Introduction to Fungi, Third Edition

158 CHYTRIDIOMYCOTA
Fig 6.21 Life cycle diagram of Eu-Allomyces as exemplified by A. macrogynus. A diploid sporothallus may produce diploid mitospores
from a colourless, thin-walled papillate mitosporangium, and haploid meiospores from a thick-walled pitted meiosporangium in
which meiosis occurs. Meiospores germinate to form a haploid gamethothallus which produces two different gametangia and
releases haploid gametes of two kinds, small carotenoid-rich (shaded) ‘male’gametes and larger colourless ‘female’ones.Upon
copulation, a diploid zygote gives rise to a sporothallus. Alternatively, if failing to pair up, the female gametes may function as
zoospores, in which case they give rise to a new gametothallus. Small open circles represent haploid nuclei whereas diploid nuclei
are drawn larger and split. It should be noted that many field strains of A. macrogynus have a higher ploidy level, e.g. alternating
between diploid (small circles) and tetraploid (large split circles) conditions. Key events in the life cycle are plasmogamy (P),
karyogamy (K) and meiosis (M).
branches (Fig. 6.20g). Within these thin-walled
sporangia the nuclei undergo mitosis. Initially
the nuclei are arranged in the cortical region
of the cytoplasm, but later they migrate and
become uniformly spaced apart. Movement of
the nuclei is controlled by forces generated by
actin microfilaments whilst their spacing and
positioning is controlled by microtubules (Lowry
et al., 1998). Cleavage of the cytoplasm around
the nuclei to form diploid colourless zoospores
is initiated by the formation of membranes seen
first at the plasmalemma, then extending into
the cortex to form a complex membranous
network (Fisher et al., 2000). The process of
cytokinesis, i.e. the extension and fusion of
Fig 6.22 Chemical structure of the hormone l-sirenin which
attracts male gametes of Allomyces macrogynus.The structure
of parisin, attractive to female gametes, does not seem to have
been elucidated as yet.
membranes, seems to be mediated principally
by the actin component of the cytoskeleton
(Lowry et al., 2004). According to Barron and
Hill (1974), the development of the cleavage