Introduction to Fungi, Third Edition

TRICHOMYCETES
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Simulium in the place of eggs (Moss & Descals,
1986; Lichtwardt, 1996). A similar stage has
been reported from Simulium infected with
Genistellospora homothallica (Labeyrie et al., 1996).
Ovarian tissue is invaded by the fungus, growing
in a parasitic mode. Cysts of H. melusinae dissected
from ovaries are surrounded by a membranous
sheath. They are ellipsoidal and, on germination,
form two germ tubes, one at each pole, ending as
a spherical knob, the generative cell initial. A
single generative cell develops from each initial
and forms a terminal trichospore (Fig. 7.47d). The
chlamydospores are deposited among egg masses
and infection of young larvae results from
ingestion of trichospores produced by them. The
ovarian chlamydospores therefore represent
a ‘missing link’ in the life cycle of Harpellales.
Adult blackflies do not contain trichomycete
thalli because at the final ecdysis (moult)
before pupation, the cuticular lining of the
larval gut is shed.
Sexual reproduction
This occurs by the production of zygospores and
has so far been reported only in Harpellales. In
H. melusinae zygospores are rarely detected,
possibly because they are associated with the
last stage of development of the Simulium larval
host before pupation and are shed at ecdysis.
Zygospore formation is preceded by conjugation
between swollen cells on adjacent thalli (see
Fig. 7.47e; Lichtwardt, 1967). From one of the
conjugating cells a zygosporophore grows out,
and from this a biconical zygospore develops.
The biconical shape, which is characteristic of
the Harpellales, is possibly adapted to passage
through the insect gut. In some members of the
group zygospores bear polar filamentous appendages,
but these are absent in Harpella (Moss &
Lichtwardt, 1977). The cytological details of
zygospore formation have not been fully worked
out but in H. melusinae the zygospore, zygosporophore
and the two conjugant cells each contain
a single nucleus. Moss and Lichtwardt (1977) have
speculated that the four nuclei might have been
derived from meiotic division of a diploid zygote
nucleus within the fused conjugants. On this
hypothesis the conjugant cells would be interpreted
as gametangia, a situation markedly
different from that found in other zygomycetes.
Relationships
On the basis of similarities in serological reactions,
septal pore structure and in sporangial
morphology, it has been suggested that the
Harpellales are related to the Kickxellales, an
order of mostly dung- and soil-inhabiting saprotrophic
zygomycetes (Moss & Young, 1978).
However, the evidence for a phylogenetic relationship
between these two groups is conflicting.
K.L. O’Donnell et al. (1998) and Gottlieb and
Lichtwardt (2001) have attempted to correlate
morphological criteria with molecular data (18S
rDNA) but found only poor support, whereas
Tanabe et al. (2004), comparing a range of DNA
sequences, found a strong link between
Harpellales and Kickxellales (see Fig. 7.1).