3.10 The untrue fungi

In our discussion so far, we have sometimes used the description ‘true fungi’ to refer to those organisms that belong to Kingdom Fungi. This implies that there are some organisms that might be called ‘untrue fungi’ because they look like fungi but are not fungi. And, well, yes this is the case (Rossman & Palm, 2006).

Water moulds. This is an informal grouping that includes the most ancient fungi and fungus-like organisms:

  • The Chytridiomycota, water moulds that are the ancestral group of the true fungi (Kingdom Fungi) and which we have already discussed [CLICK HERE if you need a reminder].
  • Oomycota & Hyphochytriomycota - water moulds that are not true fungi, but have affinities to the algae. They are now placed in Kingdom Chromista (as subphylum Pseudofungi; Cavalier-Smith, 2018), or (our preference) Kingdom Straminipila (Beakes et al., 2014).

Phylum Oomycota (Beakes et al., 2014) consists of about 600 species in 90 genera, placed in the following Orders:

  • Leptomitales; example genera: Apodachlyella, Ducellieria, Leptolegniella, Leptomitus
  • Myzocytiopsidales; example genus: Crypticola
  • Olpidiopsidales; example genus: Olpidiopsis
  • Peronosporales; example genera: Albugo, Peronospora, Bremia, Plasmopara
  • Pythiales; example genera Pythium, Phytophthora, Pythiogeton
  • Rhipidiales; example genus: Rhipidium
  • Salilagenidiales; example genus: Haliphthoros
  • Saprolegniales; example genera: Leptolegnia, Achlya, Saprolegnia
  • Sclerosporales; example genera: Sclerospora, Verrucalvus
  • Anisolpidiales; example genus: Anisolpidium
  • Lagenismatales; example genus: Lagenisma
  • Rozellopsidales; example genera: Pseudosphaerita, Rozellopsis
  • Haptoglossales; example genera: Haptoglossa, Lagena, Electrogella, Eurychasma, Pontisma, Sirolpidium.

The Oomycota produce biflagellate zoospores bearing one whiplash and one tinsel-type flagellum on each zoospore (see chapter 2 in Carlile et al., 2001). The genera Saprolegnia, Achlya, Phytophthora and Pythium are prime examples.

Table 1. Some differences between Oomycota and true fungi

Character

Oomycota, Kingdom Straminipila

Eumycota in Kingdom Fungi

Flagella, when present

Biflagellated cells with anterior tinsel and posterior whiplash flagella

Uniflagellate or multiflagellate cells with posterior whiplash flagella

Microfibrils in cell walls

Cellulose (also present in algal and plant cell walls)

Chitin or chitosan

Protein in cell wall characterised by

Hydroxyproline

Proline

Sterol in membrane

Cholesterol, demosterol

Ergosterol

Ploidy throughout most of life cycle

Diploid

Usually haploid or dikaryotic, rarely diploid

Cristae in mitochondria

Tubular

Plate-like (like most animals)

Golgi

Stacks of cisternae, similar to those in plants and algae

Simple cisternae which are not-stacked

Intermediate in lysine biosynthesis

α-ε-diaminopimelic acid (DAP)(as in algae and plants)

a-aminoadipic acid (AAA)(as in euglenoid protozoa)

NAD-linked isocitric dehydrogenase

Absent

Present

Base pair at the very base of helix 47 in the variable regions of the 18S rRNA sequences

AU

UA

Although these are primitive organisms, at least in terms of their evolutionary position with respect to Fungi, they are highly adapted to their life style. This is illustrated by the behaviour of zoospores of Oomycota.

  • Temperature influences the fate of the zoosporangium in Phytophthora infestans (cause of Potato Blight): below 15°C the zoosporangium forms zoospores, but above 20°C it forms a germ tube (so in the cold water of the soil the zoospores will swim to find new hosts; in the warm sunshine the sporangium will infect the plant.
  • After release, zoospores typically swim for many hours; zoospores of Phytophthora megasperma swim at 88 µm sec-1 at 15°C; so it only takes them 11 seconds to cover a distance of 1 mm.
  • Zoospores may show amoeboid movement when in contact with a solid substratum; allowing slow-speed targeting on the host.
  • Zoospores show tactic movements. A ‘taxis’ is a movement towards or away from a stimulus (tropism is growth towards or away from a stimulus).
  •  Zoospores of Phytophthora palmivora are negatively geotactic (they swim upwards; which is where the nice newly-formed host leaves and buds will be found).
  • Zoospores of Pythium aphanidermatum have a positive chemotaxis to roots.

Hyphae of Oomycota are also positively chemotropic (hyphae of true fungi are not chemotropic, although they do exhibit other tropisms, particularly autotropism).

Saprolegnia is another important genus. Saprolegnia species are parasites of freshwater fish and fish eggs, and can cause economic damage on fish farms. Reproduction is mainly asexual, but the life cycle (Fig. 5) includes a sexual phase.

Diagrammatic life cycle of Saprolegnia
Fig. 5. Diagrammatic life cycle of Saprolegnia

Hyphal branches become modified into long zoosporangia separated from the hypha by septa. Biflagellate zoospores released from a zoosporangium swim for a while and then encyst. Each eventually gives rise to a secondary zoospore, which also encysts and then germinates to produce a new mycelium.

For sexual reproduction, compatible oogonia and antheridia develop on the same diploid mycelium. Meiosis occurs within these gametangia. In mating, antheridia grow toward the oogonia and develop tubular processes called ‘fertilisation tubes’, which penetrate the oogonia. Male nuclei travel through the fertilisation tubes to fuse with the female nuclei within (karyogamy). Following karyogamy, a thick-walled zygote, called the oospore, is produced. The oospore germinates into hyphae, which then produce a zoosporangium.

Pheromones in Achlya

  • Achlya bisexualis is heterothallic. Sterols are involved in the hormonal mechanisms regulating sexual reproduction in Achlya (as they are in animals).
  • The female mycelium produces antheridiol, which induces male hyphae to make antheridial branches.
  • Antheridial hyphae are attracted by antheridiol (this is a chemotropism) to the female mycelium.
  • Male mycelium produces the hormone oogoniol (another sterol), which induces female mycelium to produce oogonia.
  • Antheridia grow around the oogonium, producing fertilisation tubes (plasmogamy) and enable karyogamy which results in oospore formation. So even these primitive organisms have a sterol hormone mechanism for cell targetting.

Other fungus-like members of Kingdom Straminipila are:

  • Phylum Hyphochytriomycota (Beakes et al., 2014), which are microscopic organisms that form a small thallus, often with branched rhizoids, occurring as parasites or saprotrophs on algae and fungi in freshwater and in soil. The whole of the thallus is eventually converted into a reproductive structure. Only 23 species (in 6 genera) are known and are placed in Order Hyphochytriales (example genera are: Hyphochytrium, Rhizidiomyces).
  • Phylum Labyrinthulomycota (Beakes et al., 2014), in which the feeding stage comprises an ectoplasmic network and spindle-shaped or spherical cells that move within the network by gliding over one another. They occur in both salt- and freshwater in association with algae and other chromists. There are about 45 species in 10 genera, placed in the two Orders Labyrinthulales (e.g. Labyrinthula) and Thraustochytriales (e.g. Thraustochytrium).

Slime moulds. The organisms known as slime moulds are now all placed in Kingdom Protozoa. They do not form hyphae, and they generally lack cell walls, being capable of ingesting food particles by phagocytosis. The slime moulds fail to meet normal definitions of fungi, but they produce fruiting bodies which have a superficial resemblance to those of fungi, and this is why they have been called ‘moulds’ and have been studied by mycologists and included in most textbooks on mycology. They are placed in three phyla in the Protozoa:

  • Order Plasmodiophoromycota (included in the Phylum Phytomyxea) are obligate intracellular symbionts or parasites of plant, algal or fungal cells living in freshwater or soil habitats. They have multinucleate, unwalled plasmodia. There are about 15 genera with 50 species in the Order Plasmodiophorales; example genera are Plasmodiophora, Polymyxa and Spongospora. Plasmodiophora and Spongospora cause serious plant diseases (Bulman & Braselton, 2014).
  • Phylum Myxomycota (in the Supergroup Amoebozoa) are free-living unicellular or plasmodial amoeboid slime moulds. A total of 900 species assigned to 80 genera, and seven Orders, among which are the Dictyosteliales (e.g. Dictyostelium), Physarales (e.g. Didymium, Physarum, Fuligo) and Stemonitales (e.g. Stemonitis) (Stephenson, 2014).
  • Phylum Acrasiomycota (in the Supergroup Excavata) are generally saprotrophic amoeboid slime moulds, found on a wide range of decaying plant material. A total of 12 species assigned to 6 genera in the single Order Acrasiales (e.g. Acrasis, Copromyxa) (Stephenson, 2014).

Finally in the Phylum Choanozoa (Kingdom Protozoa) are placed two groups of organisms previously misclassified as trichomycete fungi. They are now placed in Class Mesomycetozoea making up the Orders Amoebidiales and Eccrinales. They are intimately associated with arthropods, insects, millipedes and crustaceans and have a coenocytic thallus attached to the host by a holdfast.

Updated July, 2019