Chromosomal differentiation through an Alpine hybrid zone in the grasshopper Chorthippus parallelus

The grasshopper Chorthippus parallelus is genetically differentiated over i ts range into at least five major geographical subdivisions. Two of these s ubdivisions, designated as subspecies, meet and form a hybrid zone in the P yrenees. These subspecies differ for a wide range of morphological, behavio ural and chromosomal characters, which vary clinally across the zone. A fur ther geographical subdivision exists within Italy. Here we present the firs t evidence for chromosomal divergence between populations of the grasshoppe r north and south of the Alps. Across two Alpine cols (Col de Larche, betwe en France and Italy; Passe de Resia, between Austria and Italy), these popu lations differ in both structural and functional components of the X-chromo some. Northern Alpine individuals possess an active nucleolar organizing re gion (NOR) at the distal end of the X-chromosome and an associated region o f heterochromatin (C-band). Both these features are absent from individuals from the south of the Alps. However, all individuals examined carry distal ly located rDNA on the X-chromosome. Clinal transition was examined in the distal C-band in transects through the two cols. The dine centres are rough ly coincident with the tops of the cols. Both dines were of similar widths (Col de Larche, 21.88 km; Passe de Resia, 24.05 km), and therefore much wid er than those for an X-Linked distal C-band in the Pyrenean hybrid zone. Th is suggests that there are different selective pressures on the cytogenetic characters in the Alps. The results are discussed in the context of the hi storical population dynamics of the species in relation to the climatic cha nges associated with the Pleistocene ice ages.