Effect of chromatin modification on meiosis: wheat, a model for polyploid crops

This project will address an important problem which has hampered the efficient exploitation of the genetic diversity held within wild relatives of wheat. Being able to work with wild relatives so that beneficial characteristics can be introduced into wheat will be a major scientific achievement and dramatically improve the way breeders can generate new varieties of wheat with increased performance.

Some wild relatives are adapted to thrive under different climatic conditions to that of domestic wheat, or they carry natural resistance to important diseases and/or carry other important characteristics which could influence yield. What we want to do is to produce the tools which will allow the exploitation of this diversity and genetically introduce these favourable characteristics into wheat. In doing so we will be enable wheat breeders to, amongst other things, improve wheat performance in a sustainable way, increase yield, introduce disease resistance and drought tolerance.

In a small number of cases this has already been achieved. What stops wild relatives being used efficiently? The wild relative and the wheat chromosomes must align and efficiently exchange (recombine) during meiosis. Without recombination, there isn't the opportunity to introduce the genetic diversity of wild relatives into wheat. The Ph1 locus substantially reduces recombination between wild relative and wheat chromosomes. Ph1 even reduces recombination between chromosomes derived from wheat landraces where they are significantly diverged. This makes gene transfer by recombination during meiosis difficult in the case of wild relatives, or inefficient in the case of landraces.

So how can we overcome this problem? In wheat, the Ph1 locus regulates recombination. Understanding this Ph1 regulation will provide us with an insight into this process. It will provide us with an understanding of how recombination sites are selected in wheat, and leading from this , how the process can be altered and tailored for specific needs, thus enabling us manipulate it for plant breeding.

Most related chromosomes of wild relatives of wheat exhibit extensive gene synteny along their chromosome length. Moreover the genes on related chromosomes exhibit more than 95% homology at the sequence level. Despite this level of similarity, there is little recombination between wild relative and wheat chromosomes at meiosis due the presence of the Ph1 locus. The wild relative chromosomes and wheat chromosomes differ in their repetitive content and the level of transcription from their genes. Deletion of the Ph1 locus allows the chromosomes to behave like homologous chromosomes and recombine. Recombination involves the initiation of double strand breaks within genic regions and then repair of breaks. Recent data indicates that Ph1 is affecting chromatin structure and this affects the distribution and processing of double strand breaks. Thus Ph1 is affecting the genic regions in some way enabling them to be recognised, or not, in terms of the recombinational machinery.

By exploiting next generation sequencing and cell biological approaches, we will identify what is being altered in the genic regions by the presence or absence of Ph1 at the onset of meiosis. We will access whether there is altered distribution of double strand breaks or their processing, chromatin structure or methylation of and transcription from the genic regions. From this information, we will treat developing wheat anthers prior to meiosis with drugs which themselves induce similar changes and then score the resulting effects on chromosome pairing during metaphase I, to confirm the association of the particular change with a pairing effect.

In the past there are examples where the wild relatives of wheat have been successfully exploited as a novel source of genetic variation for traits in wheat breeding programmes. However, the introgression of genes from wild relatives into wheat is very time consuming and inefficient and therefore fell out of favour. Recently, international breeding centres have again started to exploit wheat's wild progenitors, to generate synthetic hexaploid Triticum aestivum, in order to create a "synthetic wheat" breeding programme. Some 25% of elite lines of wheat generated by CIMMYT are derived from crosses to these synthetic wheat genotypes. Having exploited this approach successfully, many now argue that to increase yield production in wheat it is imperative to revisit the exploitation of genetic variation available in the wild relatives in breeding programmes. As a result of this, a number of private sector breeders are encouraging the reestablishment of wheat alien introgression in the UK public sector.

To facilitate the transfer of genetic variation via wheat/alien introgression, research is required to increase the speed and enhance the efficiency of the process. In brief, wheat/alien introgression involves the hybridisation of wheat with a wild relative followed by repeated backcrossing to generate lines of wheat carrying an alien chromosome on which a target gene is located. A series of further crosses to specific genotypes/mutant lines are then required before the chromosome of the alien species can recombine within those of wheat, allowing the transfer of the target gene to wheat without linked deleterious effects.

There are some key barriers to the exploitation of wild species in breeding programmes. One of these barriers is the ability to induce exchange between the wild relative chromosomes of wheat and the wild species. The presence of the Ph1 locus suppresses this exchange. Deletion mutants of the Ph1 locus exist. However once the Ph1 locus is removed, then the chromosomes undergo extensive rearrangement. Thus within an alien introgression programme, one is trying to induce exchange between the wild relative chromosome with a rearranged wheat chromosome. It is important to develop an approach from our understanding of the Ph1 locus, where its effect can be switched off with one generation to allow the wheat and wild relative chromosomes to exchange. Thus the present aims to understand how Ph1 is affecting the genic regions where recombination occurs. Based on this understanding, the present proposal aims to exploit drugs to overcome the Ph1 effect.

The expected outcome of this project will the creation of tools which enhance the ability of breeders to have access to the genetic potential of wild wheat species by enhancing exchange between wild relative and wheat chromosomes. It will provide an indication of how the level and distribution of recombination may be altered in wheat. It will also allow the development of a skills base, particularly in cytogenetics, a dying art in the UK which can be utilised by the next generation of scientists involved in wheat breeding and the larger wheat breeding community.