DIFFERENTIATION AND DEVELOPMENT 103
A. fumigatus, and N. crassaare seen to have a pattern
of hydrophobic rodlets on their surface, but the rodlets
are absent when the hydrophobin genes are inactivated
by targeted gene disruption. Identical rodlets are
formed when solutions of pure hydrophobins are
allowed to dry. Talbot (2001) recently reviewed the struc-
ture and roles of hydrophobins. We return briefly to
this subject later in this chapter.Sexual developmentSexual reproduction in all organisms involves three
fundamental events: the fusion of two haploid cells
(plasmogamy) so that their nuclei are in a common
cytoplasm; nuclear fusion (karyogamy) to form a
diploid; and meiosisto produce recombinant haploid
nuclei. Depending on the fungus, these events can
occur in close succession or separated in time; also, they
occur at different stages of the life cycle according
to whether the fungus is normally haploid or diploid.
These points were outlined in Chapter 2 (see Figs 2.1,
2.8, 2.14, 2.15, 2.16).
Two further points must be made. First, some
fungi are homothallic(self-fertile) but many are het-
erothallic(outcrossing). in which case sexual repro-
duction is governed by mating-type(compatibility)
genes. Often there are two mating types, governed by
a single gene locus (bipolar compatibility); in these
cases the alternative genes are not an allelic pair but
usually are quite different from one another so they
are called idiomorphs. Some Basidiomycota have two
mating-type loci (tetrapolar compatibility) with multi-
ple idiomorphs at each locus. In these cases a successful
mating occurs between two fungal strains that differ
from one another at each gene locus. In most fungi
the mating-type genes are regulatory genes, producing
protein products that bind to DNA and control the
expression of several other genes.
The second point is that the sexual spores of many
fungi function as dormant spores (e.g. zygospores,
oospores, ascospores, and some basidiospores). So sex-
ual reproduction serves an important role in survival,
and the sexual spores are typically produced at the onset
of unfavorable conditions for growth. Fungi that do not
undergo regular sexual reproduction sometimes produce
alternative survival structures such as chlamydospores,
sclerotia, or melanized hyphae. Other fungi adopt
different strategies. For example, Pythium oligandrum
produces sexual spores by parthenogenesis, and
Saccharomyces cerevisiaeand a few other Ascomycota
undergo regular mating-type switchingto ensure
that there will always be a mixture of the two mating
types (aor αα) in a population. In this case, every time
that an acell buds, the parent cell switches to ααwhile
the daughter cell remains a. In fact, wild populations
ofS. cerevisiaeare always diploid because the aand
ααcells fuse and undergo karyogamy, then the diploid
cell buds to form further diploids which can respond
rapidly to unfavorable conditions by undergoing
meiosis and producing ascospores. For this reason, all
the laboratory strains used by yeast geneticists have
been mutated so that they do not undergo mating-type
switching, and this enables the sexual crosses to be
controlled experimentally.
Against this background, the rest of this chapter
will focus on two topics: the roles of the mating-type
genes, especially in hormonal regulation of mating
(reviewed by Gooday & Adams 1993), and the devel-
opment of fruiting bodies of Basidiomycota because
these are the most advanced differentiated structures
in the fungal kingdom.Mating and hormonal controlChytridiomycota
Most of the information on the control of sexual
reproduction in Chytridiomycota has come from stud-
ies on Allomycesspp. (see Fig. 2.1 for the life cycle).
These fungi are homothallic (self-fertile), but they
produce motile male and female gametes (sex cells)
from different gametangia. The female gametes are
larger, hyaline and they release a pheromone, sirenin
(Fig. 5.22), to attract the male gametes. The male
gametes are small, and orange colored due to the pres-
ence of a carotenoid pigment. Sirenin is a powerful
attractant, active at concentrations as low as 10−^10 M,
but optimally at 10−^6 M. Compounds that attract cells
at these concentrations can be assumed to alter the
swimming pattern by binding to a surface-located
receptor (Chapter 10). Although there is no informa-
tion on this possible receptor, the male gametes are
known rapidly to inactivate sirenin, which could aid
their movement up a concentration gradient. It is
interesting that both sirenin (from the female gametes)
and the carotenoid pigment of the male gametes are
produced from the same precursor isoprene units
[CH 2 =C(CH 3 )−CH=CH 2 ] (see Fig. 7.14). So this is an
example where cells of the same genetic make-up
show different biochemical properties because they have
been produced in different gametangia, separated by a
complete cross wall.Oomycota
The Oomycota can be homothallic (e.g. most Pythium
species) or heterothallic with two mating types. But in
all cases a single colony produces both the “male” and
“female” sex organs (antheridia and oogonia – see Fig.
2.34), so the mating-type genes govern compatibility,