Membrane adhesions between the flagella of mating-type "plus" and "minus" gametes of Chlamydomonas reinhardi are shown to stimulate a rapid change in the ultrastructure of the flagellar tips, designated as flagellar tip activation (FTA). A dense substance, termed fibrous tip material (FTM), accumulates between the flagellar membrane and the nine single A microtubules of the tip. The A microtubules then elongate, growing into the distal region of the tip, increasing tip length by 30%. This study describes FTA kinetics during normal and mutant matings, presents experiments designed to probe its role in the mating reaction, and offers the following conclusions: (a) FTA is elicited by agents that cross-link flagellar membrane components (including natural sexual agglutinins, antiflagellar antisera, and concanavalin A) but not by flagellar adherence to polylysine-coated films. (b) FTA is reversed by flagellar disadhesion. (c) Gametes can undergo repeated cycles of FTA during successive rounds of adhesion/disadhesion. (d) FTA, flagellar tipping, and sexual signaling are simultaneously blocked by colchicine and by vinblastine, suggesting that tubulinlike molecules, perhaps exposed at the membrane surface, are involved in all three responses. (e) FTA is not blocked by short exposure to chymotrypsin, by cytochalasins B and D, nor by concanavalin A, even though all block cell fusion; the response is therefore autonomous and experimentally dissociable from later stages in the mating reaction. (f) Under no experimental conditions is mating-structure activation observed to occur unless FTA also occurs. This study concludes that FTA is a necessary event in the sexual signaling sequence, and presents a testable working model for its mechanism.

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