Flagellar membrane agglutination and sexual signaling in the conditional Gam-1 mutant of Chlamydomonas

The temperature-sensitive gametogenesis-defective mutant, gam-1 is sex-limited, expressed only in mating type minus (mt-), and can sexually agglutinate but not fuse at the restrictive temperature (35°C) with gametes of wild type (wt) mt⁺. Thin-section, freeze-cleave, and scanning electron microscopy reveal that the gam-1 phenotype is dependent on both the temperature at which the cells undergo nitrogen starvation (and therefore gamete formation) and the temperature at which the cells are maintained during the 12 h before mating. Under all conditions of gametogenesis at 35°C, each gam-1 cell produces a normal-appearing membrane-associated mating structure that fails to activate in response to flagellar agglutination. Varying with the conditions of gametogenesis, on the other hand, are the agglutination and signaling properties of the gam-1 flagella. The two mutant phenotypes displayed by gam-1 have been denoted gam-1 -I and gam-1 -II. An agglutination reaction involving gam-1 -I cells does not result in activation of the wt mt⁺ mating structure. A more stable agglutination reaction, which can result in activation of the wt mt+ mating structure, is characteristic of gam-1 -II cells, but because the gam-1 mt- mating structure still fails to activate, cell fusion is precluded. We conclude that the gam-1 mutation affects flagellar component(s) involved in establishing an effective, signal-generating agglutination reaction.