Genetic map construction and QTL mapping of resistance to blackleg (Leptosphaeria maculans) disease in Australian canola (Brassica napus L.) cultivars.

Genetic map construction and identification of quantitative trait loci (QTLs) for blackleg resistance were performed for four mapping populations derived from five different canola source cultivars. Three of the populations were generated from crosses between single genotypes from the blackleg-resistant cultivars Caiman, Camberra and (AV)Sapphire and the blackleg-susceptible cultivar Westar(10). The fourth population was derived from a cross between genotypes from two blackleg resistant varieties (Rainbow and (AV)Sapphire). Different types of DNA-based markers were designed and characterised from a collection of 20,000 EST sequences generated from multiple Brassica species, including a new set of 445 EST-SSR markers of high value to the international community. Multiple molecular genetic marker systems were used to construct linkage maps with locus numbers varying between 219 and 468, and coverage ranging from 1173 to 1800 cM. The proportion of polymorphic markers assigned to map locations varied from 70 to 89% across the four populations. Publicly available simple sequence repeat markers were used to assign linkage groups to reference nomenclature, and a sub-set of mapped markers were also screened on the Tapidor x Ningyou (T x N) reference population to assist this process. QTL analysis was performed based on percentage survival at low and high disease pressure sites. Multiple QTLs were identified across the four mapping populations, accounting for 13-33% of phenotypic variance (V (p)). QTL-linked marker data are suitable for implementation in breeding for disease resistance in Australian canola cultivars. However, the likelihood of shifts in pathogen race structure across different geographical locations may have implications for the long-term durability of such associations.

Impact of Cre1, Cre8 and Cre3 genes on cereal cyst nematode resistance in wheat.

The cereal cyst nematode (CCN; Heterodera avenae), a root disease of cereal crops, is a major economic constraint in many wheat (Triticum aestivum)-growing areas of the world. The objective of this study was to assess the impact of the Cre1, Cre8 and Cre3 genes on CCN resistance. A population of 92 doubled-haploid (DH) lines derived from a cross between wheat cvs. Frame and Silverstar as well as 1,851 wheat breeding lines were screened for CCN resistance at the Primary Industries Research Victoria (PIRVic). A second population of 9,470 wheat breeding lines was screened at the South Australian Research and Development Institute (SARDI). Cre3 had the largest impact on reducing the number of female cysts, followed by Cre1 and Cre8. There was no significant difference in number of cysts between DH lines with or without the Cre8 marker, suggesting that the marker is not perfectly linked to Cre8. The estimated heritabilities were 0.32 in the DH population, 0.48 in the PIRVic data set and 0.32 in the SARDI data set, which confirm that this is a trait of low heritability. The repeatability of CCN resistance improved with an increase in the number of plants assessed per line-up to ten. However, 85-88% of the improvement was achieved with the assessments of the first five plants.