Abstract
Following the emergence of the Ug99 lineage of Puccinia graminis f. sp. tritici (Pgt) a collective international effort has been undertaken to identify new sources of wheat stem rust resistance effective against these races. Analyses were undertaken in a collection of wheat genotypes gathered from across Africa to identify stem rust resistance effective against the Pgt races found in Eastern and Southern Africa. The African wheat collection consisted of historic genotypes collected in Kenya, South Africa, Ethiopia, Sudan, Zambia, Morocco, and Tunisia, and current South African breeding lines. Both Bayesian cluster and principal coordinate analyses placed the wheat lines from Sudan in a distinct group, but indicated a degree of genetic relatedness among the other wheat lines despite originating from countries across Africa. Seedling screens with Pgt race PTKST, pedigree information and marker haplotype analysis confirmed the presence of Sr2, Sr36, Sr24, Sr31, and Lr34/Yr18/Sr57 in a number of the lines. A genome-wide association study (GWAS) undertaken with Diversiry Arrays Technology (DArT) and stem rust (Sr) gene associated markers and Stem Area Infected (SAI) and Reaction Type (RT) field phenotypes, collected from trials carried out across two seasons in Kenya in 2009 and in South Africa in 2011, identified 29 marker-trait associations (MTA). Three MTA were in common between SAI and RT, with the biggest effect MTA being found on chromosome 6AS. Two wheat lines, W1406 and W6979 that exhibited high levels of adult plant stem rust resistance were selected to generate bi-parental mapping populations. Only the MTA on chromosomes 6AS and 3BS, and the locus Lr34/Yr18/Sr57 were confirmed following QTL mapping. Additional stem rust resistance QTL, not detected by the GWAS, were found on chromosomes 2BS, 2DL, 3DL, and 4D.
Highlights
Across Africa wheat consumption has increased considerably since the mid 1990’s, faster than any other major food grain
Lines were assessed for growth type, homozygosity and stem rust resistance, and two lines of each accession were selected for further greenhouse seed multiplication in South Africa
In all seasons a broad variation in stem rust reactions was observed within the African wheat collection
Summary
Across Africa wheat consumption has increased considerably since the mid 1990’s, faster than any other major food grain. This has resulted in a growing reliance on wheat imports, as wheat production in Africa has failed to keep up with demand (Mason et al, 2012). Durum wheat has been grown in the Abyssinian highlands of Ethiopia, and along the Nile valley in Egypt and the Sudan for thousands of years (Bonjean and Angus, 2001). Bread wheat was introduced into Southern Africa by the Dutch in the Seventeenth Century and by missionaries into East Africa, including Kenya and Tanzania in the Nineteenth Century (Bonjean and Angus, 2001). Wheat yields in Africa have tended to be low, but recent wheat simulation analyses suggest that there may be potential for profitable, competitive, wheat production in several African countries (Shiferaw et al, 2012)
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