WAR of the MICROWORLDS
How do I kill Thee? Let me count the Ways! (# 5)
Olpidium vermicola: the posteriorly flagellate zoospores are attracted to a nematode eggs by chemical secretions. The zoospore produces a spherical cyst on the 'shell' of the egg. The 'shell' is then penetrated by means of a narrow peg, and the contents of the cyst transferred inside where it grows rapidly to form a solitary, nonseptate thallus. Each infection produces a separate thallus. The micrograph below is the result of three separate infections. CHYTRIDIOMYCOTA. For further information on this species see Barron, G.L. 1986. Mycologia 78: 972-975.
Nematodes are motile and can move at a fair clip on the micro scale. For most of the fungi that attack nematodes, the mobile nematode is lured to the site of attack by the stationary fungus.
However, mature nematodes (gravid females) lay large numbers of eggs in the environmental niche that they are occupying. Nematode eggs are attacked as an important or exclusive food source by a number of fungi from several different classes. The eggs are are rich in nutrients and leak small quantities of chemicals through the outside wall of the egg ('shell') to the exterior. These compounds act as signal chemicals to predatory organisms including fungi. The fungus uses these signal chemical to locate and attack the egg. Fungi do this in two ways :
1) by hyphae growing towards the egg (slow) or 2) by zoospores that swim towards the egg (fast).
Zoosporic fungi, therefore, seemingly have a particular advantage as egg parasites as they are capable of relatively rapid directional movement and can follow gradients of signal chemicals leaking from the eggs to seek out and attack their prey.
Most fungus spores are passive (non motile) and can persist for weeks, months, or even years and remain viable with minimal loss of energy reserves. Spores of some Slime Moulds have been germianted after sixty years storage at ambient temperatures as herbarium specimens. One disadvantage of spores that are released as zoospores, however, is that they are constantly using energy for movement and must locate and attack their food source in a very short period of time (measured in hours) before their energy supply is exhausted (note: there are some biological solutions to this problem that will be covered in a later section).
click on thumbnails for larger image 1. 2. 3.
Olpidium vermicola (Fig. 1) is a zoosporic fungus that attacks nematode eggs (its only known substrate). The posteriorly uniflagellate zoospores (Fig. 2) are attracted to the egg by chemical secretions and encyst on the outer wall ('shell' ) forming a small sphere. The fungus then penetrates through the egg wall and the fungus protoplasm is transferred into the interior of the egg as a tiny thallus. The thallus secretes enzymes to break down the contents of the egg then absorbs these nutrients and grows to fill the interior. At maturity the thallus morphs into a zoosporangium and produces one or two exit tubes that grow out through the egg wall to the exterior (Fig. 3). The protoplasm inside the thallus is multinucleate. Some protoplasm cleaves around each nucleus to form spores each of which produces a long, posterior, whiplash, flagellum and the zoospores now swim through the exit (evacuation) tube to track down and attack additional eggs. Zoospores after release will provide a welcome snack for other predatory organisms (amobea, rotifers, tardigrades, copepods, mature nemtodes, etc.). If they survive all these hazards they may run out of energy before they locate an egg and perish from starvation. A few reach their target! The zoosporangium in an infected egg can produce several hundred zoospores.
When an egg is attacked simultaneously by several zoospores then a number of smaller thalli will develop inside the egg. Multiple attacks cause competition for the food source and this results in a change in development of the thalli. Instead of producing zoosporangia some or all of the thalli produce thick-walled resting spores.
Resting spores can withstand long periods of difficult conditions (cold, heat, drought etc.) but will eventually be stimulated to germinate at some later date by a suitable signal to start this game of life and death once more.
Images copyright George Barron from MycoAlbum CD (contains these pictures and a 1000 more)
Comment on MycoaAlbum CD by DH - ' a magnificent accomplishment '