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 .KEY WORDS microtubules " membranes cell-cell recognition -sensory cilia agglutination The mating reaction of the biflagellate Chlamydomonas reinhardi (16) initiates when mating-type "plus" (mt+) and "minus" (mt) gametes are mixed together. The resulting interactions can be subdivided into seven stages, all of which can be completed within 30 s. (a) The cells adhere to one another via mi-specific flagellar surface agglutinins, thought to be glycosylated membrane polypeptides (6, 57). (b) Pairs of adhering cells move their agglutinins out to their respective flagellar tips, a "tipping" response that brings the apical J. CELL BIOLOGY

Experiments have explored the possible relationships between the flagellar surface motility of Chlamydomonas, visualized as translocation of polystyrene beads by paralyzed (pf) mutants (Bloodgood, 1977, J. Cell Biol . 15 :983-989), and the capacity of gametic flagella to participate in the mating reaction . While vegetative and gametic flagella bind beads with equal efficiencies and are capable of transporting them along entire flagellar lengths, beads on vegetative flagella are primarily associated with the proximal half of the flagella whereas those on gametic flagella exhibit no such preference . This difference may relate to the "tipping" response of gametes during sexual flagellar agglutination (Goodenough and Jurivich, 1978, /. Cell Biol. 79 :680-693) . Colchicine, vinblastine, chymotrypsin, cytochalasins B and D, and anti-ß-tubulin antiserum are all able to inhibit the binding of beads to the flagellar surface. Trypsin digestion and an antiserum directed against whole Chlamydomonas flagella have no effect on the ability of flagella to bind beads, but the beads remain immobile . These results suggest that at least two flagellar activities participate in surface motility : (a) bead binding, which may involve a tubulin-like component at the flagellar surface; and (b) bead translocation, which may depend on a second component (e .g . an ATPase) of the flagellar surface. Surface motility is shown to be distinct from gametic adhesiveness per se, but it may participate in concentrating dispersed agglutinins, in driving them toward the flagellar tips, and/or in generating a signalto-fuse from the flagellar tips to the cell body . Directly supporting these concepts is the observation that bound beads remain immobilized at the flagellar tips during the "tip-locking" stage of pf x pf matings, and the observation that bound ligands such as antibody fail to be tipped by trypsinized flagella .Movement of marker particles on the surface of biological membranes has been widely used to study membrane fluidity and cell migration (1,3,4,10,13,23,24,26) . Recently, discussion has centered on possible connections between membranes and underlying cytoskeletal structures (i .e., axonemes, microtubules, microfilaments, and actomyosin complexes) (11,12,31) . It therefore becomes of interest to learn whether surface particle movements are driven by known components of the cytoskeleton, and whether these movements can serve to transmit biological information to the cell interior.We have addressed these questions by analyzing surface motility associated with the flagellar membrane of the unicellular eukaryote Chlamydomonas reinhardi. As described and discussed in detail by Bloodgood and co-workers (7,8), the flagella will bind polystyrene microspheres, small fragments of debris, or bacterial cells suspended in methyl cellulose, and these will be rapidly transported up and down the length of the flagellum . Each bead moves linearly along the flagellum, and if several beads are bound to the same flagellum they can pass ea...

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