Development and validation of molecular markers for Phytophthora medicaginis resistance in lucerne

Abstract

Lucerne (Medicago sativa ssp. sativa L.) is one of the most important forage legumes worldwide, and Australia's most valuable perennial forage legume. Phytophthora root rot, caused by the oomycete pathogen Phytophthora medicaginis is a lethal disease of lucerne in Australia and America. The disease is a major productivity constraint when cultivars without adequate resistance levels are grown in conditions conducive to high disease pressure. The research presented here examined Phytophthora resistance in lucerne genotypes selected from Australian cultivars, however the findings are of relevance worldwide due to the extensive use of international (USA) germplasm in Australian lucerne breeding programs in particular.A broad-based synthetic approach, first described in1942, has been the most commonly used process in lucerne breeding. However, this approach presents an inherent set of limitations which are highlighted by the yield plateau observed in lucerne in the absence of disease and pest pressures.Improved breeding strategies have been proposed to address the limitations of the current approach and the constraints imposed by lucerne biology. These improved breeding strategies utilise synthetics based on far fewer parents than are currently being used, providing greater possibilities for making genetic gain for multiple quantitatively inherited traits.However, a paucity of information exists on the number, genomic location and diversity of P. medicaginis resistance loci available in the M. sativa gene pool for breeding Phytophthora resistant lucerne. This information would underpin selection of parental genotypes in narrow-based breeding schemes aimed at addressing the current yield stagnation in lucerne. One previous report described the mapping of Phytophthora resistance loci in the lucerne genotype W116, however the molecular marker systems employed did not enable chromosome number assignment to the linkage groups generated, and the markers have limited comparative mapping potential. The research presented here sought to address the current knowledge gap in the lucerne-Phytophthora pathosystem. It determined the genetic basis for P. medicaginis resistance in lucerne clone WA272 by assigning chromosome locations to the resistance loci identified and generated an alignment of the resistance loci identified with those in W116.An autotetraploid mapping population of 182 individuals was generated and phenotyped for P. medicaginis reaction using established inoculation procedures. The population was genotyped using SSR markers from existing M. sativa and M. truncatula genetic linkage maps. Large effect QTLs iii for resistance to P. medicaginis were detected in the WA272 x D mapping population, and using the linkage map developed, these were located to WA272 linkage groups 2, 5, 6 and 7. The large effect QTL identified on linkage groups 2 and 6 through interval mapping, and the best single markers from linkage groups 5 and 7 collectively explained 94%, 92% and 76% of the phenotypic variation for the cotyledon...