Temporal variation in the chromosomal structure of a hybrid zone and its relationship to karyotypic repatterning

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A temporal analysis of the chromosomal structure of the hybrid zone in the grasshopper Caledia captiva has revealed that, over a period of six generations, the position of the zone has remained unchanged when assessed in terms of chromosomal frequencies. In complete contrast however, chromosomal genotypic frequencies have changed dramatically and asymmetrically over the same period. The frequencies of chromosomal heterozygotes have been significantly reduced on one side of the zone accompanied by increases in the frequencies of homozygous metacentric chromosomes. These asymmetrical genotypic changes are also reflected in a complete reversal of the patterns of gametic disequilibria (Tr2) across the zone. It is proposed that undirectional selection has favoured a metacentric karyotype on one side of the zone during a major climatic change.The structure of the hybrid zone involves two major and independent features. First, as a secondary consequence of hybridisation, recombinational change in F1 hybrids disrupts the internal organisation within chromosomes. This results in the production of inviable F2 and backcross progeny and hence, explains the structure of the zone in terms of the sharp change in chromosomal frequencies. Secondly, the asymmetrical nature of the gametic disequilibria between chromosomes represents the direction of selection which favours an acrocentric Torresian karyotype in dry years and a metacentric Moreton karyotype during mesic years. Variation in both chromosome structure and embryonic weight is associated with the predictability of the environment. The acrocentric Torresian karyotype and its associated larger embryos are correlated with a univoltine life history in drier, unpredictable habitats. A similar pattern exists within the Moreton subspecies in the form of a chromosomal cline in S.E. Australia. At the southern limit of this cline the karyotype is totally acrocentric, the life history is univoltine and the embryos are the same weight as the Torresian.It is speculated that variation in chromosomal structure, in terms of the relationship between centromeres and telomeres, may provide a mechanism for altering cellular phenotype through changes in such factors as replication patterns or chromatin packaging which may act quite independently of the informational content of the chromosome.