GEOGRAPHICAL-DISTRIBUTION OF B-CHROMOSOMES IN THE GRASSHOPPER EYPREPOCNEMIS-PLORANS, ALONG A RIVER BASIN, IS MAINLY SHAPED BY NONSELECTIVE HISTORICAL EVENTS

The analysis of 19 populations of the grasshopper Eyprepocnemis ploran s, collected along four rivers belonging to the Segura basin (Mundo, B enamor, Taibilla and Segura itself), has shown that the presence of B chromosomes ends abruptly in each river, coinciding with the existence of a narrow pass in which this grasshopper cannot live because of the absence of the appropriate habitats. The existence of a broad inland region lacking grasshoppers with B chromosomes suggests that B chromos omes arose after the first colonization of the Iberian Peninsula by E. plorans specimens from North African populations. The B chromosome se ems to have spread upstream along each of these four rivers until reac hing geographical barriers that have impeded its advance and thus have preserved the non-B chromosome region. The available evidence indicat es that the observed geographical distribution of the B chromosome pol ymorphism in this zone was shaped mainly by historical nonselective ev ents.