Note: This program first aired on August 20, 2016.
There’s fungus among us. Though it has been a dry summer, in
the past few weeks, right after each heavy rain, on the trails I run I see
mushrooms pushing their way up out of the forest floor. Russulas and
Lactariuses, coral fungus and boletes, an occasional amanita and delicious
chanterelles. And those are just the groups I can identify with relative ease.
Mushrooms are the reproductive structures of certain kinds
of fungus, ascomycetes and basidiomycetes. There are many other kinds of fungus
out there as well, but they don’t make mushrooms (think mold and yeast and a
bunch of stuff that is essentially invisible to human eyes). As the reproductive structures of ascomyctes
and basidiomycetes, they emerge when environmental conditions favor fungal
growth. The timing of these appearances gives a clue as to what those favorable
conditions are. It has been a dry summer, dry enough that all of the organic
matter that makes up the upper layers of the forest floor is dry, and we’ve
experienced a few small forest fires. Scary stuff in our dense, low fire
frequency eastern forest. The mushrooms
we see emerge after damp weather are the result of the action of billions of
fungal filaments below ground in the soil. These filaments, called mycelium make
up the bulk of fungal biomass, at least in terms of the mushrooms we see in the
forest. The mushrooms are truly only the tip of the ice berg.
Mycelium are made up of even smaller individual filaments
called hyphae, and grow through the soil in the forest feeding on organic
matter. Like the fine hair like roots of plants, these microscopic fillaments
don’t do well when the soil is very dry, their movement and metabolism are
aided by the water that makes the soil damp. Hence, a nice flush of rain that wets the forest soil results in a boom
of mycelial activity, and it is when compatible mycelium meet up underground
that a mushroom results. Rapid increases in mycelium increases the chance of
these meetings, hence mushrooms appearing overnight after wet weather. The
mushroom’s only job is to create spores, which can result from sexual
reproduction between those two compatible mycelia, and are a dispersal
mechanism for fungus. Tiny and airborn, spores can travel great distances on
air currents, and if they land in the right spot, can germinate and form a new hyphal
strand. If that strand of hyphae finds what it needs it continues to grow and
becomes multistranded mycelium. If it runs into another mycelium from the same
species, and they are compatible mating types, they will merge and share their
genetic information, and build a mushroom from this conjoined mycelium. In
special cells in the mushroom (typically on the gills underneath the cap) meiosis
will occur and the spores that are formed will contain a mix of genetic
information unique from either parent.
If that is the job of the mushroom, what is the job of all
that mycelium in the forest soil? We’ll answer that question next week.
References:
Mostly books this time around:
David Arora, Mushrooms Demystified
George Barron, Mushrooms of Northeast North America
Lawrence Millman, Fascinating Fungi of New England
Elizabeth Noore-Landecker, Fundamentals of the Fungi, 4th
ed.
James, Timothy (2007). "Analysis of mating type locus
organization and synteny in mushroom fungi: Beyond model species". In J.
Heitman; J. W. Kronstad; J. W. Taylor; L. A. Casselton. Sex in Fungi: Molecular
Determination and Evolutionary Implications. Washington DC: ASM Press.
pp. 317–331.
