Introduction to Fungi, Third Edition

INTRODUCTION
77
Fig 5.2 Schematic drawing and terminology of sexual
reproductive organs in the Oomycota. Modified from Dick
(1995).
obtained from several species (see Fig. 5.24). At
the onset of encystment, adhesive and cell wall
material is secreted by the synchronized fusion
of pre-formed storage vesicles with the zoospore
plasma membrane (Hardham et al., 1991;
Hardham, 1995), thereby providing a rare example
of regulated secretion in fungi. Constitutive
secretion by growing hyphal tips is more commonly
associated with their mode of life.
Some members of the Oomycota have no
motile spore stages but can be readily related
to groups still producing them.
5.1.3 Sexual reproduction
The life cycle of the Oomycota is of the
haplomitotic B type, i.e. mitosis occurs only
between karyogamy and meiosis. All vegetative
structures of Oomycota are therefore diploid
(see Figs. 5.3 and 5.19). This is in contrast to the
Eumycota in which vegetative nuclei are usually
haploid, the first division after karyogamy being
meiotic. Sexual reproduction in Oomycota is
oogamous, i.e. male and female gametangia are
of different size and shape (Fig. 5.2). Meiosis
occurs in the male antheridia and in the female
oogonia, and is followed by plasmogamy (fusion
between the protoplasts) and karyogamy (fusion
of haploid nuclei). Numerous meioses can occur
synchronously, so that true sexual reproduction
can actually happen within the same protoplast
(Dick, 1990a). Heterothallic species of Oomycota
display relative sexuality, i.e. a strain can produce
antheridia in combination with a second
strain but oogonia when paired against a third
(see pp. 86 and 95). Steroid hormones play
an important role in sexual reproduction (see
Fig. 5.11).
The mature oospore contains three major
pools of storage compounds (Fig. 5.2; Dick, 1995).
The oospore wall often appears stratified, and
this is due in part to a polysaccharide reserve
compartment, the endospore, which is located