52 CHAPTER 3
(a) (b)
(c)Fig. 3.4Electron micrographs of hyphae of Athelia (Sclerotium) rolfsiiprepared by freeze substitution. (a) Part of the
apical compartment, about 7μm diameter, showing the distribution of major organelles: Spitzenkörper (S); mitochondria
(M); tubular vacuoles (Va); nucleus (N). (b) The Spitzenkörper at the extreme tip of the hypha consists of a cluster of
apical vesicles (AV ) with contents of varying electron density, surrounding a vesicle-free zone; a microtubule (MT ) extends
to the plasma membrane (P). Next to the plasma membrane are bundles of F-actin (filasomes, F). The wall consists of at
least two layers of different electron density ( WL 1, WL 2). (c) Subapical region of the hypha, showing mitochondria,
tubular vacuoles, endoplasmic reticulum (ER), and Golgi bodies (G) (Courtesy of R. Roberson; see Roberson & Fuller
1988, 1990.)
- Nutrient-poor conditions near the center of a
colony create exactly the opposite effect, in which
hyphae grow towards one another and fuse at the
points of contact. As seen in Fig. 3.6, this again
involves precise orientation of hyphal tip growth,
because the “homing response” is extremely accurate
and always leads to tip-to-tip fusion. We have no
knowledge of the factors involved.- Hyphal anastomosis only occurs between members
(colonies) of the same species, and seldom, if ever,
between unrelated species. Even within a species,
strains that are genetically different can fuse, but this