Mark Bower explains a complicated arrangement of reproduction between a fungus and two sets of wasps, a ménage à trois of lower orders.
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| Cerrena unicolor - Mark Bower |
A Three Way Relationship….It’s Complicated!
At first glance, these mushrooms look like any number of boring “bracket” fungi one
encounters while hiking in the Ozarks. However, this particular species offers a bit more interest than is usual. The green cap contrasting with the white cap margin is unusual, and surprisingly, the under surface of the cap does not have the expected round pores, but displays a “maze-like” pattern.
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| Maze pattern under Cerrena unicolor -Mark Bower |
It didn’t take long to identify this moss-covered, maze-pored polypore as the Mossy Maze Polypore, Cerrena unicolor. Thrilled already with this discovery, I became excited beyond all reason after reading about its ecological relationships. This fungus produces copious amounts of sexual (diploid) spores on its maze-like pores.
These spores are dispersed into the environment, and if by chance they land on a suitable substrate (in this case, dead hardwood) they can germinate. After germination, the fungal hyphae infiltrate the dead wood and break it down with cellulase and lignase enzymes. When the substrate’s nutritional content is exhausted, or whenever the fungus darn well pleases, it develops its fruiting bodies and the cycle begins anew.
So far, it all seems normal. The dispersal of sexual spores into the ecosystem by a fungus is a common occurrence. It turns out, however, that asexual reproduction is far more common in the Fungal Kingdom. Asexual (haploid) spores are produced by the fungal mycelium. This is where a couple of wasps enter the picture.
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| Tremex columba female - Bill Shehan CC |
A female Horntail Wasp (usually Tremex columba in the Ozarks) stores the asexual spores and tips of mycelia of Cerrena unicolor in an abdominal gland called a mycangium, which is connected to its ovipositor. The wasp locates a hardwood tree which is sick or dying, and bores hundreds of holes through the bark into the sapwood with her ovipositor. As she is depositing her eggs into the newly drilled holes, the fungal spores and mycelial tips are injected as well. The eggs wait about a month to hatch, allowing the fungus to infiltrate the wood, digesting it in the process. After hatching, the wasp larvae tunnel through the wood, feeding mainly on the fungal mycelium. This can go on for as long as one or two years before formation of the pupae. The adult wasps will emerge from the pupal stage July through August, chewing their way to the surface, and carrying their fungal baggage.
This two way relationship between the fungus and the Horntail Wasp benefits both parties. The fungus gets to be efficiently transported from tree to tree; the wasp larva has an excellent source of nutrition, thanks to the fungus.
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| M. macurus - MJ Hatfield - CC |
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| Michael Battenberg CC |
But wait! There is more to this fascinating story! The clever fungus actually secretes pheromones from beneath the bark of its host. This attracts an incredible parasitoid wasp, the Giant Ichneumonid Wasp, Megarhyssa macrurus (or M. atrata). This wasp has as its single host, the larvae of the Horntail Wasp. Its gigantic ovipositor (seen to the right) drills into the Horntail larval tunnels. It then stings and paralyzes a larva, which allows the parasitoid wasp to deposit an egg into it. The egg hatches and feeds on the host larva, devouring it in a few weeks. It then pupates and emerges as an adult the following spring.
From all appearances, the fungus is in charge of this three part relationship. It uses the Horntail Wasp to spread it from tree to tree, and it uses the Giant Ichneumonid Wasp to control the population of Horntail Wasp larvae which are feeding on the fungus, a case of inter species birth control.
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Mark has identified 436 species of fungi on our land on Bull Creek. I can't even identify 436 different people. He will be back on the blog soon.
