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

Abstract

SUMMARYAn 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)F hybrids have very different distribution patterns whereas the (LE XTT)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 X LE)FI 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 x LE)FI hybrids have the same mean cell chiasma frequency as both the MAX and TI' taxa and the (LE XTT)FIhybrids. 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 pattern.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 IT, 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.

Gene flow across a chromosomal tension zone. I. Relicts of ancient hybridization

“…In order to investigate the components of hybrid inviability involved in the products of Moreton x Torresian crosses, Coates and Shaw (1982) examined the pattern of meiotic recombination in F1 hybrids. This analysis demonstrated a major perturbation in the positioning of chiasmata that was directly attributable to the pericentric inversion differences between these two taxa.…”

“…The deviation from the expected frequencies was entirely the result of intrachromosomal interactions . Secondly, Coates and Shaw (1984) carried out experimental crosses involving the Torresian and Moreton chromosomal forms and a third chromosomal form named the "Lakes Entrance" taxon. This latter taxon is characterized by an acrocentric karyotype similar to that of Torresian individuals, but it is allozymically indistinguishable from Moreton, and it contains the numerous interstitial C-bands common in the Moreton subspecies (Coates and Shaw, 1984).…”

“…and Coates and Shaw (1982) found the hybrid breakdown to be caused partially by differences in centromere positions of homologous chromosomes from the two taxa, which alter the distribution of positions of crossing-over in heterozygous meiosis; this crossing-over in turn is presumed to disrupt physically integrated gene complexes. The effect of the centromere position differences has been estimated to account for 46 per cent of the inviability of Torresian/Moreton F2 hybrids; Coates and Shaw (1984) suggested that the remaining hybrid breakdown may be associated with "genic" differences between the taxa. Previously, allozyme differences Daly eta!., 1981), restriction-fragment length polymorphisms (RFLPs) of the ribosomal DNA (rDNA) (Arnold et a!., 1987a) and mitochondrial DNA (mtDNA) (Marchant, 1988), and variation in the cytological distribution and associated DNA sequences of heterochromatin (Shaw et a!., 1976;Arnold and Shaw, 1985;Arnold, 1986) which distinguish Torresian from Moreton, have been described.…”

“…(1976). C-band chromosome analysis of grasshoppers from other sites within our study area has previously been carried out by Coates and Shaw (1984;Shaw, unpublished data). Marchant (1987).…”

“…800m) hybrid zone along the 200 km common boundary of their ranges (Shaw eta!., 1980). and Coates and Shaw (1982) found the hybrid breakdown to be caused partially by differences in centromere positions of homologous chromosomes from the two taxa, which alter the distribution of positions of crossing-over in heterozygous meiosis; this crossing-over in turn is presumed to disrupt physically integrated gene complexes. The effect of the centromere position differences has been estimated to account for 46 per cent of the inviability of Torresian/Moreton F2 hybrids; Coates and Shaw (1984) suggested that the remaining hybrid breakdown may be associated with "genic" differences between the taxa.…”