Cysteine as a fusion inhibitor in the sexual mating reaction of the green alga Chlamydomonas eugametos

SUMMARY Sexual mating of the biflagellated green alga Chlamydomonas eugametos is initiated by flagellar adhesion. This is followed by the formation of vis-a-vis pairs in which two cells of opposite mating type are connected by a slender cytoplasmic bridge. This neck slowly widens and, finally, zygotes are formed. Vis-a-vis pair formation was reversibly inhibited by cysteine. This effect was only observed when cysteine was present during the first few minutes of mating. Cysteine specifically prevented the protrusion of a plasma papilla between the flagellar bases; other early mating reactions tested were not affected. When cysteine was added to vis-a-vis pairs, the neck did not widen. It is argued that cysteine inhibits an autolysin with a protease character involved in dissolving the cell wall between the flagellar bases during early mating and in widening the neck of vis-a-vis pairs.