New structural features of gametes are presented and several physiological aspects of mating and gamete fusion in the fungus Allomyces macrogynus are described. In the posterior region of the male gametes, where fusion is thought to occur, thin-section electron microscopy demonstrated the presence of microfilaments that were associated with a ruffling of the otherwise smooth plasma membrane. When the cells were treated with the microfilament-disruptive drug cytochalasin B, fusion was inhibited due to the interaction of the drug with the female gamete. Freeze-fracture replica analysis of the posterior region of the gamete plasma membrane showed an area devoid of membrane particles (termed the flagellar collar) at the basal end of the flagellum. Physiological analysis of gamete mating and fusion showed a requirement for divalent cations, and the time required for gamete mating could be lengthened or shortened by decreasing or increasing, respectively, the divalent cation concentration. Diphenhydramine and chloroquine also increased the time required for mating or completely prevented fusion. Tertiary amine local anaesthetics produced the same type of results, as did trypsin and Pronase. Phospholipases were without an inhibitory effect. When the gametes were washed free of the drugs prior to mating, it was seen that the female gametes were more sensitive to cell surface changes than the males. In addition, 2-h-old female gametes showed a 70% loss of fusion ability with young male gametes while the reverse experiment showed only a 20% decrease. These observations support the concept that gamete mating and fusion may occur at the flagellar collar area and that mating in A. macrogynus is governed, at least in part, by cation and protein interactions.

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