Wednesday, March 18, 2015

Arthrobotrys -- a carnivorous fungus!

Source: Society for General Microbiology (www.sgm.ac.uk)
In the fungal lab last week we were introduced to Arthrobotrys oligospora, a nematophagous fungus. When we first looked at our agar-filled petri dishes inoculated with Arthrobotrys, we just saw pretty standard hyphae. But after introducing a population of nematodes into the dish the fungus went through some spectacular changes.

After a day we began to see specialized nematode-trapping structures:

A photo from an Arthrobotrys plate in our lab. This is looking across the surface of the agar, with the arrow pointing out the adhesive networks of hyphae the fungus uses to trap nematodes; they stick up above the surface to ensnare prey.
Source: http://www.uoguelph.ca/~gbarron/index.htm
The "adhesive" on these loops binds strongly to sugar compounds on the surface of the poor nematode who happens into their hyphal booby trap. Some fungi even secrete chemicals that attract nematodes to these snares.

Other nematophagous fungi utilize other specialized trapping structures like constricting rings (shown at right). These rings are made up of three connected cells that swell rapidly when a nematode passes through, squeezing the unsuspecting passerby to death.

So you've captured a nematode -- now you've got to digest it! I'll quote directly from the Society for General Microbiology to explain how the fungus goes about this:
Once ensnared, the fungus pierces the nematode’s cuticle using a narrow penetration peg which swells inside the host to form an infection bulb that the hyphae grow from. Fungal enzymes break down the contents of the nematode and the nutrients are transported elsewhere within the hyphal system for growth or spore production. Growth does not occur at the site of the hyphal trap. This phase usually takes 1–3 days, before hyphae grow out of the cadaver and sporulate.

Pretty wicked, huh? This is what the nematodes on our plates looked like a few days after capture:

The remains of three nematodes that met an unfortunate end in Botany Lab #8. But their lives weren't for naught -- check out the Arthrobotrys conidiophore at top center (marked by the red dot)!
Scanning electron micrograph of nematodes (tan tubes) trapped by the adhesive loops of A. oligospora (source: www.sgm.ac.uk)

Single conidiophore








Conidiophore forest!
All those nutritious nematodes fueled a large asexual reproductive spurt in our Arthrobotrys. At right (top) you see a single conidiophore (asexual reproductive structure), viewed from the side on one of our plates. Right (bottom) shows a group of them.


















And check out the conidiophore "forest" swaying in the breeze! (Sorry for the poor video quality.)

video



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