Effect of experimental methodology on estimation of density at sex initiation in cyclically parthenogenetic rotifers

Abstract

Sexual reproduction is density-dependent in several cyclically parthenogenetic rotifer species and induced by a chemical signal produced by the rotifers. As population density increases, the chemical accumulates in the medium until reaching a threshold that triggers sex. The density threshold for sex initiation is of ecological and evolutionary importance due to the large impact that timing of sex has on fitness in cyclical parthenogenetic rotifers. Here we study the effects of experimental methodology in combination with intrinsic features of the biological system on the estimation of the density required to produce enough of this chemical to initiate sex (i.e., density threshold). We have developed a theoretical model describing the dynamics of both population density and the concentration of the chemical signal. Model analysis showed that the time at which sex is recorded in an experiment, the initial density (i.e., experimental volume) and the population growth rate can affect the estimation of density thresholds. Results from an experiment in which density at sex initiation was tested for 29 clones of Brachionus plicatilis at two culture volumes, revealed that density threshold is negatively dependent on the experimental volume. We conclude that in order to estimate density thresholds so that they are comparable across studies, experimental methodology should be standardized.