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DIFFERENTIATION AND DEVELOPMENT 99

Fig. 5.17(a,b) Detached sporangia of Phytophthora infestans, incubated at 12°C. The sporangial contents cleave to

produce zoospores, then the apical papillum of the sporangium breaks down and the zoospores are released by

squeezing through the narrow opening.

(a) (b)

protein synthesis on 70S ribosomes, as occurs in

bacteria. Griffin & Coley-Smith (1975) investigated this

by addingradiolabeled streptomycin to P. humuli. Up

to 95% of the label remained on or near the cell

surface and could not be removed with water, but it

was readily displaced by calcium ions. Consistent

with this, streptomycin acts like a divalent cation in

solution, so it might interfere with calcium-mediated

processes by competing for calcium-binding sites on

or near the cell surface.

Conidia

Conidia are formed in a variety of ways (see Fig. 2.12;

Fig. 5.18) but always “externally” on a hypha or a coni-

diophore rather than by cytoplasmic cleavage within

a sporangium. For example, conidia can be produced

by the swelling of a hyphal tip followed by septation

(Thermomyces lanuginosus; Fig. 5.18), by a sequential

budding process (the “monilinia” stage of Sclerotinia

fructingena; Fig. 5.18), by hyphal fragmentation, or by

successive extrusion from flask-shaped cells (phialides)

leading to chains of spores (Penicillium expansum, Fig.

5.18). In a few fungi the conidia develop on or in more

complex structures such as a coremium (an aggregated

mass of conidiophores; Fig. 5.18) or within a flask-

shaped pycnidium, or on a pad of tissue (an acervulus

• for example, Colletotrichum musae; Fig. 5.4). These
  developmental (ontogenic) patterns have been studied
  intensively, at least partly in an attempt to find nat-
  ural relationships and thus a natural approach to the
  classification of the mitosporic fungi. Details can be
  found in Cole & Samson (1979).
  Although the patterns of conidium development
  are diverse, a basic distinction can be made between
  blasticconidia which are formed by a budding or
  swelling process and then become separated from the
  parent cell, and thallic conidia which are formed
  essentially by a fragmentation process.
  Neurospora crassacan be used as an example of
  blastic conidial development. This fungus sporulates by
  producing aerial hyphae (conidiophores) that grow for
  some distance away from the substrate then swell at
  their tips. The tip of the swelling produces a broad, bud-
  like outgrowth which swells and repeats this process,
  so that a branched chain of proconidia is formed,
  resembling the Moniliniastage of Sclerotinia fructigena