Introduction to Fungi, Third Edition

88 STRAMINIPILA: OOMYCOTA
Fig 5.9 Achlya colorata.(a d) Stages
in the development of oogonia.
(e) Six-month-old oospores
germinating after 40 h in charcoal
water.
zoospores, growth is usually renewed laterally by
the outgrowth of a new hyphal apex just beneath
the first sporangium (Fig. 5.8a), rather than by
internal proliferation.
Sexual reproduction in Achlya
Some species of Achlya are homothallic (Fig. 5.9)
whereas others are heterothallic (Fig. 5.10).
Achlya colorata, a homothallic species common
in Britain, has oogonial walls which develop
blunt, rounded projections so that the oogonium
appears somewhat spiny (Fig. 5.9d). Otherwise,
the process of sexual reproduction is similar to
that of Saprolegnia litoralis (Fig. 5.6). Germination
of oospores is often difficult to achieve with
Oomycota, but can be stimulated in A. colorata by
transferring mature oospores to freshly distilled
water (preferably after shaking with charcoal
and filtering). Germination occurs by means of a
germ tube which grows out from the oospore
through the oogonial wall. Here it may continue
growth as a mycelium (Fig. 5.9e) or may give rise
to a sporangium.
The study of heterothallic species of Achlya by
John R. Raper quickly revealed that the formation
of oogonia and antheridia by compatible
strains must be under hormonal control (Raper,
1939, 1957). A particularly readable account of
the classical series of experiments leading to the
discovery of the steroid sex hormone, antheridiol
(Fig. 5.11b), has been given by Carlile (1996b).
Several reviews of the broader role of hormones
in fungal reproduction have appeared recently
(Gooday & Adams, 1992; Elliott, 1994). If isolates
of Achlya bisexualis, A. ambisexualis or A. heterosexualis
made from water or mud are grown singly