Wheat meiosis and the Ph1 locus

The polyploid wheat genome

Wheat is an allopolyploid. It possesses two or more sets of related chromosomes as a result of doubling of chromosomes following sexual hybridization between closely related species. At meiosis, homologous chromosomes (homologues) need to be distinguished from the homoeologous (related) chromosomes (homoeologues), which have a similar gene content and order but diverge in repetitive DNA content. Hexaploid (bread) wheat contains A, B and D genomes, and tetraploid (pasta) wheat contains A and B genomes. Despite their genome complexity, both wheats behave as diploids at meiosis, so bread wheat chromosome 1A pairs with 1A, and not with either 1B or 1D, and this holds true for all seven chromosome groups. The accuracy and efficiency of the mechanisms used to achieve this diploidization have a profound influence on the fertility of the wheat plant, which is of major importance for success in breeding. The Ph1 locus on wheat chromosome 5B controls this diploidizing mechanism in wheat.

Diagram of wheat chromosome groups showing homologous & homoeologous chromosomes.

Ph1 and breeding

Many wild species carry interesting characteristics that would be useful to exploit in breeding programmes. Sexual hybridization between polyploid wheat and a wild species generally produces an interspecific hybrid containing a haploid set of polyploid and wild-relative chromosomes. Breeders make use of these crosses to introduce desirable genes from exotic germplasms. However, in many such hybrids there is only a low level of pairing and recombination between wheat and wild relative chromosomes. The major factor preventing pairing between wheat and wild species chromosomes is the Ph1 locus.

The importance of understanding the Ph1 locus

Wheat lines carrying deletions encompassing the Ph1 locus have been used for the last 25 years in breeding. Pairing can occur between related chromosomes in these lines, but the chromosomes are heavily rearranged, making it difficult for recombination to take place between the wheat and related chromosomes. What is needed is the ability to temporarily switch off the Ph1 locus to enable related chromosomes to pair. An understanding of the biology of this phenotype will help to achieve this goal.

The Ph1 locus and its effect

During meiosis, telomeres cluster in a bouquet arrangement and chromosomes pair correctly via their telomeres whether Ph1 is present or not (Griffiths et al., 2006).  In contrast, Ph1 does affectthe behaviour of centromeres.  In the presence of Ph1, centromeres are arranged in pairs on the nuclear membrane, whereas in the absence of Ph1, they are largely unpaired and are internalised in the cell.

In order for pairing and recombination to occur, wheat chromosomes must undergo chromatin remodelling at the onset of meiosis (Prieto et al., 2004; Colas et al., 2008). In the presence of Ph1, chromosome homologues must be identical or near identical for this to take place. Ph1 appears to ‘sense’ homology prior to the chromosomes coming into contact with each other. Thus, if the chromosomes are true homologues, chromatin remodelling is synchronized, allowing pairing and recombination to occur. This begins with the formation of a V-shaped paired structure with the telomeres at the apex, and follows with the chromosomes “zipping up”.  If homologous chromosomes are in different conformations they do not “zip up”, but instead associate as loop structures with the chromosomes “pegged” at points along the chromosome to give a chain-like structure before coming together (Colas et al., 2008).  If the chromosomes are homoeologous (related), however, and show too much divergence, remodelling is not synchronized and the chromosomes fail to pair and recombine. In the absence of Ph1, all chromosomes can remodel without the requirement for the presence of an identical or near identical chromosome, and this increases the chance of pairing between related chromosomes in addition to pairing between true homologues.

In situ hybridizations of wheat anther cells. Behaviour of two homologous segments on a single pair of homologous chromosomes during the meiotic cell cycle. (homologous segments labelled green, telomeres labelled red). Prior to meiosis the segments are compact & almost spherical. A-C) In a wheat line carrying Ph1, the two homologous segments elongate synchronously then intimately align, appearing to “zip up”. D) In a wheat line with Ph1 deleted, the homologous segments are not synchronized.

The Ph1 locus has been defined to a cluster of Cdk2-like (CDK2L) genes, disrupted by a segment of heterochromatin, on chromosome 5B.  These wheat CDK2L genes show close homology to Cdk2 in mammals (Griffiths et al., 2006; Al-Kaff et al., 2007) and some homology to the meiosis specific kinase Ime2 in budding yeast.  Cell biological and synapsis studies reveal that, like its mammalian and yeast counterparts, Ph1 CDK2L affects processes during replication, controls chromatin remodelling and affects the mismatch repair mechanism.

Deletion of the Ph1 locus leads to increased transcription of related Cdk2-like loci on homoelogous genomes and overexpression of the synaptonemal complex protein Asy1 (Asynapsis 1), suggesting a link between Asy1 expression and the Ph1 Cdk2-related locus.  Research evidence from Jason Able’s group (University of Adelaide) suggests that during meiosis 1, the protein encoded for by the wheat Asynapsis 1 gene (TaASY1) stimulates interactions between homologous chromosomes at all sites along their entire length, thus promoting homologous chromosome pairing. Furthermore,it is probable that Ph1 suppresses interactions between homoeologous chromosomes by regulating the level of TaASY1 expression during meiosis (Boden et al., 2008).  This regulatory role for Ph1 is consistent with its suggested identity as a Cdk-like gene.

Szwarcwort-Cohen et al. (2009) have shown that Cdk2 from mammals will compliment for the Ime2 mutant in yeast, leading to overexpression of the yeast homologue of Asy1, Hop1.  Ime2 is a master coordinator in yeast affecting premeiotic replication, transcription of early meiotic genes and chromosome segregation.  This suggests that homologue recognition in wheat, mammals and yeast may be linked by a conserved process across these species involving a master meiotic coordinator Ph1/Cdk2/Ime2 affecting premeiotic replication and transcription of early meiotic genes.

Future work

Our working hypothesis is that the Ph1 Cdk2L genes in wheat act in a similar way to Cdk2 genes in mammals and Ime2 genes in yeast, and that this explains all the phenotypic effects associated with the Ph1 locus in wheat. Therefore, part of our future work is aimed at testing this functional similarity by examining the biochemical effect of CDK2L and identifying interacting factors. Other future strategies involve temporarily switching off the Ph1 effect to allow pairing between the chromosomes of wheat and its wild relatives (See chemical modification of chromosome pairing).

Summary of the Ph1 effect in wheat

Summary of the Ph1 effect in a wheat-rye hybrid

The cell biological studies developed in wheat to understand the effect of the Ph1 locus were undertaken in collaboration with Peter Shaw and his group. This collaboration has now been expanded to include proteomics approaches in wheat for studying the Ph1 effect. 

For a recent review of the role of Ph1 in homologue recognition and chromosome pairing and a comparison with similar mechanisms in other organisms see Moore and Shaw, 2009 (reference below).

References

Moore G and Shaw P (2009) Improving the chances of finding the right partner. Curr Opin Genet Dev.  In Press

Boden SA, Langridge P, Spangenberg G, Able J (Early View Date: November 2008) TaASY1 promotes homologous chromosome interactions and is affected by deletion of Ph1. The Plant Journal DOI: 10.1111/j.1365-313X.2008.03701.x

Szwarcwort-Cohen M, Kasulin-Boneh Z, Sagee S, Kassir Y (2009) Human Cdk2 is a functional homolog of budding yeast Ime2, the meiosis-specific Cdk-like kinase. Cell Cycle 8 1-8 [Epub ahead of print]

John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK

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