Introduction to Fungi, Third Edition

BLASTOCLADIALES
159
membranes is induced by the availability of free
water. Zoospores are released from the sporangia
after dissolution of the plugs blocking the exit
papillae. Since nuclear division in the thinwalled
sporangia is mitotic, these are termed
mitosporangia, and the diploid swarmers they
release are mitospores. The mitospores, after a
swimming phase, encyst and are capable of
immediate germination, developing into a
further diploid asexual thallus.
The second type of zoosporangium is the dark
brown, thick-walled, pitted resting sporangium
(meiosporangium), formed at the tips of the
branches. Meiotic divisions within these sporangia
result in the formation of the haploid
meiospores, which develop into sexual thalli.
The life cycle of a member of the subgenus
Eu-Allomyces is thus an isomorphic alternation
of gametothallic and sporothallic generations
(Fig. 6.21). Comparisons of the nutrition and
physiology of the two generations show no
essential distinction between them up to the
point of production of gametangia or sporangia.
Emerson and Wilson (1954) have made cytological
and genetic studies of a number of
collections of Allomyces. Interspecific hybrids
between A. arbuscula and A. macrogynus have
been produced in the laboratory, and it has
been shown that the fungus earlier described
as A. javanicus is a naturally occurring hybrid
between these two species. Cytological examination
of the two parent species and of artificial
and natural hybrids showed a great variation
in chromosome number. In A. arbuscula the
basic haploid chromosome number is 8, but
strains with 16, 24 and 32 chromosomes have
been found. In A. macrogynus the lowest haploid
number encountered is 14, but strains with
28 and 56 chromosomes are also known. The
demonstration that these two species each
represent a polyploid series was the first to
be made in fungi. The wild-type strain of
A. macrogynus appears to be an autotetraploid
which, after meiosis, produces diploid gametothalli
(Olson & Reichle, 1978).
The behaviour of the hybrid strains is of
considerable interest. As seen above, the parent
species differ in the arrangement of the primary
pairs of gametangia, A. arbuscula being
hypogynous whilst A. macrogynus is epigynous.
Following fusion of gametes derived from different
parents, zygotes formed, germinated and
gave rise to sporothalli. The meiospores from the
hybrid sporothalli had a low viability (0.1 3.2%),
as compared with a viability of about 63% for
A. arbuscula meiospores, but some germinated
to form gametothalli. The arrangement of the
gametangia on these F 1 gametothalli showed
a complete range from 100% epigyny to 100%
hypogyny. Also, in certain gametothalli the ratio
of male to female gametangia (normally about
1:1) was very high, with less than one female
per 1000 male gametangia. It was concluded
from these experiments that, since intermediate
gametangial arrangements are found in hybrid
haploids, this arrangement is not under the
control of a single pair of non-duplicated allelic
genes, but that a fairly large number of genes
must be involved. Hybridization in some way
upsets the mechanism which controls the
arrangement of gametangia in the parental
species. By treating meiospores of A. macrogynus
with DNA extracted from gametothallic cultures
of A. arbuscula, Ojha and Turian (1971) have
demonstrated an inversion of the normal gametangial
arrangement, i.e. a proportion of the
DNA-treated meiospores developed colonies
with hypogynous antheridia instead of the
normal epigynous arrangement. Similar inversions
were also obtained in converse experiments.
In an isolate of the naturally occurring
hybrid A. javanicus, Ji and Dayal (1971) have
shown that although copulation between anisogamous
gametes results in the formation of
sporothalli bearing thin-walled and thick-walled
sporangia, the swarmers from the thick-walled
sporangia rarely develop into gametothalli, but
into sporothalli. This is not surprising for a
hybrid, and is possibly due to a failure of meiosis
in the thick-walled sporangia.
Sub-genus Cystogenes
A life cycle different from Eu-Allomyces is found
in Allomyces moniliformis and A. neo-moniliformis.
There is no independent gametothallic generation,
but this stage is probably represented by
a cyst (C. M. Wilson, 1952). The asexual thalli
resemble those of subgenus Eu-Allomyces, bearing