A grasshopper hybrid zone as a window into the evolution of sterility

The observation of pervasive patterns of reproductive isolation across species pairs suggests that generalities, or ‘rules’, underlie speciation in all animals and plants. The first is Haldane’s rule, or the preferential sterility of hybrid males; and the second is the large X-effect, or the disproportional role of sex chromosomes in species differences. Understanding how these rules first evolve before speciation is complete remains a challenging task because they are often observed between taxa that no longer hybridize in nature. We address this using two recently diverged subspecies of grasshoppers, Pseudochorthippus parallelus parallelus, and P. p. erythropus, that follow the ‘two rules of speciation’ while interacting in a hybrid zone.