Fearless fungi

I had to realize, we had an extended break so there was no class this week either. Therefore, I'll tell you about an exciting story I've read. My flatmate wants to be a mycologist (a biologist whose main focus is on fungi) and he drew my attention to a phenomenon which is in connection with microbiology. As this discipline deals with almost anything that is on the 'small scale' there are many fungi within our scope.

Fruit body of the Amylostereum areulatum. Source.

If you have read the article about the autumn leaves on Small Things Considered, this topic will not be totally new. What I intend to present is another way of symbiosis between an animal and a tiny organism. There is a certain wasp called the Sirex noctilio  which has much to do with fungi, namely the Amylostereum areolatum. As all insects, this Sirex goes through metamorphosis, it has clearly distinctive life stages in which the animals differ completely in their appearance. First they lay their eggs which later develop to larvae (just think of a caterpillar) vaguely said 'after a while' these larvae create a layer around themselves and form a pupa. As the animal inside undergoes many structural changes, this berrylike thing hatches and the adult wasp, moth, etc. reveals itself. We call the adult insects imagos. 

Tunnels made by the larvae. Source.

So, the imagos of this species lay their eggs with a special tool, an ovipositor (an egglaying tube) into a tree's trunk. There is a special sack in the imago's (females only) body which is connected to the tube where the eggs run out of the insect's body. This sack is the so called mycetangium containing fungi cells. As the eggs 'roll out' into the tree trunk, these fungi attach to them. There are many cases when the imagos create tunnels inside the tree, but now it is different. It is the  Sirex larvae that make their way inside the tree. However, they require the presence of the Amylostereum because they cannot digest cellulose and lignin (both are components of the plant cell wall). This shouldn't be new for you, just consider herbivorous animals. They also have many microorganisms dwelling in their guts, in order to digest cellulose. Furthermore, we don't need to go that far! We humans have a plethora of bacteria within our intestinal tract for similar kinds of purposes. So! Here, it is the other way around, these digestion helper guys are outside (!) the animal. As the larva crawls its way in the tree, it spreads the fungi onto the tunnels' walls. As the fungi begins the degradation of the tree tissue the tunnels become full of accessible food for the larvae.

It is interesting that the imagos have nothing to do with fungi during their lifetime, so every female must gain a fungi population within their body in their larval stage. So, during the pupal stage and until egg laying they keep their fungi fellows in their mycetangium (which produces some essential mucus for keeping the fungi alive). 

Sirex noctilio. Source.

But why is this good for the fungi?? The tree bark serves as a fence for the plant in protecting it from diseases. This ovipositor tube of the insect is a perfect tool for delivering not just the eggs but the fungi straight into the inner part of the tree trunk. Moreover, these tunnels serve as an ideal habitat for the Amylosterem, lacking any competitor fungi species. Think back to our lab practice, there were many times when we had the goal to create populations of only ONE certain microbe species. Basically that is what the Sirex does for the Amylostereum; they are clever, aren't they? 

A. A Sirex imago
B. This is a part of the wasp's abdomen,
containing the mycetangium (round bag) and the outer 'stick'
is the ovipositor which is pierced into the tree trunk.
C. The mycetangium from a closer look. The inner vesicle contains
and produces the mucus, in the outer part you can see fungi
cells
D. Fungi cells in themselves.
This picture is from the book: Erzsébet Jakucs and László Vajna:
Mikológia