The chromosomal component of reproductive isolation in the grasshopper Caledia captiva: III. Chiasma distribution patterns in a new chromosomal taxon

Abstract

An analysis of chiasma distribution patterns among two classes of F1 hybrids produced by crossing a new chromosomal taxon, Lakes Entrance (LE), to both the Moreton (MAX) and Torresian (TT) taxa, has demonstrated that, when compared to their parental taxa, the (MAX × LE)F1 hybrids have very different distribution patterns whereas the (LE × TT)F1 hybrids have similar distribution patterns. Chiasmata in the Lakes Entrance and Torresian taxa, and their F1 hybrids generally show proximal-distal patterns of localisation in five of the eight largest autosomes although some subtle statistical differences were detected between the F1 hybrids and the parental taxa in those chromosomes. The highly significant differences in chiasma distribution patterns between the (MAX × LE)F1 hybrids and their parental taxa in chromosomes 1, 2, 4, 5, 6 and 8 can be directly attributed to pericentric heterozygosity. In these cases most recombination is localised in the interstitial and distal regions of the chromosomes. Although pericentric heterozygosity would be expected to result in a reduced mean cell chiasma frequency, the (MAX × LE)F1 hybrids have the same mean cell chiasma frequency as both the MAX and TT taxa and the (LE × TT)F1 hybrids. This appears to be due to the presence of exchanges (scored as chiasmata) within the pericentric re-arrangement region. The data strongly suggest that these exchanges are U-type following straight non-homologous pairing at pachytene rather than the result of crossing over following homologous pairing within an inversion loop. In gross stained meiotic material U-type exchanges were in 15·5 per cent of cells scored. The analysis of chiasma distribution in the F1 hybrids from crosses between the chromosomally divergent but genically equivalent MAX and LE taxa provides further substantive evidence that the dramatic change in the pattem.of recombination in chromosomally heterozygous F1’s disrupts intrachromosomal organisation resulting in the generation of recombinant progeny incapable of completing embryogenesis. In comparison the lack of any noticeable change in the recombination system in the F1 hybrids from crosses between the genically divergent but chromosomally similar taxa, LE and TT, suggests that the F2 inviability in this case is most likely a consequence of recombination between genically divergent genomes involving whole chromosome segregation rather than extensive intrachromosomal recombination. © 1984 The Genetical Society of Great Britain.