Understanding Wolbachia acquisition and co-divergence of hosts and their associated bacteria: Wolbachia infection in the Chorthippus parallelus hybrid zone

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.

Effects of habitat structure and land-use intensity on the genetic structure of the grasshopper species Chorthippus parallelus

Land-use intensity (LUI) is assumed to impact the genetic structure of organisms. While effects of landscape structure on the genetics of local populations have frequently been analysed, potential effects of variation in LUI on the genetic diversity of local populations have mostly been neglected. In this study, we used six polymorphic microsatellites to analyse the genetic effects of variation in land use in the highly abundant grasshopper Chorthippus parallelus . We sampled a total of 610 individuals at 22 heterogeneous grassland sites in the Hainich-Dün region of Central Germany. For each of these grassland sites we assessed habitat size, LUI (combined index of mowing, grazing and fertilization), and the proportion of grassland adjoining the sampling site and the landscape heterogeneity (the latter two factors within a 500 m buffer zone surrounding each focal site). We found only marginal genetic differentiation among all local populations and no correlation between geographical and genetic distance. Habitat size, LUI and landscape characteristics had only weak effects on most of the parameters of genetic diversity of C. parallelus ; only expected heterozygosity and the grasshopper abundances were affected by interacting effects of LUI, habitat size and landscape characteristics. The lack of any strong relationships between LUI, abundance and the genetic structure might be due to large local populations of the species in the landscape, counteracting local differentiation and potential genetic drift effects.