Understanding And Manipulating Recombination

Meiotic recombination is one of the principal forces creating the genetic diversity that drives evolution and is the fundamental instrument underlying most crop breeding programmes. A greater understanding of the control of recombination in crop plants would enable manipulation of this process to improve the speed and accuracy of plant breeding. This would be particularly useful for crop species such as wheat, barley, oats and forage grasses (such as Lolium and Festuca) that all exhibit a non-random pattern of recombination relative to the gene distribution in their genomes whereby chiasmata appear to be preferentially targeted to the ends of the chromosomes.

Figure 1: Seven paired barley chromosomes at Metaphase I showing ring bivalent formation with distal chiasmata.
Figure 1: Seven paired barley chromosomes at Metaphase I showing ring bivalent formation with distal chiasmata.

This means that large areas of the chromosome around the centromeric region rarely recombine, even though these regions represent substantial proportions of the physical maps of the chromosomes. As an example in barley chromosome 3H the lack of recombination in the centre of the chromosome means that 70% of the physical map is represented by a region that makes up only 5% of the genetic map. Importantly the near co-linearity of barley 3H and rice chromosome 1 means that the syntenic relationship shown between them indicates that this region of low recombination is not gene-poor and may well contain 50% of the genes on chromosome 3H.

Thus the genes in this area are inherited together as a large linkage block, preventing the generation of novel gene combinations and useful variation that could be exploited in breeding programmes. Even small changes in the crossover frequency and distribution, particularly changes that promote recombination in centromeric regions, could therefore have a significant effect on the efficiency of breeding in these crops by breaking up some of the extensive linkage blocks.

Figure 2: (a) Relationship between the physical and genetic maps of barley chromosome 3H and the physical map of rice chromosome 1 showing the effect of the skewed distribution of recombination in barley. (b) Graphical genotypes of a range of barley cultivars showing patterns of reduced recombination in the centromeric region of 7H. Cultivars are arranged horizontally and genes are vertical in linear order with alternative SNP alleles coloured red or green.
Figure 2: (a) Relationship between the physical and genetic maps of barley chromosome 3H and the physical map of rice chromosome 1 showing the effect of the skewed distribution of recombination in barley. (b) Graphical genotypes of a range of barley cultivars showing patterns of reduced recombination in the centromeric region of 7H. Cultivars are arranged horizontally and genes are vertical in linear order with alternative SNP alleles coloured red or green.

Unfortunately the tools to manipulate this fundamental process in breeding programmes do not currently exist and understanding of the control of recombination in grasses is fragmentary at present. However considerable advances in understanding have been made in studies using the model plant Arabidopsis and we are involved in collaborative projects to take the recent knowledge gained in Arabidopsis on the control of meiotic recombination and apply it to barley.

In particular the recently funded BBSRC LoLa project ‘Meiosis in barley: manipulating crossover frequency and distribution’ involves collaboration with Professor Chris Franklin and Dr Sue Armstrong at the University of Birmingham, Dr Glyn Jenkins at the University of Aberyswyth and locally with Professor Claire Halpin at the University of Dundee. We will also be collaborating with Dr Sue Armstrong and Professor Chris Franklin at the University of Birmingham in an EU funded project (FP7-KBBE-2007-1-1-03) entitled ‘Systematic Analysis of Factors Controlling Meiotic Recombination in Higher Plants’ (MeioSys).

Also of relevance to this work is the ongoing BBSRC-CSI project (‘The establishment and application of a forward genetic resource for the development of efficient breeding strategies in grass and cereals’ BB/E006736/1) that is exploring the syntenic relationships and detailed mapping data between barley and Lolium and wheat that will facilitate the transfer of any findings in barley to the other less tractable grass/cereal crops.

PrintFor further information on this work please contact Isabelle Colas (isabelle.colas@hutton.ac.uk) from the James Hutton Institute.