New insight into Wolbachia epidemiology: its varying incidence during the host life cycle can alter bacteria spread.

Wolbachia is an obligate endosymbiont whose spread depends mainly on its capacity to alter host reproduction by, for instance, cytoplasmic incompatibility. Several mathematical models have been developed to explain the dynamics of bacterial spread, because of its applied interest. However, some aspects of the host's and bacterium's biology have not been considered in modelling: for instance, changes in Wolbachia proportions during the host's life cycle have been observed in several species, including Drosophila sp., Nasonia sp. and Aedes sp. (Diptera), but also in the grasshopper Chorthippus parallelus (Orthoptera), the species studied in this article. These changes influence the proportion of incompatible crosses and, consequently, infection prevalence in subsequent generations. In this paper, we are interested in ascertaining whether these changes in the infection proportions during the host's life cycle can influence the dynamics of the spread of these bacteria. We have examined its consequences using a mathematical model to predict the evolution of Wolbachia infection frequencies. The simulations were validated by experimental field data from C. parallelus. The main outcome is that those changes above mentioned might affect long-term infection spread, with possible consequences for the current distribution of Wolbachia and the way it affects its host's reproduction.