Introduction to Fungi, Third Edition

SAPROLEGNIALES
89
Fig 5.10 Achlya ambisexualis.
(a) Male and female mycelia grown
on hemp seeds and placed together
in water for 4 days. Note the
formation of antheridial branches
on the male and oogonial branches
on the female. (b) Fertilization,
showing the diclinous origin of the
antheridial branch.
Fig 5.11 Sterols from Achlya spp.Fucosterol (a) is one of the
most abundant sterols in Oomycota and precursor to the sex
hormones antheridiol (b) and oogoniol (c).
on hemp seed in water, reproduction is entirely
asexual, but when certain of the isolates are
grown together in the same dish, it becomes
apparent within 2 3 days that one strain is
forming oogonia, and the other antheridia. The
development of oogonia and antheridia occurs
even when the two strains are held apart in the
water or separated by a cellophane membrane or
by agar. This suggests that one or more diffusible
substances are responsible for the phenomenon.
As compatible colonies approach each other, the
first observable reaction is the production of fine
lateral branches behind the advancing tips of the
male hyphae. These are antheridial branches.
By growing male (antheridial) strains in water
in which a female (oogonial) strain had been
grown previously, Raper (1939) showed that
the vegetative female mycelium was capable
of initiating the development of antheridial
branches on the male. The reverse experiment
showed no effect on female colonies in medium
in which undifferentiated male colonies had
been grown. The role of the vegetative female
colony as initiator of the sequence of events
leading to sexual reproduction was confirmed by
ingenious experiments in micro-aquaria consisting
of several consecutive chambers through
which water flowed by means of small siphons.
Male and female colonies were placed alternately
in successive chambers, so that water from a
male colony would flow over a female colony and
so on. If a female colony was placed in the first
chamber, the male colony in the second chamber
reacted by developing antheridial hyphae. If,
however, a male colony was placed in the first
chamber, the male colony in the third chamber