#045: Mushroom Morphology: Sequestrate Fungi

The sequestrate fungi* are an unnatural grouping of mushroom-forming basidiomycetes that have adapted to life in desert areas by keeping developing spores inside the fruiting body. Because of this they are included among the gasteroid fungi.  Like the other gasteromycetes, sequestrate fungi cannot form ballistospores (see FFF#013), and cannot forcibly discharge their spores.  Sequestrate fungi have a stipe and a cap-like head, similar to toadstools.  However, the head never fully opens and the spore-bearing surface remains enclosed by the cap.  The gleba (fertile tissue) does not develop regular gills or pores.  Instead, the gleba forms irregular pockets that are sometimes gill-like in appearance.  This is a highly diverse group of fungi, so there are many variations on this basic structure.  Some sequestrate fungi partially open their caps, while others never do.  Some have reduced stipes and are barely held above the ground.

There are two main ways that sequestrate fungi disperse their spores.  The first is by attracting animals, mostly small mammals like squirrels.  These animals are attracted to the scents made by the mushrooms.  The fruiting bodies are either eaten or carried off by the animals, thus dispersing the spores.  Because the animals are attracted by smell, most of these fungi are dull in color (except in New Zealand — visit the first link below to find out more).  The second way that sequestrate fungi disseminate their spores is by using the wind.  These fungi are woody and stand upright.  Once all the spores are mature, the exterior gradually breaks down to release the spores.  During this time, spores may also be dispersed short distances by water during rainstorms.

Sequestrate fungi are thought to have evolved to keep developing spores moist in arid or montane environments.  The dry conditions made it advantageous for mushrooms to not fully develop their caps.  The enclosed spores would be kept moist and more spores in total could be produced.  Sometime after this the cellular mechanism for ballistospory was lost.  Eventually the gills or pores of the fertile tissue became fused with each other and less organized overall.  Since the mushrooms were no longer dropping spores to the ground, the hymenium did not have to be characterized by highly organized, vertical sections.  Some sequestrate fungi have gone further and no longer develop a stipe.  However, most of these fungi still have a columella running through the center of the gleba, much as a stipe would.  The false truffles have followed this evolutionary pattern to the extreme.  False truffles have enclosed spores, lack a stipe, and – unlike sequestrate fungi – produce below-ground fruiting bodies.  Check back next week for more on false truffles!

As mentioned previously, the sequestrate fungi do not share a common evolutionary lineage.  It is evident that this morphology has evolved multiple times (possibly more than 30!), making any coherent discussion of ecology or taxonomy of the group almost impossible.  Phylogenetic studies have shown that sequestrate fungi have evolved from: Russulales/Russulaceae/Russula, Russulales/Russulaceae/Lactarius, Agaricales/Cortinariaceae/Cortinarius, and Boletales/Boletaceae, among others.  The one thing that can be said of the group is that the species appear with the greatest abundance and diversity in arid and montane regions, which supports the theory that they evolved to successfully disperse their spores in dry conditions.  In North America, sequestrate fungi are most abundant in the west and southwest.

* A note about the name: the sequestrate fungi have gone through a few name changes over the years.  They were originally known as “pouch fungi,” but this was replaced by the more scientific term “secotioid fungi” because many of these fungi are found in the genus Secotium.  It quickly became clear that this group contains mushrooms from many different evolutionary lineages, so the more general term “sequestrate fungi” was introduced in 1992.  Some people consider this term to include all basidiomycete gasteroid fungi as well as truffles (which are ascomycetes).  Others consider it to include all basidiomycetes that follow the evolutionary pattern discussed above, which includes false truffles and the gasteromycetes that do not fit into any other categories.  In this post I am specifically discussing above-ground basidiomycetes that do not fit into any other gasteromycete morphological groups.  I will discuss their below-ground relatives (false truffles) next week.

See further:

http://sporesmouldsandfungi.wordpress.com/2013/05/17/pouch-secotioid-or-sequestrate/ (includes an interesting discussion of the evolutionary pressures on sequestrate fungi in New Zealand)

http://www.mykoweb.com/systematics/literature/The%20Secotioid%20Syndrome.pdf (this is an academic paper)

http://oregonstate.edu/dept/botany/mycology/bot461/class/lecture10.html (scroll down to the bottom for the bullet-point version of this post)

http://en.wikipedia.org/wiki/Secotioid