The chromosomal component of reproductive isolation in the grasshopper Caledia captiva - I. Meiotic analysis of chiasma distribution patterns in two chromosomal taxa and their F hybrids

A comparison of chiasma distribution patterns between two chromosomal taxa, Moreton and Torresian, and their F hybrids demonstrates highly significant differences between all chromosomes analysed. In chromosomes 1, 2, 4, 5, 6 and 8 these differences can be directly attributed to pericentric heterozygosity in the F hybrid. In chromosomes 7 and 8 where there is no pericentric heterozygosity these differences may be due to heterozygosity for interstitial and terminal bands of hetero-chromatin or possibly undetected paracentric rearrangements. The F hybrids also have a significantly lower mean cell chiasma frequency. The Moreton and Torresian taxa differ significantly in chiasma distribution pattern in chromosomes 1, 2, 4, 5, 6 and 8 and both Moreton populations analysed have a significantly lower mean cell chiasma frequency than the Torresian population. In addition the two Moreton populations, (MMX) and (MAX), differ significantly in the chiasma distribution pattern in chromosomes 1 and 2 and the chromosomally more polymorphic population (MMX) has a significantly lower mean cell chiasma frequency. There is some evidence that the differences in both chiasma distribution and frequency between these two populations may be due to genetic differences in addition to the effects caused by chromosomal polymorphism. It has been shown that in general there is a substantial reduction in recombination in the intersitial regions of most chromosomes in the Moreton and particularly the Torresian taxon because of a proximal-distal localisation of chiasmata. In the F hybrid, however, nearly all recombination events are located within these interstitial regions. This provides support for the hypothesis that the frequent placement of chiasmata in regions of normally low recombination may disrupt the internal coadapted genetic environment of both chromosomal forms resulting in non-functional recombinant progeny in the next generation. The recombination data in this study also provide a basis for an empirical test of this hypothesis. © 1982 Springer-Verlag.