Growth, Differentiation and Sexuality

Fig. 17.3. Life cycle of the dimorphic human pathogen
Cryptococcus neoformans, based on Kwon-Chung (personal
The tip cells of these filaments swell to form the
basidia in which meiosis occurs (Kwon-Chung
1975).
Aseconddevelopmentalpathwayoccursin
both MATα and MATa cells under conditions
of desiccation and nitrogen limitation (Wickes
et al. 1966; Tscharke et al. 2003), which is asexual
and known as monokaryotic or haploid fruiting
(Wickes et al. 1966). Cells of either mating
type produce filaments, which typically produce
branch-like projections at each septum. According
to Kwon-Chung (personal communication), these
projections are nucleate and appear to be potential
branches, as shown in Fig. 17.3, but other authors
maintain they are anucleate, and refer to them
as unfused clamp cells (see Hull and Heitman
2002). Haploid fruiting in both mating types is
stimulated by pheromone signalling, suggesting
that it may have a role in promoting mate fusion
(Wang et al. 2000). Occasionally, mating gives rise
to diploid cells that are thermally dimorphic – at
37 ◦ C the cells are yeast-like but at 25 ◦ C they are
Mating Type Genes in Basidiomycetes 361
communication). The filamentous phase is also known as
Filobasidiella neoformans (Kwon-Chung 1975)
filamentous and resemble the haploid filaments
produced by MATα and MATa cells (Sia et al.
2000). All mycelial filaments differentiate basidia
as swellings of the hyphal tip cells. In matings,
the basidium is the cell in which meiosis occurs;
the four nuclei produced by meiosis remain in
the basidium, and nuclei from subsequent mitotic
divisions are incorporated into chains of up to
40 basidiospores that are a mixture of MATα
and MATa mating types (Kwon-Chung 1980).
In haploid fruiting, the spores produced are all
of one mating type and, until recently, it was
assumed that these were derived from entirely
mitotic events. However, Lin et al. (2005) have
now shown that both diploidisation and meiosis
may occur during monokaryotic fruiting in MATα
strains. Significantly, different MATα cells can
fuse, so that this so-called monokaryotic pathway
can lead to genetic recombination between
individuals of the same mating type, something
that the authors emphasise as being particularly
important, in view of the greater virulence of