untitled

Fig. 2.15Stages in the life cycle of Neurospora crassaand many similar fungi in the Ascomycota.

(b) (c) (d)

Ascogonium

(a)

New crosier

ascus initial

Septation

hyphal fusion

Nuclear

division

Crosier

formed

Fig. 2.16Early stages in the production of

asci from an ascogenous hypha. (a) The

ascogenous hypha grows for some distance

then curls back at its tip to form a crosier.

(b) Synchronous nuclear division occurs as

the crosier bends back upon itself. (c) Septa

are laid down so that the penultimate cell

(located at the top) has one nucleus of each

mating type. This cell will extend and will give

rise to the first ascus, when the nuclei fuse

and then undergo meiosis and one round of

mitosis, to produce eight ascospores. Mean-

while, the tip cell of the crosier fuses with the

ascogenous hypha, and the nucleus is trans-

ferred. (d) A new branch develops from the

fused cell, and this branch forms a further

crosier, to repeat the whole process. Thus,

eventually, a large number of ascus initials are

produced, which will give rise to a large

number of asci. The final stage in this pro-

cess is the development of a fruitbody tissue

surrounding the asci. The haploid ascospores

are either Aor amating type, and eventu-

ally germinate to produce haploid colonies,

to complete the life cycle.

FB4eC02 04/20/2005 02:52PM Page 29