The control of gene flow across a narrow hybrid zone: a selective role for chromosomal rearrangement?

Abstract

Independent molecular, biochemical, and cytological markers have been used to assess the evolutionary status of a narrow hybrid zone between two genomically distinct taxa of the grasshopper Caledia captiva. Despite the extreme narrowness (<1 km) of the hybrid zone when assessed in terms of its chromosomal profile, other diagnostic nuclear and cytoplasmic markers are found at high frequencies in populations up to 400 km beyond the chromosomal limits of the zone. These data suggest that the hybrid zone is very old and capable of moving while still retaining its narrow chromosomal profile. The data also suggest that the chromosomal rearrangements are the principal components maintaining this narrow hybrid zone and that strong selective forces, acting upon the structure of the genome, are involved. Analysis of chromosomal variation has revealed that the Moreton taxon has a complex chromosomal cline in which every chromosome changes from metacentric to acrocentric or telocentric along the cline. We argue that the chromosomal rearrangements which are involved in maintaining the narrow hybrid zone are themselves playing an adaptive role within the population, both as homo- and hetero-zygotes. A model is presented to explain these patterns of genomic change along environmental gradients as an adaptation, probably at the cellular level, to establish a temporal synchrony of events during embryogenesis.