Meiosis in barley: manipulating crossover frequency and distribution

The future sustainability of UK agriculture will be dependent on the provision of new crop varieties that are able to meet future environmental and economic needs. The development of new crop varieties by plant breeding is based on harnessing the natural variation that is generated through the process of sexual reproduction and selective crossing to produce lines with novel combinations of desirable characteristics. During the formation of male and female gametes new combinations of the parental genes inherited by an individual are generated through the process of meiosis. In meiosis, homologous recombination ensures that chromosomes are accurately segregated such that each gamete gets a single complete set of chromosomes. To achieve this, transient physical links must be established between homologous pairs of parental chromosomes. This results in the reciprocal exchange of genetic information between each pair of homologous parental chromosomes, thereby generating a new combination of genes along each chromosome. Thus when male and female gametes fuse during sexual reproduction the progeny possess some characteristics of each parent and novel features that have arisen through the 'shuffling' of genes during meiotic recombination. Control of the patterns of recombination along chromosomes during meiosis in plants is therefore one of the major factors determining the outcome of plant breeding programmes. Unfortunately, it is clear that patterns of recombination can be highly skewed such that genes in some regions of the genome rarely undergo recombination. This is the case in some important grass species such as barley and wheat where it can have an adverse effect on potential breeding programmes Over the past decade studies in Arabidopsis, the model system for plant genetics, have resulted in considerable progress in our understanding of how meiosis and recombination in plants is controlled at the molecular level. Hence, this project seeks to transfer some of this knowledge to the crop plant barley and thus enable plant breeders to overcome one of the major challenges they face in the development of new varieties of this crop. This is feasible in the case of barley because we have a good understanding of barley genetics and genetic tools are in place for this crop to facilitate such a transfer. Our objectives will be to determine how meiotic recombination is controlled in barley and the basis for the skewed pattern of recombination. We will then explore strategies that could be used to manipulate the patterns of recombination that could be applied by plant breeders in their existing programmes without recourse to GM technology. If this is successful these approaches could then be applied to more complex grass crop species such as wheat and forage grasses (e.g. ryegrass) that show the same skewed distribution of recombination.

Recent progress in understanding of the control of recombination in plants offers the prospect of the ability to manipulate this process to profoundly improve the speed and accuracy of plant breeding. This is particularly relevant for certain species in the grass subfamily Pooideae such as barley, wheat and ryegrass that show a highly skewed distribution of recombination relative to gene content. Currently the tools to manipulate this fundamental process in breeding programmes do not exist and the understanding of the control of recombination in grasses is fragmentary. Hence, this project seeks to take advantage of recent advances in meiosis research in Arabidopsis and apply this to barley as a representative cereal. This will allow the coupling of cytogenetic studies to the genetic and genomic resources available for this species and permit the use of forward and reverse genetic approaches to conduct functional analyses as has proved so fruitful in Arabidopsis. The project will involve initial work to transfer the molecular cytogenetic techniques and tools from Arabidopsis to barley which will enable a thorough molecular cytogenetic analysis of barley meiosis that will provide a benchmark against which to judge other aspects of the project. In parallel to the cytogenetic work barley homologues of known Arabidopsis meiotic genes need to be fully isolated and characterised through direct molecular analysis and bioinformatics work. These preparatory strands of work will then be utilised to determine and analyse factors affecting the frequency and distribution of meiotic crossovers in barley lines using both existing and de novo mutants in both forward and reverse genetic approaches. The results from this work will inform the strategies used in the final suite of work aimed at the manipulation of recombination these will include the use of TILLING as well as transformation approaches to provide the possibility of future non-GM exploitation routes.