A hybrid zone between Chorthippus parallelus parallelus and Chorthippus parallelus erythropus (Orthoptera: Acrididae): chromosomal differentiation

Abstract

Two subspecies of the meadow grasshopper Chorthippus parallelus from narrow hybrid zones in the Pyrenees. We studied the patterns of chromosome divergence to evaluate their possible role in the maintenance of such hybrid zones under natural conditions. The two subspecies share some common features such as identical chromosome number, similar basic chromosome form, and the GC-rich nature of all the heterochromatic DNA. Similar positions for the nucleolar organizer region (NOR) in two pairs of autosomes were also observed. The main difference was the presence of an active NOR in the X chromosome in Cp. parallelus that was absent from Cp. erythropus. The pattern of banding by C-banding, acridine orange, and chromomycin A3 also showed the subspecific characteristics on this chromosome. The presence or absence of an active NOR on the X chromosome was used to analyse the hybrid zones in two different transects of the Pyrenees, and clear clines of X NOR frequencies were observed passing from Cp. parallelus to Cp. erythropus. Some meiotic abnormalities were found in individuals from the hybrid zones with the subsequent production of abnormal sperm. Differences in genetic activity involving the sexual chromosome and the male sterility found in F1 hybrids in laboratory crosses are discussed as an example of Haldane''s Rule.