Breaking the frog fungus code

frog fungus
A graduate student at UC Santa Barbara, Imani Russell is studying the genetics of the fungus responsible for mass amphibian dieoffs. Image: Lobsang Wangdu/NRS

By Kathleen Wong, UC Natural Reserve System

It’s a crisp February morning at Quail Ridge Reserve, and Imani Russell is stalking frogs. Prepared for the chill in a Golden State Warriors ski cap, insulated jacket, and full hiking boots, she is scouting the perimeter of a former cattle pond. She places each foot so carefully that her heavy duty footwear barely raises a rustle from the dried leaves carpeting the ground. She pauses at promising clumps of grass, hummocks of duff, and little shrubs to probe for her quarry with gloved hands.

Over the past year, Russell has become an expert frog hunter. A graduate student at UC Santa Barbara, she’s been capturing frogs and toads at more than a dozen different UC Natural Reserves in order to study a catastrophic wildlife disease. But today, with the mercury hovering in the low 50s and a stiff breeze in the air, the normally ubiquitous amphibians are nowhere to be found.

Russell presses on, undaunted. She knows there’s no time to waste.

frog fungus
Pacific tree frogs (Pseudacris regilla) are among the species being infected by the devastating Batrachochytrium dendrobatidis fungus. Image: Lobsang Wangdu/NRS

Amphibian devastation

Over the past two decades, amphibians around the globe have been decimated by a devastating pathogen. More than 30 percent of the world’s frog, toad, salamander, and newt species have been affected, with consequences ranging from population crashes to outright species extinctions.

The organism responsible is the Batrachochytrium dendrobatidis fungus. Also known as Bd or the chytrid fungus, some scientists feel the fungus is responsible for “the greatest recorded loss of biodiversity attributable to a disease.”

“We just don’t know why it spread as far and as fast as it did,” Russell says. “There’s so much we don’t know about how the infection works or how the hosts respond physiologically or genetically.”

Strangely, not all frogs have responded the same way to the pathogen. Some populations have weathered infection just fine, while others that may even belong to the same species have been decimated.

“I’m trying to figure out the genetic basis of the frog susceptibility to the fungus,” Russell says.

A personal connection

Russell’s interest in conservation and wildlife runs in the family. Her grandfather was a U.S. Fish and Wildlife ichthyologist. “I was always hearing about setting aside land to conserve the desert pupfish and such things,” she says.

After graduating from Mills College in Oakland, she volunteered on three different field ecology projects in Peru and Ecuador and earned a Master’s degree from the University of Michigan before joining Professor Cherie Briggs’ lab at UC Santa Barbara.

“My main passion is conservation of species and understanding wildlife diseases and how they work,” Russell says. “Knowing the life history and observations tell you a lot, but sometimes there are things you can’t see that would make a world of difference in creating disease mitigation strategies.”

Research that makes a difference

Russell wants to obtain information relevant to amphibian populations across California. “Knowing the signs for host susceptibility is the first step for management,” she says. For this reason, she is surveying frogs and toads for Bd at 13 different reserves in the UC Natural Reserve System. These sites range from the Angelo Coast Range Reserve in the north to Boyd Deep Canyon Desert Research Center in the south, and provide a range of different climate conditions and ecosystems.

Over the course of this year, she will return to sample frogs up to three more times at reserves where she finds Bd-positive frogs. She will then compare the genes active in healthy populations of frogs and toads against infected ones living on the same reserve.

“We should be able to see which genes are responsible for the host defenses or susceptibility. Then maybe we can engineer some non-invasive tests for populations threatened by Bd,” Russell says.

Night moves

Researching Bd in frogs involves adopting the schedule of the frogs themselves. While at a reserve, she and her assistants spend the daytime hours scouting field sites such as ponds or streams, then return after dark to do the actual capturing.

“A lot of frogs are just more active at night,” Russell says.

Russell and her student assistants make the most of this odd sampling schedule. “We try to go out a bit before sunset, and wait at our site for the sun to go down. Sometimes we’ll eat dinner there as we wait for the frogs to start calling.”

Many species, such as the Pacific tree frog and most toads, are easy to nab on land with a quick reach of a gloved hand. Others are more wily. Non-native bullfrogs tend to leap into the water at the first sign of danger. To catch these animals, Russell stations students armed with nets in the pond or stream before flushing the animals off the bank.

frog fungus
Russell taking tissue samples from a frog. Image courtesy Imani Russell

Snips and swabs

Russell pops each frog in a goldfish bag, the same kind used to sell fish at aquarium stores, until she’s amassed a few to sample. Then, by the light of her headlamp, she takes two types of samples from every captured animal: a skin swab for Bd, and a tissue sample from the frog itself.

Getting Bd samples is strangely akin to washing a toddler. With a cotton swab, Russell wipes the frog’s sides, thighs, toes, and even the webs of its feet.

The second sample is more intrusive: Russell must clip a couple of millimeters from a toe. This is the only way to ensure she has a good sample of frog tissue. She’ll use half to study the genetics of the frog, and half to isolate and culture the fungus.

frog fungus
Imani Russell sweeps a pond at Quail Ridge Reserve in search of frogs to sample. Image credit: Lobsang Wangdu

A welcome mat for science

Russell has captured as few as four frogs on slow nights, and up to 50 in sessions that have lasted until two in the morning.

This nighttime field schedule is one reason why Russell wanted to work at the NRS. “Since most of the reserves are closed to the public, I feel safe taking my team of undergraduate research assistants out at night.”

In addition, the reserves she is visiting all have accommodations. When a catching session is done, “we can just go to sleep somewhere that’s close by, comfortable, and warm.”

In addition, she felt the reserves were ideally suited to host scientific studies like hers.

“It was so much faster and easier to get permission to catch and take tissue samples from frogs at reserves. I might still be waiting for a permit from various parks,” Russell says.

Linking ecology and genetics

Back at UC Santa Barbara, Russell is chipping away at a mountain of lab analyses.  From the swabs, Russell can detect whether a frog is infected with Bd, and, if so, gauge how heavy its fungal load is. So far, Russell has found Bd on frogs at 13 reserves.

From part of the toe clips, she is sequencing the genes for a component of the frog immune system. MHC class II molecules stick bits of potential pathogens on the surfaces of immune system cells. This stimulates other immune system components to combat the possible infection.

An MHC molecule’s ability to recognize a pathogen is partly genetic. This is why Russell suspects these genes enable some frogs to resist Bd better than others.

Russell will then compare the MHC sequences in frogs with high Bd burdens against those with less pervasive infections. This should help her identify MHC genotypes that are superior at fungus fighting.

Eventually, she will also determine which specific genes are turned on in both the frog and any fungus it might have. She will do this by measuring levels of RNA transcripts. Produced when DNA is “read” to produce proteins, the transcripts reveal the genes re being expressed.  

Russell will do the same gene activity analysis on her fungus samples. Identifying the fungus genes turned on during an infection might open yet another window into ways to disable the pathogen.

A path to treatment

Wildlife managers have been using creative ways to inoculate frogs against Bd. For example, scientists have taken frogs from the Sierra Nevada into captivity, infected them with a less virulent strain of the fungus, and nursed them through the illness until they mount an immune response. Frogs treated this way no longer build up lethal infection loads.

But treating every frog in the state is impossible. Russell hopes her findings will help identify the most vulnerable animals.

“My goal is to have useful information to give land and wildlife managers,” Russell says. “I want to find some easy-to-measure trait and be able to say we should focus our efforts on this population as opposed to that one.”

This is why she is sampling frogs over the course of an entire year. If she discovers that pathogen loads are higher in certain weather conditions or habitat types, that will further help scientists narrow down the populations most in need of assistance.

Ultimately, Russell wants to pursue a career in applied wildlife management involving diseases such as Bd. “I think it’s really important to have people who can take these natural organisms, and be able to apply high-level lab skills to inform these management decisions,” she says. For species in the cross-hairs such as frogs, having someone with Russell’s skills in their corner could mean the difference between life and death.

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