Low levels of chromosomal differentiation between the grasshoppers Chorthippus brunneus and Chorthippusjacobsi (Orthoptera; Acrididae) in northern Spain

The grasshopper species Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) form a hybrid zone in northern Spain. These species probably diverged while isolated in southern refugia during one of the recent ice ages, and are clearly distinguished by morphology and male calling song. However, in contrast to other Chorthippus taxa that form hybrid zones in Europe, these two species cannot be reliably distinguished on the basis of characteristics of the karyotype such as heterochromatin banding patterns and composition, as revealed by C-banding and fluorochrome staining. Silver staining also reveals the presence of two autosomal nucleolar organiser regions (NORs) in both species. However, differentiation between C. brunneus and C. jacobsi was revealed on the X chromosome using fluorescent in situ hybridisation (FISH). C. brunneus individuals showed additional rDNA sequences on the X chromosome that were not observed in any C. jacobsi individuals. These sequences are not transcribed, indicating either mutational silencing of an ancestral NOR on the X chromosome, or the transposition of non-functional sequences from the autosomes. The implications of these results for the evolution of NOR number in Chorthippus are discussed.