Top left: Triticum timopheevii Top right: Triticum macrochaetum
Bottom left: Aegilops mutica Bottom right: Aegilops juvenalis
Global population levels are set to increase from 6 to 9 billion by 2050 with the result that we need to increase food production by 70% just to maintain our present nutrition levels, which already includes one billion severely malnourished and a further 100 million at near starvation level1. Since it takes 7-10 years to breed a new wheat variety, the crosses that will lead to the development of varieties in 2020 have already been made. Furthermore, although we need to increase yield in many countries, food production has levelled out either due to climate change, e.g. increased temperature, or, in crops such as wheat, a lack of sufficient genetic variation, thus limiting the ability of breeders to develop higher yielding plant varieties. The identification and exploitation of genetic variation for the development of superior high yielding varieties adapted to the changing environment is therefore of critical importance to meet the challenges of global food security.
The transfer of genetic variation from the near and distant relatives of our key crop species, wild relative gene introgression, provides a vast and virtually untapped reservoir of genetic variation for agronomically important traits that can be utilised by plant breeders for the development of superior, high yielding, adapted crops. In the monocots, wild relative gene introgression has already had a significant impact on agricultural production. In wheat, examples of key introgressions include those from Aegilops umbellulata (which saved US wheat production from catastrophic failure in 19602,3); resistance to a range of diseases, tolerance to acid soils, increased yield advantage and stability from rye (in the late 1990s a 1B/1R translocation was present in the majority of world wheat varieties and a number of the present global top varieties, e.g. "Rialto", still carry it4); a gene from Ae. ventricosa conferring resistance to eyespot has been exploited in breeding programmes5; many of the top wheat varieties in Europe, e.g. "Robigus", are derived from unknown introgressions from Triticum dicoccoides; 30% of all wheat varieties produced at CIMMYT are derived from crosses between normal wheat and “synthetic” wheat (synthetic wheat is derived from crosses between Ae. tauschii (syn. Ae. squarrosa), DD genome, and tetraploid wheat, AABB genomes followed by chromosome doubling via colchicine6).
What is wild relative gene introgression
Wild relative gene introgression, in its simplest form, involves the sexual hybridisation of different species to form an inter-specific F1 hybrid. Gene introgression occurs in the F1 hybrid (or its derivatives) when related, i.e. homoeologous, chromosomes from the two parental species (i.e. chromosomes that carry orthologous genes in essentially the same order) recombine at meiosis resulting in the generation of inter-specific recombinant chromosomes. These recombinant chromosomes are then transmitted to the next generation through the gametes. The repeated backcrossing of the F1 hybrid, or its derived amphiploid, to one of the parental genotypes results in the generation of lines which carry the majority of the genome of one species but also carry one or more chromosome segments from the other parental species.
Although the value of genetic variation from wild relative species has been clearly demonstrated, only a fraction of their full potential in breeding has so far been exploited in breeding programmes. In the past, this has been as a direct result of the lack of robust high throughput screening procedures that enable the rapid identification and characterisation of introgressed chromosome segments. However, recent advances in conventional and next generation sequencing platforms and technology in combination with the sequencing of the crop and model plant genomes, e.g. rice and Brachypodium7, are now enabling the development of strategies to fully exploit the potential of wild relative species for crop improvement8,9, e.g. King et al. (2013); Griffiths et al. (2006).
1 Gustafson JP, Borlaug NE, Raven PH (2010). World food supply and biodiversity. World Agriculture 1: 37-41.
2 Sears ER (1956). The transfer of leaf rust resistance from Aegilops umbellulata to wheat. Brookhaven Symposium Biology 9: 1-22.
3 Sears ER (1972). Chromosome engineering in wheat. Stadler Symposium 4: 23-38.
4 Ammar K, Mergoum M, Rajoram S (2004). The history and evolution of triticale. In Megoum M, Gomez-Macpherson H (eds) Triticale improvement and production. FAO Plant Production and Protection series NO. 179. FAO Rome, Italy, pp 1-11.
5 Doussinault G, Delibes A, Sanchez-Monge R, Garcia-Olmedo F (1983). Transfer of a dominant gene for resistance to eyespot disease from a wild grass to hexaploid wheat. Nature 303: 698-700.
6 Dreisigacker S, Kishii M, Lage J, Warburton M (2008). Use of synthetic hexaploid wheat to increase diversity for CIMMYT bread wheat improvement. Australian Journal of Agricultuarl Research 59: 413-420.
7 Febrer M, Goicoechea JL, Wright J, McKenzie N, Song XA, Lin JK et al. (2010). An integrated physical, genetic and cytogenetic map of Brachypodium distachyon, a model system for grass research. PLoS ONE 5: Article number e13461.
8 King J, Armstead I, Harper J, Ramsey L, Snape J, Waugh R, James C, Thomas A, Gasior D, Kelly R, Roberts L, Gustafson P, King I (2013). Exploitation of interspecific diversity for monocot crop improvement. Heredity 110:475-483.
9 Giffiths S, Sharp R, Foote TN, Bertin I, Wanous M, Reader S (2006). Molecular characterisation of PH1 as a major chromosome pairing locus in polyploid wheat. Nature 439: 749-752.