The fungi exhibit a polyphyletic range of sexual differentiation, with a variety of mating systems promoting genetic exchange and gene flow (see Carlile, 1987; Prillinger, 1987). This review considers examples of the biochemical and cytological mechanisms of fusions of cells of different mating types (Table 1).
Membrane fusion of motile gametes
Allomyces macrogynus, a member of the Chytridiomycetes, produces five types of flagellate cell in its life cycle: male and female gametes, the resultant zygote, and haploid and diploid zoospores. Of these, fusion occurs only between a male and female gamete. Both gametes are motile by means of one posterior flagellum, but there are many differences between them that must be phenotypic sexual differences, since they are both produced by the same haploid plant and must be isogenic. Only the female produces the chemotactic attractant, sirenin, and only the male (the spermatozoid) is attracted by it (Carlile & Machlis, 1965; Pommerville, 1978, 1981). Likewise, but less obviously, the male produces an attractant, parisin, which attracts the female (Pommerville & Olson, 1987). Sirenin is a sesquiterpene (Fig. 1d) and parisin shows similar chemical characteristics. The male gamete swims much more actively than the female, is bright orange with an accumulation of γ-carotene, is much smaller than the female, and has fewer and smaller mitochondria (Fig. 1a,b). It also appears to lack the ‘sidebody complex’ or ‘Stüben body’, a complex of lipid granules and microbodies of unknown function that is present in the female, and has fewer γ-like particles (Pommerville & Fuller, 1976).