title: Population dynamics of a selfish B chromosome neutralized by the standard genome in the grasshopper Eyprepocnemis plorans.
abstract: Effects of the B chromosome polymorphism of the grasshopper Eyprepocnemis plorans were analyzed in two natural populations. Postmating sexual selection, female fertility, and survival were studied. The B chromosome lacks drive and has no detectable effects on fitness. A neutral B cannot invade a population and establish a polymorphism, but the confidence limits on our estimates cannot exclude the possibility that the polymorphism is maintained by a balance between weak drive and weak selection against individuals with two and three B's. However, other lines 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 neutralizes the B drive so that it becomes near-neutral and begins a random walk toward extinction by stochastic loss. Because the B is common by the time drive disappears, the random walk is likely to continue for a long time. If in the course of the random walk a variant B with greater drive appears, then it will displace the original variant, and a new cycle of drive suppression and drift to extinction occurs. A simulation model of this process suggested that the mean time to extinction is proportional to the two-thirds power of the population size; it is much less affected by subpopulation size or the number of populations in a subdivided population.
authors.affiliation: Departamento de Genetica, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain.
authors.forename: J P
authors.initials: JP
authors.lastname: Camacho
authors.forename: M W
authors.initials: MW
authors.lastname: Shaw
authors.forename: M D
authors.initials: MD
authors.lastname: López-León
authors.forename: M C
authors.initials: MC
authors.lastname: Pardo
authors.forename: J
authors.initials: J
authors.lastname: Cabrero