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.

We analyse chromosome location of H3 and H4 histone gene clusters by fluorescence in-situ hybridization (FISH) in 35 species of Acrididae grasshoppers belonging to seven subfamilies. As in other organisms, H3 and H4 co-localized in the same chromosome region in the 11 species where double FISH was performed with the H3 and H4 DNA probes. Chromosome location of H3-H4 histone gene clusters showed high regularity in the species analysed, with all of them carrying a single H3-H4 cluster in an autosome which, in most cases, was located interstitially in the proximal chromosome third. In 17 out of the 21 species with 2n masculine = 23 acrocentric chromosomes, the H3-H4-carrying autosome was about eighth in order of decreasing size. Two of the four exceptions changed H3-H4 localization to proximal (Pezotettix giornae) or distal (Tropidopola graeca) in the eighth-sized autosome, but the remainder (the two Eyprepocnemis species) showed the H3-H4 cluster distally located in the second-sized autosome. All 14 species with 2n masculine = 17 chromosomes (including three long metacentric autosome pairs, five acrocentric autosome pairs and an acrocentric X chromosome) carried an interstitial H3-H4 cluster in the short arm of the smallest of the three long metacentric pairs. These results suggest that chromosome location of H3-H4 histone gene clusters seem to be highly conservative in Acrididae grasshoppers. The change in H3-H4 location from the acrocentric medium-sized autosome in the 2n masculine = 23 karyotype to the long metacentric autosome in the 2n masculine = 17 karyotype is most parsimoniously explained by common ancestry, i.e. by the involvement of the H3-H4-carrying acrocentric in the centric fusion that gave rise to the smallest of the three long metacentric autosomes of 2n masculine = 17 species.

Long-run evolution of B chromosomes is mainly made up by an evolutionary arms race between these selfish genetic elements and the standard genome. The suppression of B drive is one of the clearest expressions of genome defense against B chromosomes. After drive neutralization, the B is condemned to extinction unless a new variant showing drive can emerge and replace it. This paper reports the first empirical evidence for the substitution of a neutralized B variant by a new selfish B variant. Such a polymorphism regeneration has recently taken place in a natural population of the grasshopper Eyprepocnemis plorans.

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A G banding technique combining trypsin and hot saline treatments was used to analyze the chromosomes of two grasshopper species, Eyprepocnemis plorans and Locusta migratoria, both of which contain both standard and supernumerary heterochromatin. Although this technique does not produce G bands like those in mammalian chromosomes, it serves to characterize heterochromatic regions whose nature has been inferred from other banding techniques (C, N, CMA, and DAPI banding). The light regions revealed by G banding contain GC-rich DNA sequences, the more prominent of which coincide with nucleolus organizer regions (NORs). Furthermore, the proximal heterochromatin in E. plorans was heterogeneous, and the standard and supernumerary heterochromatin showed conspicuous differences in organization. Supernumerary heterochromatin is an exception to the regular patterns shown by the standard heterochromatin. The findings are related to the mechanism of action of these banding techniques.Key words: banding techniques, grasshoppers, Eyprepocnemis plorans, Locusta migratoria.