,” Zamudio says. “The endemic line is much less pathogenic. It can infect frogs but it doesn’t usually kill them. Now that we found these potential virulence factors — things like enzymes that break down proteins and might be involved in the actual process of invading the frog’s skin — we can cross-check and look for them in other Bd lineages.”
“The easiest way to become different from something else is to change who you mate with.”
By understanding Bd, Zamudio and her colleagues hope to contribute to the discovery of a way to stop the fungus before it decimates the world’s frogs. One potential route is to immunize the frogs. “We can’t go around vaccinating frogs with a needle,” Zamudio says. “But there are other ways to vaccinate, for instance, simple exposure. One reason to compare the endemic lines to the virulent lineage is because the endemic lineage could potentially be used as a vaccination agent.”
The Amazonian Glass Frog — Mating and Species Diversity
Along with her work on chytrid fungi, Zamudio also pursues a variety of research looking at mating systems and parental care, especially in amphibians. “Mating systems are important,” she says. “Because if you think of species diversifying to become independent of each other, often that entails some limit on how they reproduce, and mating behavior underlies that. The easiest way to become different from something else is to change who you mate with.”
In one study, with her collaborator Anyelet Valencia Aguilar, a PhD student in Brazil, Zamudio looked at parentage and the mating system of an Amazonian glass frog. The males of the species establish territories where the females deposit their eggs, leaving the males to do all the parental work of hydrating the clutch and guarding it until hatching. The researchers discovered that the more clutches a male had, the more likely he was to attract another female. Some males even increased their odds of attracting females by staying near a foreign clutch of eggs and pretending it was theirs.
“Reproductive strategies like this are part of species diversity,” Zamudio says. “Understanding how a species goes about making these kinds of reproductive and parenting decisions, and how those animals are related allows me to say, ‘This evolved here in this tree of life of frogs and may have been one of the factors that differentiated these two sister species.’”
Becoming a Herpetologist
Zamudio always loved animals. Before she decided to become a herpetologist, however, she had planned to study monkeys and become the Jane Goodall of the New World. As an undergraduate, things took a different turn. Zamudio worked for a herpetologist who was studying lizards, and she found them much more accessible.
“I worked mostly on reptiles at first,” she says, “but I was working in Brazil, which has the richest frog fauna in the world; and I was drawn to their diversity. That’s where all the questions that spark my interest come from.”
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