The role of calcium in the Chlamydomonas reinhardtii mating reaction

The mating reaction of Chlamydomonas reinhardtii entails a rapid series of cell-cell interactions leading to cell fusion. We have demonstrated (Pasquale, S. M., and U. Goodenough. 1987. J. Cell Biol. 105:2279-2293) that cAMP plays a key role in this process: gametic flagellar adhesion elicits a sharp increase in intracellular cAMP, and presentation of dibutyryl-cAMP to unmated gametes elicits all known mating responses. The present study evaluates the role of Ca²⁺ in this system. We document that the mating- induced increase in cAMP, and hence the mating responses themselves, are blocked by a variety of drugs known to interfere with Ca²⁺-sensitive processes. These data suggest that Ca²⁺-mediated events may couple adhesion to the generation of cAMP. Such events, however, appear to be localized to the flagellar membrane; we find no evidence for the mating-related increase in cytosolic free Ca²⁺ that has been postulated by others. Indeed, by monitoring the length of the Ca²⁺-sensitive centrin-containing nucleus- basal body connector, we show that cytosolic free Ca²⁺ levels, if anything, decrease in response to cAMP signaling. We confirm a previous report that Ca²⁺ levels increase in the mating medium, but document that this represents a response to augmented cAMP levels and not a prelude. Finally, we show that IP₃ levels remain constant throughout the mating reaction. These results are discussed in terms of the various signal transduction systems that have now been identified in Chlamydomonas.