Developing tools for introgression into wheat where recombination is not possible

This project will address an important problem which has hampered the 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 other wild relatives being used? The reason is that similarity of gene order, particularly at the chromosome ends (telomeres), is necessary to allow the chromosomes to align efficiently and recombine during the process of meiosis. Without recombination there isn't the opportunity to introduce the genetic diversity of wild relatives into wheat. Recombination Is possible in some cases but many wild relatives have rearranged their chromosomes compared with wheat chromosomes, making gene transfer difficult, if not impossible, by recombination during meiosis. So how can we overcome this problem? What we want to do is to exploit special genes, known as gametocidal genes that are found in some wild species. These genes were discovered when breeding to produce wheat lines that had an additional segment of a chromosome from a wild relative, because some chromosomes from the wild relative were found to transmit preferentially to the offspring. These chromosomes have been termed 'cuckoo' chromosomes (or gametocidal chromosomes). Gametocidal genes on these chromosomes induce chromosomal breakages which frequently result in translocations, or exchanges, between the chromosomes of wheat and those of the wild species. This strategy provides a route for the transfer of genes from chromosomes of wild species into wheat. However it is laborious and requires extensive experience in cytogenetics. We want to be smarter in the use of this system and so we need to understand its biological basis therefore the aim of this project is to identify the genes responsible for controlling the gametocidal effect on the 4S chromosome of Aegilops sharonensis. All breeding is currently based on chromosome assortment and the ability of chromosomes to undergo meiotic recombination. Identifying the biological basis for the gametocidal effect will enable an alternative system to be more effectively exploited and deployed in plant breeding. This will ultimately enhance the genetic diversity in wheat and in particular the pool of wild species which can be exploited for wheat improvement.

Gene introgression from wild relatives into wheat through recombination relies on the chromosomes of both parents exhibiting synteny along their length, particularly in the telomeric regions. This restricts which wild species can be exploited as many have rearranged their chromosomes relative to wheat. To exploit the genetic diversity in wild relatives a novel method is needed to facilitate the introgression of beneficial traits to improve wheat. We propose to develop a novel method by identifying and understanding gametocidal genes, the genes that have been found to be responsible for preferential transfer of segments of wild chromosomes into wheat when using conventional introgression techniques of backcrossing. There are at least two tightly linked genes, termed breaker and inhibitor, which are involved in preferential transmission. This system can be exploited to induce, and recover, translocations between wheat chromosomes and the alien donor. We will study the gametocidal genes on the group 4 chromosomes of Aegilops sharonesis, as they produce a major effect. The strategy to identify the gametocidal genes is that used to successfully define the Ph1 locus (Griffiths et al 2006). The gene content of the gametocidal region will be established using genome synteny with rice and Brachypodium. This information provides markers with which to saturate the Aegilops sharonensis gametocidal region, thereby providing an initial framework for fine mapping. A physical map of the region covering the gametocidal genes is then established using a BAC library of Aegilops sharonensis. By placing the breakpoints of deletions known to either encompass, or not, the gametocidal genes first onto rice/Brachypodium, and then onto the physical map of the Aegilops sharonensis region, we will define a minimum deletion region containing the gametocidal genes. Sequencing will reveal the gene content of the region and allow us to identify candidates for the breaker and inhibitor genes

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. These barriers include the ability to make the initial hybrid and the time taken to induce exchange between both collinear and non-collinear chromosomes of wheat and the wild species. The advent of novel approaches for tackling these problems now means that a wheat/alien introgression programme can, once more, be considered. The ability to enhance the efficiency of making the initial hybrid has been addressed (see attached BBSRC interim report) and the ability to induce exchange between collinear chromosomes has also been largely addressed (also see the second BBSRC interim report attached). This leaves the ability to induce exchange between non-collinear chromosomes as an outstanding requirement. This problem will be addressed through this present proposal; to clone the genes responsible for the gametocidal effect. Understanding the genes involved and biology of this character will inform on whether such genes are present in wheat already but lack gametocidal action (which seems likely) or are novel to the wild B genome species. If they are present in wheat already, then the easiest route of deployment would be to induce their activity in wheat. Thus in a wheat-wild relative interspecifc hybrid, their activity could be induced in the gametes and the translocation of the alien segment carrying the trait of interest onto a wheat chromosome selected for in the resulting progeny. The expected outcome of this project will the creation of tools to allow breeders to have access to the genetic potential of wild wheat species that possess non-collinear chromosomes and which cannot normally be introgressed into wheat. It will also allow the development of a skills base, which can be utilised by the next generation of scientists involved in wheat breeding and the larger wheat breeding community.