Sex,population dynamics and resting egg production in rotifers

The interaction between sexual reproduction and population growth in the rotifer Brachionus plicatilis was examined using exponential and logistic growth models. A computer simulation was used to explore the effects of the frequency of sex and the proportion of a female's daughters reproducing sexually on population growth rate and resting egg production. Within the parameters of the simulation, the proportion of sexual daughters required for maximum resting egg production increased as the frequency of sex decreased. Adding density-dependence to the model also shifted this optimum towards a higher proportion of sexual daughters. When population growth was unconstrained by density, the optimum proportion of sexual daughters varied between 10–50%, depending on the frequency of sex. This compares to 25–64% when the greatest density restraints were applied at K = 500 females. As the population growth rate (r) increases, the optimum proportion of sexual daughters increases nonlinearly and these effects are exaggerated as sex becomes less frequent. A compilation of published data from laboratory populations of four Brachionus plicatilis strains reveals that the average proportion of sexual daughters in 21% and in close agreement with that predicted by simulation. The limitations of the simulation and its generalizability to other rotifers are discussed.