untitled

usually leads to rapid death of the fused hyphal

compartments – a reaction governed by hetero-

karyon incompatibility (het) loci (see Figs 9.6, 9.7).

• The hyphae of Oomycota seldom if ever exhibit
  “vegetative” anastomosis, but they do undergo
  hyphal fusions when they produce sexual hyphae of
  opposite mating types; the development of these is
  controlled by sex hormones (see Fig. 5.22).

The structure of yeasts

Several fungi grow as budding, uninucleate yeasts,
rather than as hyphae. Common examples include
the bread yeast, Saccharomyces cerevisiae(Ascomycota,
Fig. 3.7), Cryptococcus spp. (Ascomycota, Fig. 3.7),

and Sporobolomyces roseus (Basidiomycota). Some

other fungi are characteristically dimorphic(with two

shapes): they switch between a hyphal and a yeast form

in response to changes in environmental conditions

(Chapter 5). A good example is the genus Candida

(Ascomycota), including the common species Candida

albicans, which can be a significant pathogen of humans

(Chapter 16). Therefore, yeasts are not fundamentally

different from hyphal fungi; they merely represent a

different growth form, adapted to spread in nutrient-

rich water films and similar types of environment.

As shown in Fig. 3.8, each yeast cell has a single

nucleus and a typical range of cytoplasmic organelles,

including a large, conspicuous vacuole and usually

several phase-bright lipid bodies. The vacuole and the

lipid bodies are often the only structures clearly visible

FUNGAL STRUCTURE AND ULTRASTRUCTURE 53

Fig. 3.5(a–e) Stages in the development of a fun-
gal colony from a germinating spore. The broken lines
in (e) represent narrow anastomosing hyphae near
the center of the colony.

Fig. 3.6Videotaped sequence of anastomosis of two hyphae of Rhizoctonia solani. The times shown are minutes after
the start of video recording. The upper hypha had stopped growing at t 0 but began to produce a branch (arrowhead)
at t 6 in response to the approaching hyphal tip. The hyphal tips met at t 14. Dissolution of the tip walls and complete
hyphal fusion was achieved at t 32. (From McCabe et al. 1999.)