Nuclear DNA introgression across a Pyrenean hybrid zone between parapatric subspecies of the grasshopper Chorthippus parallelus.

Two parapatric subspecies of the European grasshopper Chorthippus parallelus meet along the Pyrenees and form a hybrid zone. A nuclear DNA sequence marker (cpnl-1), involving the presence or absence of a 5 bp insertion, was found to differentiate between the two subspecies along either side of the High Pyrenees but further electrophoretic and sequence analyses revealed that considerable mixing of the subspecific genomes had occurred towards the western and eastern ends of the Pyrenees. The cline for this marker was relatively narrow in two adjoining western central high cols (Peyrelue: 9.7 km and Portalet: 13.3 km) but was significantly wider in another central high col towards the east (Quillane: 42.3 km), indicating that a different combination of forces has been operating on this locus in different regions of the Pyrenees.