The Chlamydomonas Fus1 protein is present on the mating type plus fusion organelle and required for a critical membrane adhesion event during fusion with minus gametes.
Gametes of C. reinhardi lack the cell wall which vegetative cells possess. Just below the cell apex gametes form a. fertilization tubule which is up to 2 fi long and 0-2 /* in diameter; its plasma membrane and that of the apex have slender tubular projections. At the base of the fertilization tubule regularly lies the choanoid body, a collar-shaped cytoplasmic organelle; the plasma membrane overlying the body appears as an electron-dense ring. Gametes possess two 'free' basal bodies in addition to the basal bodies of the two flagella. In the initial stage of union the conjugating cells are connected by the fertilization tubule whose plasma membrane is continuous with that of both copulants. At one end of the tubule lies a conspicuous choanoid body, but at the other end is a small structure which possibly is a homologue of the choanoid body. Subsequently, the fertilization tubule shortens and widens until finally no tubule exists and the apical ends of the two protoplasts adjoin. The merging cells then bend like a jack-knife and lateral alignment of the protoplasts occurs. This fourflagellated zygote becomes motile at about the time when the flagellar bases of the former gametes seem to approach each other and when fibrillar elements of the flagellar roots come into contact. In the motile zygote the nuclei do not fuse but remain ensheathed in the cup-shaped plastids of the two gametes. A mating of plus (+ ) and minus ( —) strains cultured, respectively, for high and low starch content suggested that gametes of only the plus ( +) mating type contain the choanoid body. Since it appears that the gamete containing the choanoid body also produces the fertilization tubule, it is inferred that gametes of only the plus (+ ) mating type produce the fertilization tubule. Should further investigation support this inference, it would be established that there is a structural basis for designating the plus ( +) mating type as male and the minus ( —) type as female. Fertilization involves fusion of the gamete membranes through the mediation of a specialized structure (the fertilization tubule) and in this respect there are similarities to certain aspects of fertilization in animal phyla. The relation of the fertilization tubule to the protoplasmic bridge of other species of Chlamydomonas is discussed.