Growth, Differentiation and Sexuality

360 L.A. Casselton and M.P. Challen
Fig. 17.2. Life cycle of the saprophytic mushroom Coprinus cinereus (Coprinopsis cinerea)
tip cell, and both nuclei take up positions adjacent
to the entrance to this cell, one inside and one in
the main cell (Iwasa et al. 1998). Both nuclei divide
in synchrony; new cell walls are laid down, creating
three cells, a dikaryotic tip cell, and uninucleate
clamp and subterminal cells. The clamp cell grows
backwards to fuse with the subterminal cell,
and the clamp cell nucleus is released to join its
partner. An observation of Buller (1931), of which
we are reminded by Badalyan et al. (2004), is that
the subterminal cell produces a projection that
growstowardstheclampcelltipandfuseswith
it. This is reminiscent of the mating projections
produced by mating Saccharomyces cerevisiae
cells in response to pheromone stimulation and,
since clamp cell fusion is a B-dependent step, it is
assumed to require pheromone signalling (Brown
and Casselton 2001). As well as being necessary
for clamp cell fusion, nuclear migration following
the initial mating cell fusion is also B gene- and,
thus, pheromone-dependent. Formation of the
clamp cell and synchronised nuclear division are
A gene-dependent (Sweizynski and Day 1960).
C. neoformans has a more complex life
cycle than do U. maydis and C. cinereus, in
that there are haploid, diploid and dikaryotic
developmental pathways (see Hull and Heitman
2002). Like U. maydis, it is a dimorphic fungus
and can exist as yeast-like cells that divide
by budding, or as filaments. The dikaryon of
this fungus is not the pathogenic stage – it is
the yeast form that infects humans and causes
meningoencephalitis and, interestingly, it is the
α mating type cells that are the most virulent
and occur more frequently in the population
(Kwon-Chung and Bennett 1978; McClelland
et al. 2004). The yeast-like MATa and MATα
cells can mate; pheromone signalling between
mates induces MATα cells to form conjugation
tubes. MATa cells do not form conjugation
tubes but may become enlarged in some strains
(Moore and Edman 1993; Davidson et al. 2000;
McClelland et al. 2004; Heitman, personal communication).
Cell fusion gives rise to a dikaryon
with binucleate cells and fused clamp connections,
similar to those we see in C. cinereus.