ONE-WAY INTROGRESSION OF A SUBSPECIFIC SEX-CHROMOSOME MARKER IN A HYBRID ZONE

A hybrid zone occurs along the ridge of the Pyrenees between two subsp ecies of the meadow grasshopper Chorthippus parallelus. C-banding reve als differences between the chromosomes of these two subspecies, espec ially for the X chromosome. One C-band is closely associated with an a ctive nucleolar organizing region (NOR) on the X chromosome of Cp. par allelus but is absent from Cp. erythropus and can be used as a marker to follow the NOR. The cline for this marker is very narrow (1 km) and shows remarkable non-coincidence with other clines. It is displaced t o the south by 5 km at Col du Pourtalet and 15 km at Col de la Quillan e. Recombinants between the marker and other subspecific bands occur b ut are infrequent. The causes and consequences of such non-coincidence of clines are discussed.