Abstract
Unmixed gametes differ structurally from vegetative cells in having a specialised cell organelle, thegamosome, which is formed just below the cell apex during gametogenesis. It resembles half a desmosome in structure, consisting of a thickened patch of plasma membrane and an underlying dense plaque (which probably has a central hole). The characteristic flagellar agglutination which occurs when plus and minus gametes are mixed appears to depend on adhesion between the flagellar surface coats of the clumping and pairing gametes. The formation of cell pairs, each consisting of one plus and one minus gamete, is followed by the breakdown of their cell walls and by a structural transformation of the gamosome, thegamosome reaction: the central part of the gamosome (of at least one mating type—the plus) evaginates to form a protruding membrane-boundedgamosomal tubule. The membrane at the tip of the plus tubule fuses with that of the minus gamosome (or possibly gamosomal tubule) to form a slender, membrane-boundedcopulation tube (about 0.15 μm thick) between the mating gametes. The tube widens and shortens as the contents of the two cells flow together inside the common plasma membrane to form a spherical quadriflagellate zygote. A crescentic body, probably a remnant of part of the plus gamosome, persists in the cells for a while.
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