SUMMARY: Gametes and zoospores of and were repelled by H, K, NH or Na as well as by Ca, Mg and La. This negative chemotaxis, which was monitored with a chemotaxis bioassay, occurred no matter what anionic counter-ion was used. The use of a swim-out assay showed that repulsion was similar for all cations and resulted in a band of zoospores that migrated farther into the tube with time. When zoospores allowed to adapt to 10-fold dilutions of 10 m-KC1 were challenged with 10 m-KC1 and their motile behaviour monitored in swim-out assays, it was found that the higher the initial concentration of KC1 incubated with the cells, the less repulsion occurred when these cells were challenged with 10 m-KC1. In addition when zoospores were incubated in a 10 m solution of one repellent (i.e. KC1, NHC1 or CaC1) and then challenged in swim-out assays with a 10 m solution of another repellant (i.e. KC1 or HC1) to determine if the cationic sites of interaction were common or different, the results indicated that the sites were the same since challenging with another cation did not cause zoospore repulsion. The repulsion mechanism was studied further by mixing 10 m-KC1 and the male pheromone attractant sirenin. These experiments, using the chemotaxis bioassay, showed that in a mixed population of male gametes and zoospores, the male cells were attracted towards the source of the sirenin while all the zoospores were repulsed by KC1. Thus it appears that the mechanism for attraction is different from repulsion or that the attraction mechanism can override the repulsion behaviour.


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