RESUMO
The propulsive force generated by Chlamydomonas mutants deficient in flagellar dynein was estimated from their swimming velocities in viscous media. The force produced by wild-type cells increased by 30-40% when viscosity was raised from 0.9 to 2 cP but decreased as viscosity was further raised above 6 cP. The biphasic dependence of force generation on viscosity was also observed in the mutant ida1, which lacks the I1 component of the inner-arm dynein. The mutant ida4, which lacks the inner-arm I2 component, was extremely susceptible to viscosity and stopped swimming at 6 cP, at which other mutants could swim. In contrast, oda1, which lacks the entire dynein outer arm, produced a fairly constant force of about one-third of the wild-type value, over a viscosity range of 0.9-11 cP. In demembranated and reactivated cell models of the wild type, the propulsive force decreased monotonically as viscosity increased. Thus the increase in force generation at about 2 cP observed in live cells may be caused by some unknown mechanism that is lost in cell models. The cell models of oda1, in contrast, did not show a marked change in force generation with the change in viscosity. These results indicate that the force generation by the outer-arm dynein greatly depends on viscosity or the velocity of movement, whereas the complete set of inner-arm dynein present in the oda1 axoneme produces a fairly constant force at different viscosities. These different properties of inner and outer dynein arms should be important in the mechanism that produces flagellar beating.