Amphibian Chytrid Fungus

AMPHIBIAN CHYTRID FUNGUS (Batrachochytrium dendrobatidis)

Image modified from Wikipedia.
Original image by Alex Hyatt (CSIRO)

     Hey, this isn't an animal. No matter. This is fine for the following reasons: 1) it's relevant for amphibian conservation, and amphibians are animals; 2) when you look at the diversity of all eukaryotic organisms, fungi and animals are close enough that they might as well be the same thing; 3) I already did one about a plant a while ago so who cares; 4) get off my back

Appearance

Take a second to picture a fungus. Unless you're a total fucking weirdo, the first thing that came to your mind was probably something like a mushroom, right? That makes sense, because most of the fungi we knowingly interact with on a regular basis are mushrooms (or yeasts, but don't get me started on fucking yeasts). They're big, they're visible, they're recognizable, and importantly, they don't move around a whole lot so you can get a pretty big look at them. Batrachochytrium dendrobatidis, or B-dendy as it shall henceforce be called because I don't want to type bahuidfpoi densposfu anymore, is the complete opposite of that. It's small, it's invisible to the naked eye, it doesn't really look like anything, and during part of its life cycle it can move. B-dendy is one of the types of fungus that has a flagellated zoospore stage, and can swim around in the water like sperm. This is one of the reasons why it's a major pathogen of amphibians - it can just fuck around in a pond for a while, which is also where amphibians like to be.

B-dendy also has a colonial multicellular stage. This means that a bunch of cells will get together, but each cell will remain independent and capable of going through its life cycle on its own, not unlike the fucking Thing from The Thing.

Points: 0.5/1, one point awarded for being weird, one half point deducted for being creepy

Behaviour

B-dendy is best known for causing chytridiomycosis, an often lethal disease of amphibians. In adult amphibians, the fungal thallus (multicellular life stage) grows inside the host's skin cells. All amphibians use cutaneous respiration (fancy science term for breathing through their skin) to some extent, so you can imagine having a fungus growing inside your skin would not be a great time. Also, amphibian skin is really, really thin and doesn't provide a whole lot of protection for the inside bits, so it's pretty easy for the fungus to just fucking go to town on their internal organs as well.

The real kicker here, though, is this: the reason b-dendy attacks amphibian skin cells is to feed on sweet, sweet keratin, a special type of protein. You've probably heard of keratin from shampoo commercials, because keratins are found in human hair (and fingernails, and skin). But. The fungus doesn't actually need keratin to survive. It can grow perfectly fine on an agar plate with different nutrients. So it's going around wrecking amphibians' shit because it feels like, I guess.

Points: 0/1 for being a dick about it

Distribution

At the time of its description in 1999, b-dendy was already found worldwide, or at least, wherever amphibians are found. That is to say, mostly in the southern hemisphere. As of 2016, chytridiomycosis was known to affect at least 700 species of amphibians, or about 14% of all amphibian species. If that doesn't sound bad to you, be aware that that number doubled between 2009 and 2016. Granted, the disease is caused by two different species of fungus, but they're both pretty much the same thing.

It's considered an invasive species everywhere, because nobody knows where exactly it came from. There are some records of what might be chytridiomycosis outbreaks around the turn of the century from both Brazil and China, which clears up nothing.

Points: 1/1 for looking out for number one and being fantastically successful

Ecology

Chytridiomycosis is one of the biggest factors in global amphibian population decline and species loss, contributing to the Holocene/Anthropocene mass extinction event that we all have the great pleasure of getting to live through. To emphasize how depressing that is, amphibians made it through the end-Cretaceous extinction event (the one that took out the dinosaurs), the end-Triassic extinction event (the one that took out the rauisuchians), the end-Permian extinction event (the "Great Dying" that took out everything that wasn't bolted down), and the end-Devonian extinction event (the one that took out the placoderms), and yet they're getting hardcore fucked by a glorified yeast. So far, an estimated 90 amphibian species have gone extinct because of the fungus, and almost 400 species have declined to the point that they will probably go extinct within our lifetime. Bear in mind that that's out of 700 or so species that are known to carry the fungus.

You may well ask, "won't it just tire itself out? If it kills all of its hosts, then maybe there will be some uninfected frogs and whatever that are spared this horror." But b-dendy is smart. It also infects the keratinized regions of other animals, like birds and crustaceans - particularly waterfowl and freshwater crustaceans that co-occur with amphibians. This is brilliant because birds can cover a lot more ground than a crayfish or a salamander and therefore act as great carriers of the disease, and because even if all the amphibians in the world get wiped out, b-dendy will persist.

Points: 1/1 for being a genius bastard

Evolution

B-dendy is one of the only chytrid fungi that is pathogenic to vertebrates, and its closest relatives are either plant pathogens, or regular old saprophytes (these are like your typical fungus that hangs out in the dirt and munches on decaying organic matter). So basically, b-dendy saw an opening and went for it. It's also evolved a similar array of infection-associated genes as the fungus that causes yeast infections in people, despite them not being closely related at all. And if there's one thing that we love over here on this stupid blog, it's convergent evolution. I have literally no choice but to stan.

On the other hand, some species of amphibians seem to be immune or at least resistant to the disease. One really gross study suggested that maybe some amphibians have proteins on their skin that disrupt the fungus' ability to take hold. This would be pretty advantageous for the amphibians, and would make b-dendy look like a chump.

Points: 0.5/1

Life History

Like all fungi, b-dendy has a life cycle that is incomprehensibly strange to us mammals, and as such, is to be treated with fear and suspicion. I'm only sort of kidding. Fortunately for you, as far as fungi go, b-dendy's life cycle is actually fairly simple because it doesn't have a sexual stage - that's right, b-dendy does not fuck.

So one of its life stages is that zoospore I talked about earlier, which you can imagine as a single, lonely sperm swimming around in the water. When one of those sad, lonely sperms lands on something delicious, like a frog, it forms a cyst and loses its tail (aka, the flagellum). It them grows rhizoids (the single cell's equivalent of roots) and starts reproducing asexually until it grows into the multicellular structure called a thallus. The thallus can then produce more zoospores, and the zoospores can also divide into even more zoospores if the occasion calls for it. So, you know, even though it doesn't get it on like Donkey Kong, b-dendy has more options for making more of itself than those of us constrained to reproduce by pivmo sex.

Points: 0.5/1

Interaction with Humans

B-dendy and people should be best friends for two reasons.

Number 1, even though we don't know where this thing came from originally, it's pretty likely that human movement has facilitated its spread across the planet. Sure, it can go on birds, and birds can go far, but like... think about what's more realistic here - birds that just happen to be carrying the fungus moving between continents, or people, who go everywhere, all the time, doing it? Also there's some evidence to suggest that the pet trade in particular helps b-dendy get around, because then people are literally taking amphibians from one part of the world, and moving them to a different part of the world. Maybe some of them escape, maybe some of them get released, maybe some kid is playing with their infected pet salamander and don't wash their hands before touching an outside salamander. Who knows. But it seems feasible.

Number 2, like I mentioned earlier, people... also... have keratins? "Keratin" is a very large family of proteins that do a similar job, and the keratins in lizard scales, for example, are very different from the keratins in human fingernails. However, amphibian keratins and mammalian keratins are fairly similar and it wouldn't be that much of a stretch for pathogenic chytrid fungi to move to people. Granted, we don't breathe through our skin, so human chytridiomycosis probably wouldn't be any worse than toe fungus, or that yeast that lives on your scalp, but still. It could happen.

Points: 0.5/1

Total Score: 4/7   B-dendy is okay

Further Reading

• CABI Invasive Species Compendium: Batrachochytrium dendrobatidis (Bd) [link]
• Joneson S, Stajich JE, Shiu S, Rosenblum EB. 2011. Genomic transition to pathogenicity. PLoS Pathogens. 7(11): e1002338 [open access]
• Lips KR. 2016. Overview of chytrid emergence and impact on amphibians. Philosophical Transactions of the Royal Society London B Biological Science. 371(1709) [open access] 
• Longcore JE, Pessier AP, Nichols DK. 1999. Batrachochytrium dendrobatidis gen et sp nov., a chytrid pathogenic to amphibians. Mycologia. 91(2): 219-227
• McMahon TA, Brannelly LA, Chatfield MWH, Johnson PTJ, Joseph MB, McKenzie VJ, Richards-Zawacki CL, Venesky MD, Rohr JR. 2013. Chytrid fungus Batrachochytrium dendrobatidis has nonamphibian hosts and releases chemicals that cause pathology in the absence of infection. PNAS. 110(1): 210-215. [open access]
• Rosenblum EB, Voyles J, Poorten TJ, Stajich J. 2010. The deadly chytrid fungus: a story of an emerging pathogen. Plos pathogens. 6(1): 1000550
• Odd organisms, fungi life cycle [link]
• Stokstad E. 2019. This fungus has wiped out more species than any other disease. Science. [link] 
• Vandebergh W, Maex M, Bossuyt F, Van Bocxlaer I. 2013. Recurrent functional divergence of early tetrapod keratins in amphibian toe pads and mammalian hair. Biology Letters. 3(9) [open access] 
• Van Rooij P, Martel A, Haesebrouck F, Pasmans F. 2015. Amphibian chytridiomycosis: a review with focus on fungus-host interactions. Veterinary Research. 46(137). [open access] 
• Woodhams DC, Ardipradja K, Alford RA, Marantelli G, Reinert LK, Rollins-Smith LA. 2007. Resistance to chytridiomycosis varies among amphibian species and is correlated with skin peptide defenses. Animal Conservation. 10(4): 409-417 [open access]