Chromosomes form into seven groups in hexaploid and tetraploid wheat as a prelude to meiosis

Abstract

SummaryHexaploid wheat possesses 42 chromosomes derived from its three ancestral genomes. The 21 pairs of chromosomes can be further divided into seven groups of six chromosomes (one chromosome pair being derived from each of the three ancestral genomes), based on the similarity of their gene order. Previous studies have revealed that, during anther development, the chromosomes associate in 21 pairs via their centromeres. The present study reveals that, as a prelude to meiosis, these 21 chromosome pairs in hexaploid (and tetraploid) wheat associate via the centromeres into seven groups as the telomeres begin to cluster. This results in the association of multiple chromosomes, which then need to be resolved as meiosis progresses. The formation of the seven chromosome clusters now explains the occasional occurrence of remnants of multiple associations, which have been reported at later stages of meiosis in hexaploid (and tetraploid) wheat. Importantly, the chromosomes have the opportunity to be resorted via these multiple interactions. As meiosis progresses, such interactions are resolved through the action of loci such as Ph1, leaving chromosomes as homologous pairs.

Homologue recognition during meiosis is associated with a change in chromatin conformation

“…Centromeres also associate in groups for meiosis in the wheat-rye hybrids 7,8 . The availability of specific probes for wheat and rye centromeres now enables us to characterise these interactions 9 .…”

“…In hexaploid wheat, centromeres pair premeiotically and then sort into 7 groups at the beginning of meiosis 8,12 and Ph1 affects the specificity of the interactions 7 . The present study clearly demonstrates the effect of Ph1 on these interactions and that the 7 centromere groups are indeed formed from the related chromosomes.…”

“…The wheat centromere, rye centromere, the rye heterochromatin knob probes used, tissue sectioning and specimen preparation, in situ hybridisation and probe preparation and the labelling of the rye segments have all been described previously 8,9,11 . Some 1500 tissue sections have been used for this study.…”

“…We collected confocal optical stacks using a Leica TCS SP as described previously 8 . The lengths of the homologous segments were measured in 3-D confocal stacks of randomly selected meiocytes in which the telomere bouquet was present.…”

Effective chromosome pairing requires chromatin remodeling at the onset of meiosis

“…In common with these structural changes in the subtelomeric regions, visualization of the overall chromosome structure by using dispersed repetitive sequences reveals that these repetitive regions also undergo extensive remodeling before pairing (7). Wheat centromeres also change conformation early in meiosis (8). At the same time as chromatin remodeling occurs, the telomeres cluster in a group termed ''a bouquet'' in many species (6,9).…”