Wolbachia infection in the chorthippus parallelus hybrid zone: Evidence for its role as a reproductive barrier

Abstract

Chorthippus parallelus parallelus (Cpp) and Chorthippus parallelus erythropus (Cpe) are two grasshopper subspecies whose distributions overlap in the Pyrenees, where they form a hybrid zone at those points where the topography and their ecological requirements allow. This hybrid zone is a consequence of secondary contact between endemic Iberian (Cpe) and continental European populations (Cpp) that expanded after the most recent glaciations from the refugia in which they had diverged genetically in allopatry. The morphological, physiological, genetic and behavioral differences between these subspecies (and their natural and laboratory-reared hybrids) have been intensively studied in recent years, for which reason this hybrid zone is considered a singular model in evolutionary biology. These studies reveal a complex pattern of factors involved in the origin, structure and maintenance of the hybrid zone. They offer an excellent panorama of evolution "in action". Wolbachia is a genus of obligate endosymbiont bacteria that induce changes in the reproduction of arthropods and nematodes. In previous studies we have reported the existence of these bacteria in individuals and populations of Chorthippus parallelus. Now we show that they produce a significant reproductive barrier in this hybrid zone, implying, as proposed elsewhere, that Wolbachia may be involved in speciation phenomena.