POPULATION-DYNAMICS OF A SELFISH B-CHROMOSOME NEUTRALIZED BY THE STANDARD GENOME IN THE GRASSHOPPER EYPREPOCNEMIS-PLORANS

Effects of the B chromosome polymorphism of the grasshopper Eyprepocne mis plorans were analyzed in two natural populations. Postmating sexua l selection, female fertility, and survival were studied. The B chromo some lacks drive and has no detectable effects on fitness. A neutral B cannot invade a population and establish a polymorphism, but the conf idence limits on our estimates cannot exclude the possibility that the polymorphism is maintained by a balance between weak drive and weak s election against individuals with two and three B's. However, other li nes of evidence favor the following model of the dynamics of the B in E. plorans. In a newly invaded population, the B has substantial drive , but the evolution of drive suppressor genes in the A chromosomes neu tralizes the B drive so that it becomes near-neutral and begins a rand om walk toward extinction by stochastic loss. Because the B is common by the time drive disappears, the random walk is likely to continue fo r a long time. If in the course of the random walk a variant B with gr eater drive appears, then it will displace the original variant, and a new cycle of drive suppression and drift to extinction occurs. A simu lation model of this process suggested that the mean time to extinctio n is proportional to the two-thirds power of the population size; it i s much less affected by subpopulation size or the number of population s in a subdivided population.