untitled

(Fig. 2.34d). The contents of the sporangium are

extruded into this vesicle, where the zoospores differ-

entiate and then are released by rupture of the vesicle.

Classification, ecology, and significance

There are estimated to be 500–800 species in the

Oomycota, classified in 5 or 6 Orders. However, some

of these Orders contain only a few organisms, which

will be mentioned where appropriate in later chapters.

Here we will focus on the two major Orders, Sapro-

legnialesand Peronosporales.

The Saprolegniales, commonly known as water

moulds, are extremely common in freshwater habitats

and in some brackish, estuarine environments. They can

often be baited by immersing hemp seeds in natural

waters, but it is difficult to free the cultures from

bacteria. Several species of Achlyaand Saprolegniaare

common saprotrophs in freshwater habitats, and

contribute to the recycling of organic matter, but S.

diclinais an important pathogen of salmonid fish.

The related Aphanomycesspp. are perhaps best-known

as aggressive root pathogens (e.g. A. euteicheson pea

and beans). But A. astaciis a pathogen of crayfish and

has virtually eliminated the native European crayfish

(the disease being known as crayfish plague). The

introduced American crayfish, however, is resistant

to the disease and has now replaced the European

crayfish in almost all sites.

The Order Peronosporalesis by far the largest and

most economically important group. It includes many

serious plant pathogens. For example, many Pythium

spp. cause seedling diseases and attack the feeder

roots of almost all crop plants (Chapter 14), but a few

Pythiumspp. parasitize other fungi and have potential

for biocontrol of plant diseases (Chapter 12). The

Peronosporales also includes all the Phytophthoraspp.,

causing diseases such as potato blight (P. infestans).

One of the most aggressive plant pathogens is

Phytophthora cinnamomi, with an extensive host range.

It causes root rot of pines, eucalypts, fruit trees, and

many other plants in warmer regions of the globe, and

can invade natural vegetation with devastating con-

sequences. A similarly aggressive species, Phytophthora

ramorum, causes sudden oak death in California

(Chapter 14) and is currently spreading across Europe,

posing a threat to many native European trees. Also

in this group are the important downy mildew

pathogens (e.g. Bremia lactucaeon lettuce, Plasmopara

viticolaon grape vines) which are obligate biotrophic

parasites (Chapter 14).

The remaining Orders include Leptomitales (a

small group of aquatic fungi, such as Leptomitis lacteus

which is common in sewage-polluted waters). This

group has several interesting and peculiar features, such

as fermentative rather than respiratory metabolism

(Chapter 8), a requirement for sulfur-containing

amino acids because they cannot use inorganic sulfur

sources, and hyphae that are constricted at intervals,

like chains of sausages. The walls of some species con-

tain significant amounts of chitin (which is unusual

for Oomycota) and the hyphal constrictions are often

plugged by cellulin granules, composed mostly of chitin.

The Order Lagenidalesis a small group of common

symptomless parasites of plant roots (e.g. Lagena

radicicola; Macfarlane 1970) or parasites of algae,

fungi, or invertebrates (e.g. Lagenidium giganteumon

nematodes, mosquito larvae, etc.). Often these cannot

be grown in culture in the absence of a host.

The cellular slime moulds

The cellular slime moulds comprise two groups of

organisms – the dictyostelid cellular slime moulds

and the acrasidcellular slime moulds. These two

groups are biologically similar to one another, but

the dictyostelids seem to be a monophyletic group,

whereas the acrasids seem to be more disparate. Both

types of organism grow and divide as haploid, unicel-

lular amoebae, which engulf bacteria and other food

particles by phagocytosis. They are commonly found

in moist organic-rich soil, leaf litter, animal dung,

and similar types of substrate. When nutrients

become depleted, the amoebae undergo a complex

developmental sequence in which many thousands of

amoebae aggregate and eventually produce a stalked

fruiting body (Fig. 2.35). This process has been

studied intensively in the dictyostelid slime mould,

Dictyostelium discoideum, which has become a model

system of cell communication and differentiation.

At the onset of starvation, a few amoebae act as

an aggregation center and produce periodic pulses of

cyclic AMP (cAMP). This causes other amoebae to join

the aggregate by chemotaxis, and the amoebae then

undergo streaming along defined tracks so that they

aggregate as a mound. Then the cells in the mound

differentiate into two sorts – the pre-stalk cells and

the pre-spore cells, each in its defined position where

it will later become a stalk cell or a spore. At this stage

the mound topples over, and the resulting slug (grex)

migrates to the surface of the substrate where it dif-

ferentiates into a fruiting body (sorocarp) with a stalk

and a sporing head. The walled spores are subsequ-

ently released and wind-dispersed.

A sexual cycle has also been described, leading to

the production of thick-walled, diploid macrocysts,

which undergo meiosis followed by repeated mitotic

divisions. Then the cytoplasm is cleaved and many hap-

loid amoebae are released to repeat the cycle. Detailed

coverage of the dictyostelids can be found in Raper

(1984).

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