WAR  of  the  MICROWORLDS  

How do I kill Thee?   Let me count the Ways!   (# 19) click here for full list

Haptoglossa mirabilis:   This fungus produces a very complex attack cell called the 'Gun Cell'.  The Haptoglossa gun cell is the most complicated cell known in the fungi and arguably more sophisticated than the 'nematocyst'. Using a harpoon-shaped projectile, it blows a  hole  through the wall of the victim then shoots a microhypodermic through the hole an injects an infective sporidium into a rotifer in a fraction of a second.   Of particular interest and significance is that some Haptoglossa species can attack rotifers, nematodes AND tardigrades.   This is surprising for such a sophisticated parasite and perhaps reveals that these three groups are closer than we believe under the skin!    OOMYCOTA?  For more information on this species see Barron, G.L. 1987.  Mycologia 79: 877

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How does it do this?

When a nematode touches a gun cell, a harpoon-shaped projectile is shot off and blows a hole through the cuticle of the nematode. An internal tubular device in the gun cell everts through this hole and forms a hypodermic-like structure that penetrates into the body of the animal. The osmotic power of the basal vacuole in the gun cell is very high. It takes in water rapidly and pumps the protoplasm and nucleus from the gun cell through the hypodermic and into the body of the host to form an infection sporidium. It takes about 1/10 sec from the time the nematode is touched until the infective sporidium is released into the body of the host.

This sporidium grows at the expense of the host to form a cylindrical "thallus" that feeds on the contents of the body which are completely consumed within a few days. The protoplasm in the infection thallus now cleaves to form zoospores that escape through exit tubes and swim off to encyst elsewhere and subsequently germinate to form clusters of gun cells.

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Harpoon-shaped Projectile in Housing of Gun Cell
of Haptoglossa mirabilis
TEM:  Jane Robb, University of Guelph)

The harpoon-shaped projectile is especially remarkable.   The head of the harpoon is laminated. This means that it is compressible. As it is pushed up the barrel of the gun it will fit tightly and prevent leakage to maintain maximum muzzle velocity. As it emerges from the muzzle it pierces the cuticle of the nematode. At the head emerges it will 'decompress' and make a hole wider than the width of the bore. This will facilitate penetration by the everting tubular "hypodermic".                                               For more information on this topic see Robb, J. and Barron, G.L. 1982.  Science 218: 1221

The gun cell is anchored to the substratum by a mucilaginous glue. It also has a swollen base. When the base is anchored the business end of the gun cell is then tilted upwards at an angle of about 30 degrees which is very suitable for contact with the nematodes and rotifers that graze bacteria in the vicinity of the cell.

The basal vacuole is the power pack for the cell. It is at high Osmotic Pressure. When the gun cell is released the pressure up front is removed and water flows in rapidly through the semipermeable membrane surrounding the vacuole. This squeezes the protoplasm and nucleus, like toothpaste, through the tubular hypodermic. The Haptoglossa gun cell is only about 15 microns long.

This is a simplified version of how the cell might function. It is only a hypothesis that attempts to explain the factual observations and fine structure of the cell.

The gun cell is only about 15 microns (=15/1000mm) long.

Photomicrographs from MycoAlbum CD by George Barron