Allozyme variation across a narrow hybrid zone in the grasshopper, Caledia captiva

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Seventeen enzyme systems comprising 23 genetic loci were assayed in the Moreton and Torresian chromosomal taxa Caledia captiva and an assessment was made of the allelic variation within and between these taxa. Heterozygosity per individual in both taxa averaged 10–12 per cent. Nei's coefficient of genetic distance between the two taxa was calculated to be 0·158 ± 0·061. Five of the 23 loci showed large differences in frequency between the taxa. ME showed a difference of 50 per cent in allelic frequency, whereas GOT-2, GPT, ICDH-1 and PGI showed differences in frequencies of 90 per cent or greater. Alleles at these loci were characteristic if not absolutely diagnostic of the races. These five enzyme loci provided additional markers for the analysis of samples taken across a transect of the hybrid zone and revealed that the major change in frequency for all loci occurred over a 200 metre interval, which coincided with the location of the major change in chromosomal frequency. However, no gametic disequilibrium between the four characteristic loci could be detected in the central populations of the hybrid zone, implying that these loci, at least, could freely segregate and recombine in hybrids and back-crosses. The highly symmetrical pattern of replacement for these loci contrasts markedly with the very asymmetrical pattern of chromosomal replacement, with high levels of one-way introgression of chromosomes from the Torresian into the Moreton race. Further, the detection of characteristic Moreton alleles, at frequencies of up to 20 per cent, in Torresian populations which contained a very low frequency of introgressed Moreton chromosomes, emphasises that a large proportion of the genome remains undefined and unrecognisable when the hybrid zone is assessed by chromosomal rearrangements alone.