Mating Structure Differences Demonstrated by Freeze‐Fracture Analysis of Fusion‐Defective Chlamydomonas Gametes

While the mating structure of unmated mating type minus ( mt ‐ ) gametes of Chlamydomonas reinhardtii has few intramembrane particles (IMPs), activation results in movement of IMPs to its center. Analysis of freeze‐fractured replicas of wild type (wt) mt ‐ and 3 mt ‐ fusion‐defective mutants, gam ‐1, gam ‐10 and gam ‐11, before and after activation with wt + flagella, provides a basis for suggesting that some of the IMPs in mt ‐ mating structures, particularly a subset of particles that partitions to the E face, may be fusion‐controlling molecules. Unmated gametes of gam ‐10 show a full range of images, from particle‐free to fully activated, with both the P and E face of the mating structure revealing approximately twice as many IMPs as those observed on wt . Unactivated gametes of gam ‐1 and gam ‐11 appear identical to wt ‐ . After activation, the mating structures of all of these gametes appear to have approximately the same number of IMPs. If the sizes of particles for these mutants are compared to wild type at the restrictive temperature, all 3 mutants have significantly smaller IMPs on the E face; before mating, in the plasma membrane and after mating, in the mating structure. At 34° C, the gam ‐1‐II mating structure appears to be missing most of the particles from 15.5 to 16.5 nm in diameter, while all gametes with the ability to fuse have an equivalent percentage of their mating structure particles in this size range. The possibility that an IMP in this size range represents a protein that may be responsible for gamete fusion is discussed.