Introduction to Fungi, Third Edition

AQUATIC HYPHOMYCETES (INGOLDIAN FUNGI)
693
Tumularia aquatica (Fig. 25.20b) has pear-shaped
or broadly fusiform blastoconidia which separate
by septal dissolution. The teleomorph of this
fungus is Massarina aquatica which forms pseudothecia
on submerged wood in streams.
25.2.4 The significance of spore shape
in aquatic fungi
We have seen that tetraradiate conidia may
develop in a variety of ways and are produced
both by Ascomycota and Basidiomycota.
Tetraradiate propagules have also been found
as secondary conidia of Entomophthoraceae
attacking aquatic insects (see Fig. 7.42; Descals
et al., 1981), and as basidiospores in the marine
fungi Digitatospora marina (Homobasidiomycetes)
and Nia vibrissa (gasteromycetes) (Doguet, 1962,
1967). The brown alga Sphacelaria also forms
tetraradiate propagules. There is thus ample
evidence for the view that this type of structure
has evolved repeatedly in aquatic environments.
The functional significance of convergent evolution
of tetraradiate and sigmoid spore shapes has
been discussed by Webster (1987). Experimental
studies have shown that tetraradiate propagules
are more effectively trapped by impaction onto
underwater objects than spores of more conventional
shape. This is because a tetraradiate
propagule making contact with a surface will
achieve a three-point landing, a very stable form
of attachment. Attachment is aided by secretion
of mucilage at the tips of the arms which quickly
develop ‘appressoria’ and germ tubes, but the
fourth arm not in contact with a surface fails
to differentiate in this way (Read et al., 1991,
1992a,b; Jones, 1994). Once appressoria have
been formed, the propagules are very resistant
to detachment.
Trapping efficiency may not be the only
advantage of a tetraradiate propagule because
these spores may also be found among leaves in
terrestrial habitats (Bandoni, 1972; Park, 1974).
Fig 25.20 (a) Margaritispora aquatica,
conidiophores, phialides and
phialoconidia. (b) Tumularia aquatica,
conidiophores and blastoconidia.