The contributions of host population size and maternal transmission rate to fluctuating Wolbachia frequencies

Many insects and other animals host maternally transmitted endosymbionts that alter host fitness and reproduction. The facultative endosymbiont Wolbachia is found in roughly half of all insect species, but population frequencies fluctuate across time and geography for unknown reasons. This is especially true for w Mel Wolbachia in Drosophila melanogaster and related " w Mel-like" strains that fluctuate over timescales as short as a month. Recent work suggests these fluctuations may be due to temperature effects that disrupt maternal transmission in some or all female hosts under certain conditions. We simulated temporal Wolbachia dynamics in the new R package wlbcmodeler to test whether fluctuations can be solely explained by finite-host population stochasticity or, alternatively, by perturbation of maternal transmission rates. Across a range of biologically plausible parameters, we show that declining transmission rates increase variance and decrease mean Wolbachia frequencies over time, and these effects outweigh the impact of Wolbachia effects on host fitness (i.e., fecundity) and Wolbachia -induced cytoplasmic incompatibility. Large temporal fluctuations can arise due to stochasticity or changes to maternal transmission rates that alter Wolbachia equilibria. These simulated dynamics may explain fluctuations of w Mel-like Wolbachia over years-long periods, but additional factors (e.g., insect demography) likely contribute to fluctuations over shorter timescales.