Components of reproductive isolation between the closely related grasshopper species Chorthippus biguttulus and C. mollis

Abstract

The formation and maintenance of species in nature is accomplished by the evolution of reproductive isolating mechanisms. To understand patterns of speciation and coexistence between species, it is crucial to obtain a complete synopsis of reproductive isolating barriers. We identified multiple reproductive isolation barriers between two closely related species, Chorthippus biguttulus and Chorthippus mollis, and quantified their respective contributions to isolation. To this end, we produced hybrids and backcrosses in the laboratory to examine reproductive isolation at multiple stages in the life history. The prezygotic barriers, based on chemical and acoustic cues, are currently the strongest impediments to gene flow between C. biguttulus and C. mollis. This indicates an important role for sexual selection in the maintenance of species isolation. However, extrinsic and intrinsic postzygotic isolation barriers also contributed to reproductive isolation. The virtual sterility of F1 hybrid males was based on both the dysfunctionality of their testes and intermediate behavioral mating traits (behavioral sterility). This study demonstrates that a cascade of reproductive isolating mechanisms reduces the gene flow between C. biguttulus and C. mollis. Our results further demonstrate that the courtship display in these species consists of multimodal signals and indicates a key role of chemical cues in reproductive isolation and speciation in grasshoppers. The identification of species isolating mechanisms is essential for understanding the processes that drive speciation. A profound understanding of reproductive isolating mechanisms and their fitness consequences in a system is required to predict a specific speciation scenario. Members of the grasshopper subfamily Gomphocerinae show an enormous diversity and complexity in courtship behavior, especially in acoustic displays. Here, we demonstrate that non-acoustic and postzygotic isolation mechanisms also strongly contribute to reproductive isolation between the closely related grasshopper species, C. biguttulus and C. mollis. In addition, our results suggest that ongoing gene flow between this species pair is unlikely and that sexual selection plays an important role in the maintenance of species isolation.