The division of fungi known as the club fungi, Basidiomycota, includes some of
the most familiar fungi. Mushrooms, puffballs, and shelf fungi are all members
of this group, as are the plant rusts and smuts. This group, which contains
approximately 15,000 known species, is distinguished by the presence of a club-
shaped reproductive organ called the basidium. This organ is most likely
derived from the ascus found in Ascomycota (see Ascomycota
Structure), with which it shares
several characteristics. Both originate as a binucleate, dikaryotic
structure and serve as a site for karyogamy and meiosis. They differ in
that the basidium bears its spores outside while the ascus retains them inside
the structure. The role of the basidium in sexual reproduction is discussed in
Heading .
Structure
Figure%: Structure of the Basidiomycota
Haploid spores grow into cottony tangles of hyphae called mycelia.
These mycelia usually grow under the surface until they meet up with another
mycelium. The two join (plasmogamy) and produce a series of binucleate,
dikaryotic hyphae that reach above the ground and form the fruiting body
or basidioma. The cells of the basidioma cannot divide by normal mitosis
because they have must produce two daughter cells each with a copy of both
parental nuclei. This is accomplished through the formation of a clamp
connection.
Figure%: Formation of a Clamp Connection
In this process, a bulging pocket forms in the hyphal cell wall at a point
between the two nuclei. This pocket will eventually form the clamp. Both nuclei
(a and b) then divide mitotically. These divisions are oriented
such that the a' nucleus is positioned in the clamp pocket and both the
a" and b' nuclei are toward the tip of the hypha, while the
b" nucleus takes a posterior position. Next, a cell wall forms between
the clamp, the posterior cell and the tip of the hypha. The tip now has a
complete cell with two nuclei, but the posterior cell and the clamp each have
only one nucleus. This is remedied when the clamp curves back toward the hypha
and merges with the posterior cell.
A feature used to identify Basidiomycota, aside from the presence of basidia, is
the degree of separation between individual cells. Basidiomycota have more
septate hyphae than Zygomycota, though their septae are perforated, allowing
cytoplasm to flow freely between cells.
Reproduction
Like all fungi, Basidiomycota can undergo both asexual and sexual reproduction.
Asexual Reproduction
Basidiomycota reproduce asexually by either budding or asexual spore formation.
Budding occurs when an outgrowth of the parent cell is separated into a new
cell. Any cell in the organism can bud. Asexual spore formation, however, most
often takes place at the ends of specialized structures called
conidiophores. The septae of terminal cells become fully defined, dividing
a random number of nuclei into individual cells. The cell walls then thicken
into a protective coat. The protected spores break off and are disbursed.
Sexual Reproduction
Figure %: Sexual Reproduction in the Basidiomycota
Sexual reproduction in Basidiomycota takes place in the fruiting body, in
specialized structures called basidia. The basidia is itself formed by
plasmogamy between mycelia from two different spores. Plasmogamy results in
binucleate hyphae, that is, hyphae with two types of nuclei, one from each
parent. In the gills of the fruiting body, some cells undergo fusion of these
two nuclei. These now diploid cells are the basidia. The diploid phase is
very brief. Soon after fusion, meiosis takes place, resulting in four haploid
nuclei. The nuclei then migrate to the terminus of the basidium and form four
individual projections. These projections are then separated by cell walls to
become spores.