"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 6, 2014

Ballocephala sphaerospora

This is the second post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2014. This particular post was written by Danielle Mills Waterfield on a paper published all the way back in 1951 on a fungus that infects everyone's favourite cuddly extremophile - the tardigrade (you can read the previous post about bizarre copepods that infect sea slugs here). 
Photo by Bob Goldstein & Vicky Madden
What can survive extreme pressures greater than the bottom of the Mariana Trench, withstand very high levels of radiation, and even survive in the vacuum of space? Is it a bird, is it a plane? No, it is a kind of microscopic organism call the tardigrade.

It is also called the water bear as resembles a kind of adorable multi-legged. furless teddy bear that you can’t really hug because it is too small. The tardigrade has a strong defence against most life threatening circumstances it encounters (dehydrating and dying for a while until conditions improve), but this almost invincible micro-beast is not so resistant to certain types of threats. Like every other living thing, this little creature can fall victim to merciless parasites.

In 1950 Charles Dreshler was given a leaf mold gathered form a roadside near Oxford. He observed the mold under a microscope and grew the sample in a Petri dish. To his surprise he found tardigrades being killed by a parasitic fungus. The fungus he discovered was Ballocephala sphaerospora, a member of the order Entomophthorales which has a name that literally means ‘insect destroyer’. But this fungus infects more than just insects - it also infects worms, mites and even tardigrades.

Firstly the fungal spores can and do stick to anywhere on the cuticle of the tardigrade, this is strange though because there is no evidence of an adhesive surface or other structure on the spores that allow them to stick to a tardigrade, yet they are still able to attach themselves. A while after establishing contact with the water bear, the fungus begins its takeover. The tardigrade's cuticle is still a barrier, but the fungus has a way of bypassing that. The spore develops what is called a ‘germ tube’, a long outgrowth that penetrates into the tardigrade's body.

After the germ tube is inserted into the body, the fungus starts to grow what looks like branches all through the inside of the tardigrade. It continues to spread as it feeds; the branches taking up all the room inside, squishing and crushing the animalcule's organs, and it eventually kills the tardigrade. However, some the branches becomes abjointed and will drift within the body of the host becoming much like a harmless floating husk. This continues until the water bear's organs fail, but now that the host is dead, asexual reproduction can take place!

The branches of the fungus' hyphae grow outwards from within, pushing back out through the tardigrade’s cuticle like a sowing needle and face upwards. From here, the hyphae start growing little bud like structures that fill with more of the tardigrades fleshy fluids for energy until finally, once full, the bud is walled off becoming its own little spore calla a conidia. That little conidia, can start another fungal infection elsewhere by either falling off and lying in wait for an unsuspecting tardigrade to walk into it and stick on, or wait until there are tardigrades nearby and conditions are favourable before falling off. Entomophthorales also have another trick up their sleeve - the little conidia spores have the ability to shoot off into the air via a rupture at their base. This allows the fungi to spread further and find neighbouring tardigrades, restarting the cycle as the fungus continues its reign of takeover.

Drechsler, C. (1951). An entomophthoraceous tardigrade parasite producing small conidia on propulsive cells in spicate heads. Bulletin of the Torrey Botanical Club 78: 183-200.

This post was written by Danielle Mills Waterfield

P.S. For a superb illustration of Ballocephala sphaerospora by Lizzie Harper, click here.

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