The grasshopper Chorthippus parallelus (Cp) has two distinct subspecies which meet along the Pyrenees forming a hybrid zone. As Cp is rarely found above 2000 m, the contact between the two subspecies must occur through valleys crossing the Pyrenees perpendicularly or at both ends of the mountain ridge. The contact zones in two valleys have already been studied in detail, Col de Porta-let (Central Pyrenees) and Col de la Quillane (Eastern Pyrenees), and this paper analyses the structure of the contact zone at the western end of the Pyrenees. The study has been carried out using different chromosome markers obtained by C-banding and silver staining, both related to the sex chromosome. Although the structure of this contact zone, analysed using these two markers, is similar to that of the contact zones occurring in other valleys, it is much wider and some remarkable peculiarities in the shapes of the clines have been detected. Additionally, hybrid-related sex chromosome markers detected in Portalet have not been found in the present sample.
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