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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.

Wolbachia is one of the best known bacterial endosymbionts affecting insects and nematodes. It is estimated that it infects 40% of insect species, so epidemiologically it may be considered a pandemic species. However, the mechanisms by which it is acquired from other species (horizontal transmission) or by which it coevolves with its hosts as a result of vertical transmission across generations are not known in detail. In fact, there are few systems in which the codivergence between host and bacterium has been described. This study goes in deep in the Wolbachia infection in the grasshopper Chorthippus parallelus. This well-known system allows us to investigate the mechanism of acquisition of various Wolbachia strains in a new host, and the bacterial genomic changes during bacterial-host codivergence: We describe the genetic diversity of Wolbachia strains infecting both subspecies of C. parallelus and analyse their phylogenetic relationship. We also show the emergence of new bacterial alleles resulting from recombination events in Wolbachia infecting hybrid hosts. Our data suggest that F strains detected in this grasshopper have co-diverged with its host, versus a more recent horizontal transmission of B strains. According with this, we discuss the potential role of Wolbachia in the dynamics of the grasshopper hybrid zone and in the divergence of the two grasshopper subspecies since the origin of their hybrid zone.