Abstract
Fusion of plasma membranes between Chlamydomonas reinhardtii gametes was studied by freeze fracture electron microscopy of unfixed cells. The putative site of cell fusion develops during gametic differentiation and is recognized in thin sections of unmated gametes as a plaque of dense material subjacent to a sector of the anterior plasma membrane. The overlying membrane proves to be readily recognized in replicas of unmated gametes as a circular region roughly 500 nm in diameter which is relatively free of 'regular' plasma membrane particles on both the P and E fracture faces. The morphology of this region is different for mating type plus (mt+) and mt- gametes: the few particles present in the center of the mt+ region are distributed asymmetrically and restricted to the P face, while the few particles present in the center of the mt- region are distributed symmetrically in the E face. Each gamete type can be activated for cell fusion by presenting to it isolated flagella of opposite mt. The activated mt+ gamete generates large expanses of particle cleared membrane as it forms a long fertilization tubule from the mating structure region. In the activated mt- gamete, the E face of the mating structure region is transformed into a central dome of densely clustered particles surrounded by a particle cleared zone. When mt+ and mt- gametes are mixed together, flagellar agglutination triggers similar activation events, but the tip of each mt+ fertilization tubule proceeds to fuse with an activated mt- region. The fusion lip is seen to develop within the particle dense central dome. It is concluded that these mt- particles play an active role in membrane fusion.
Original language | English |
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Pages (from-to) | 144-160 |
Number of pages | 17 |
Journal | Journal of Cell Biology |
Volume | 72 |
Issue number | 1 |
DOIs | |
State | Published - 1977 |
Externally published | Yes |