VI Functional Gene Analysis and Molecular Tools
Exploiting Comparative Biology and Genomics to Understand a Trait in Wheat, Ph1
Abstract For hexaploid wheat to be highly fertile, only true homologues must pair at meiosis, rather than the highly related chromosomes present. The mechanism, which restricts this pairing, must have arisen rapidly on wheat's polyploidisation, to ensure stability and fertility. From the analysis of Ph1, which is the major locus restricts this pairing, tweaking Cdk-type phosphorylation levels is one way to provide such a control.
Meiosis is a cell division process that ensures gametes carry the correct number of chromosomes, without a doubling of chromosome number. During meiosis, chromosome numbers are halved, leading to haploid gametes, a process that is crucial for the maintenance of a stable genome through successive generations. The process to achieve an accurate segregation of the homologous chromosomes (homologues) starts in pre-meiosis as each homologue is replicated and the respective products, sister chromatids are held together as via specific cohesion proteins. Then at the start of meiosis, each chromosome must recognize its homologue from amongst all the chromosomes present in the nucleus and associate or pair, and then recombine with that homologue. The homologues are observed as paired at Metaphase I. The homologues are then separated and segregated to two different daughter cells. In the next round of division, the sister chromatids are then separated, moving to one of four haploid cells. The accuracy of recognition and segregation of the homologues has a profound effect on overall fertility. This is more complicated in polyploids because of the greater number of related chromosomes. Polyploid fertility depends on the efficiency by which they behave as diploids during meiosis by restricting pairing to true homologues, despite the presence of related chromosomes (homoeologues). The mechanism, which restricts this pairing, must arise rapidly on polyploidisation to ensure fertility of the new polyploid plant. So what controls the difference between diploid and polyploid pairing in a polyploid species such as wheat?
The hexaploid wheat (AABBDD, 2n = 42) carries a diploid set of 42 chromosomes composed of three ancestral genomes, A, B and D. The 42 chromosomes can be divided into seven groups of six chromosomes (two chromosomes from each of the ancestral genomes). In hexaploid wheat, chromosome 1A must pair with 1A at meiosis and not with 1B or 1D. In wheat, a single locus, Ph1, (Pairing homoeologous 1) on the long arm of chromosome 5B has a major controlling effect (Okamoto 1957; Riley and Chapman 1958; Sears and Okamoto 1958). Both Riley and Sears experimented with both haploids and wheat-wild relative hybrids, which lack homologues and only possess homoeologues. They observed that, when chromosome 5B was deleted in both haploids and wheat-wild relative hybrids, that there was a level of pairing between the related chromosomes at Metaphase I. They recognized that a locus on chromosome 5B was responsible for the major effect on polyploid pairing in wheat. Using deletions, Sears further defined the effect to the long arm of wheat chromosome 5B (Sears 1977). Thus Ph1 is defined as a deletion phenotype.