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PHYLUM BASIDIOMYCOTA

INTRODUCTION TO THE BASIDIOMYCOTA

Basidiomycota (ba-si-di-o-mi-KO-ta) is made of two Greek roots that mean little base (basidion - βάσηδιον); and fungus (mykes -μύκης).  The reference is to the basidium becoming the base for the basidiospores.

The Basidiomycota are those organisms that produce sexual spores (basidiospores) on basidia (Figure A), often these are borne on distinctive basidiocarps or basidioma (Figure B).  The phylum typically has an extended dikaryophase in which the distribution of two nuclei to the daughter cells is facilitated by the formation of a clamp connection (Figure C), which is similar to the crozier of the Ascomycota.  The most notable difference between the ascomycotes and basidiomycotes is the extended dikaryotic phase of some of the basiomycotas [See the life history of a typical holobasidiomycete mushroom].  The dikaryon can grow for many years and extend over many acres.  For example, a mycelium of Armillaria ostoyae, the root rot fungus, covers about 2,200 acres in the Blue Mountains of Oregon, making it one of the largest organisms on earth (confirmed by DNA fingerprinting).  Also, given average growth rates for this species, a mycelium that large would have to be about 8,000 years old.

The successful mycelium that begins to grow from a basidiospore is haploid, but likely encounters another haploid filament fairly quickly.  Thus, the dikaryotic mycelium grows as an expanding circle.  The mycelium also feeds on all available organic matter and nutrients during its expansion.  Thus, the center of the circle becomes played out, and the hyphae die.  So, older dikaryotic mycelia expand as a ring, which also is the source of the basidiocarps.  Thus, common mushroom basidiocarps often emerge in a circle, sometimes called a fairy ring.

I once had someone call me up about a "mushroom problem" in his yard.  He had mushrooms come up every year, even though he went out faithfully and picked them before they could shed their spores.  I found the fundamental misunderstanding of the nature of the fungal organism to be very amusing.  I responded that trying to rid himself of mushrooms by picking them was like trying to kill an apple tree by picking the apples.  Unfortunately, he took a very different message from the one that I intended.  I discovered a year later that he had covered his lawn with fungicide.

The basidiomycote fungi range from common mushroom forms to some of the world's most important plant pathogens. The rusts (Subphylum Urediniomycotina, Figure D) and smuts (Subphylum Ustilagomycotina, Figure E) are the main groups of plant pathogens, some of which are of major economic importance.  The rusts (e.g. wheat rust and white pine blister rust) alternate between two hosts and have 5 different kinds of spores in their life cycles.  In the case of Puccinia graminis, karyogamy occurs in the teliospore after which it undergoes meiosis.  Upon germinating, the haploid filament has four cells, each of which has one of the meiospores and releases them as basidiospores.  After landing on an appropriate host (in this case a barberry plant), the haploid basidiospore germinates and begins to grow in the leaf tissue.  The mycelia of the different mating types develop clusters of spermogonia and receptive flexuous hyphae (functional female).  Fusion of spermagonia and receptive hyphae yield dikaryotic hyphae, which produce chains of asexual spores (aeciospores) within aecia.  The aeciospores germinate if they land on an appropriate grass (e.g. wheat).  Then, the dikaryotic hypha grows through the wheat, generally killing the plant.  They erupt on the sides of the infected wheat stalks with orange-red uredeniospores in uredinia.  These spores can infect more wheat, which in a monoculture, can lead to the loss of the entire crop very quickly.  As the wheat plant is dying, the dikaryotic filament makes a different kind of spore, a teliospore (Figure D).  This is the spore that can remain in the soil, over winter, and then produce basidiospores in the spring [See the life history of Puccinia].  The smuts (e.g. corn smut-Figure E- and oat smut) infect single hosts and have much simpler life histories [See the life history of Ustilago].  However, they are no less devastating to the crops that they infect.  

The Basidiomycotina is the most diverse subphylum in the Basidiomycota, and  the taxa include common tree ears, jelly fungi,  fairy clubs, toadstools, bracket fungi, puffballs, mushrooms, and polypores (Figures F-T).  Typically, they can be differentiated on the basis of their basidiocarps, which show great disparity in form.  In general, the basidiospores are the dispersive spores and they are launched by hydrostatic pressure between the sterigma and the basidiospore on the basidium.  In these cases, they are called ballistospores.  However, others have adopted different strategies.  Phallus, (Figure I) for example, emerges as a typical mushroom, but the cap becomes deliquescent (gelatinous) with the basidiospores embedded in the gel.  The cap also begins to emit the odor of rotting meat.   Flies that are attracted to the odor get spores caught on their feet and bodies.  Then, being flies, they might go to feed on dung or rotting organic matter, perfect growth media for Phallus.  Others, like the puffballs (Figures J & S), form basidiocarps that do not open to the outside.  The inside of puffball undergoes autolysis and forms a mass of loose spores that are shed in puffs when the outer covering (peridium) is touched.  A single giant puffball can harbor millions of basidiospores, and the mucelium can produce several each year for the life of the mycelium (For the sake of argument, say 10 years).  That would be more spores from one organism than all of the people on earth.  Yet, it might replace itself with only one or two successful mycelia.  This mantra of "many try, few succeed" is the basis of natural selection.

Many of the basidiocarps are strikingly beautiful, and some of them are quite tasty.  The polypore, Boletus edulis (Figure N), is sometimes called the King of Mushrooms because of its flavor and texture.  The champignon, Agaricus bisporus (Figure R), is the most common table mushroom in the US, and almost half of them are grown in Pennsylvania.  Hunting for wild edible mushrooms can be fun and rewarding; however, many basidiocarps are poisonous, and a few are deadly. The Avenging Angel (Amanita phalloides) produces very toxic alkaloids, such as phalloidine and α-amanitine, which in sufficient concentrations, can destroy a healthy liver over a period of three days.  Only a liver transplant can save the unlucky diner who mistakes Amanita phalloides for Amanita caesarea.  Indeed, the alkaloids are so dangerous that they can move from one mushroom to another if poisonous and nonpoisonous mushrooms are put together in the same basket.

SYNOPTIC DESCRIPTION OF THE BASIDIOMYCOTA

SYSTEMATICS OF THE BASIDIOMYCOTA

Like the taxonomic system of the Ascomycota, the Basidiomycota has seen important reexamination of many traditional associations and taxa.  The phylum is comprised of three major monophyletic groups according to molecular, ultrastructural, and biochemical evidence (Boeckhout et al. 1993, 1995; Fell et al. 1995; McLaughlin et al. 1995; Prillinger et al. 1990; Swann and Taylor 1993, 1995; Wells 1994; Wolters and Erdman 1986; Knudson 1995; and Hibbet et al. 1997). The Urediniomycotina (=Teliomycotina) represent the oldest separation and are sisters to the rest of the fungi (Ustilagomycotina and Basidiomycotina).  That relationship has been confirmed by the SSU and LSU r-RNA analyses of Lutzoni et al. (2004).  As with other groups which have parasites as their most "primitive" forms, I have reservations about claims that place the complex parasitic taxa of the Urediniomycotina at the root of the tree of the Basidiomycota.  With this reservation I offer this system, a modification of Knudsen (1995) with 3 subphyla, 4 classes, and 34 orders.

HIERARCHICAL CLASSIFICATION OF THE BASIDIOMYCOTA

This page is maintained by Jack R. Holt. Last revised 03/12/2008.