Mosaicism for Sister Chromatid Heterogeneity in Sex Chromosomes from Hybrids of two Subspecies of Chorthippus Parallelus (Orthoptera: Acrididae)

Abstract

Cytogenetically hybrid forms of the grasshopper subspecies Chorthippus parallelus erythropus (Cpe) and Chorthippus parallelus parallelus (Cpp) occur in a Pyrenean hybrid zone (HZ) ( GOSALVEZ et al. 1988; BELLA et al. 1993; SERRANO 1994). The pure subspecies have the same chromosome complement of 8 pairs of autosomes (3 large submetacentrics, 4 medium acrocentrics, and a single small acrocentric) plus the sex chromosomes (female XX, male XO) (GOSALVEZ et al. 1988). Most of the subspecific chromosomal differentiation occurs on the sex chromosomes (GOSALVEZ et al. 1988). Those of Cpe invariably have a positive pericentromeric Cband (C) and another interstitial band (E). Those of Cpp also always have a pericentromeric C-band, indistinguishable from that of Cpe, but possess a distal band (P) instead of the interstitial E band of Cpe. The P band is closely related to an active nucleolar organiser region (NOR), and so its absence in pure Cpe implies a lack of NOR activity in the sex chromosomes. In situ hybridization shows this to be due to the absence of rDNA genes (SERRANO 1994). A further band, H, localized interstitially between the E and P band locations, has been described in some individuals from hybrid populations in the HZ. All of the 8 possible parental and recombinant patterns of the E, P and H bands have been observed in individuals from hybrid populations ( SERRANO 1994). C-banding patterns of more than 4000 individuals (approximately 1000 pure and 3000 hybrids) have been studied (GOSALVEZ et al. 1988; BELLA et al. 1990, 1992, 1993; RUBIO 1992; FERRIS et al. 1993; SERRANO 1994; BURO et al. 1994; MASON et al., unpubl. data). Two of these individuals, both from hybrid populations, exhibited intra-individual variation of sister-chromatid heterogeneity in the E