untitled

small droplet, termed the hylar droplet, is seen at

the base of the spore, then the spore is “popped” from

the sterigma into the space between the gills, where it

falls vertically, free from the fruitbody. After a period

of dormancy the haploid basidiospores germinate

to produce monokaryotic hyphae, which repeat the

whole cycle.

One of the most intriguing aspects of this life cycle

is the manner in which the dikaryon develops. This

is shown diagrammatically in Fig. 2.20. The dikaryotic

tip cell extends and synthesizes protoplasm until it

attains a critical cytoplasmic volume. Then a small

backwardly-projecting branch arises, and the two nuclei

divide synchronously – one of the nuclei dividing

along the axis of the hypha, and the other dividing so

that one of the daughter nuclei enters the branch. The

branch is then cut off by the development of a sep-

tum (cross-wall) and a septum also develops in the main

axis of the hypha. The effect of this is to create a tip

cell with two nuclei of different compatibility types, a

penultimate cell with a single nucleus, and a branch

with a single nucleus, isolated by septa. The branch then

fuses with the penultimate cell, by dissolution of the

wall, and the nucleus migrates into the penultimate cell,

restoring the dikaryotic condition. This process occurs

every time the nuclei divide. The resulting “bumps” on

the hyphae are easily seen by light microscopy, and are

termed clamp connections(Fig. 2.20). However, not

all members of the Basidiomycota produce clamp con-

nections, and yet they still grow as dikaryons, suggesting

that other regulatory mechanisms are involved.

If we refer back to the development of asci in Fig.

2.16, we see a remarkable similarity in the behavior

32 CHAPTER 2

Fig. 2.19Scanning electron micrograph of the basidia of

a mushroom, Coprinus cinereus. Note the inflated basidia,

each with four sterigmata, and the basidiospores, some

of which have already been released, or collapsed during

preparation of the specimen. (Courtesy of Dr Chris

Jeffree.)

Germination

Oidia

Homing

Plasmogamy

Dikaryotic

hyphae

Fruitbody

(Karyogamy)

Basidium

Basidiospores

Monokaryotic

hyphae

Fig. 2.18A generalized life cycle of a mush-

room; see text for details. (Courtesy of Maria

Chamberlain.)

basidiospores produce a dikaryotic mycelium that is

homothallic (self-fertile). However, the vast majority of

mushroom-producing fungi are heterothallic – about

90%. We will discuss the compatibility systems of

these fungi in Chapter 5.

The way in which the basidiospores are released

is not fully understood, but just before their release a

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