ALBERT

Notes: Abstract The spatial distribution of chromosomes has been analysed in radial metaphase neuroblast cells in F1 hybrid embryos generated by crossing individuals of the Moreton and Torresian (TT) chromosomal taxa of the grasshopper Caledia captiva. The Moreton individuals were of two kinds depending on whether they carried an acrocentric X (MAX) or a metacentric X (MMX). No significant associations were detected between any pair of homologous chromosomes in either male or female (MAX x TT) and (MMX x TT) F1 hybrids. This result was supported by data which showed that the mean separation between homologues is greater, although not significantly so, than the mean separation between non-homologous chromosomes within the two Moreton genomes. Indeed, in a number of cases, genome separation was clearly observed in radial metaphase preparations from these F1 hybrids. By comparison the analysis of pairwise associations between non-homologous chromosomes within the MMX and MAX Moreton genomes revealed a number of significant associations and dissociations which strongly suggests that at least some chromosomes in these genomes are organised non-randomly at metaphase. Of particular interest was the highly significant X-5 association in the MMX genome since in a previous study X-5 rearrangements were found to occur repeatedly among different backcross progeny involving Moreton x Torresian F1 hybrids. Additionally a comparison of the organisation of chromosomes in the MAX and MMX genomes, which differ primarily by the type of X chromosome, revealed that in a number of cases pairs of chromosomes are arranged very differently with respect to each other. The distribution of chromosomes on the hollow spindle was also analysed to investigate whether a specific spatial ordering of chromosomes exists within these Moreton genomes based on the association of pairs of short arms and pairs of long arms of most similar length (the Bennett model). The twelve chromosomes in both genomes were uniquely ordered in a single chain. However, because of computing limitations, only the ordered arrangement of chromosomes 1–10 was investigated. An analysis of 48 cells in the MMX and 38 cells in the MAX genomes showed that the predicted order in the ten chromosome sub-set in each genome did not rank in the top 20% of the 181,440 possible orders. This suggests that, although there is a good evidence that some non-homologous chromosomes may be associated non-randomly at metaphase in these genomes, they do not appear to show a specific, ordered arrangement as predicted by the Bennett model. The significance of the observed non-random organisation of chromosomes in the MMX and MAX genomes is discussed in relation to the generation of novel chromosome rearrangements in Moreton x Torresian F1 hybrids and the evolution of the Moreton and Torresian genomes.