Association and introgression mapping of quantitative traits in Lolium perenne: flowering time, forage quality and abiotic stress tolerance

Most important traits subject to genetic improvement in crop plants, including forage grasses, are governed by small effects of many genes. We are using two strategies, namely association and introgression mapping, for dissection and fine mapping of flowering time, forage quality and abiotic stress tolerance in the temperate forage grass Lolium perenne. Both complement conventional QTL mapping based on F2 or backcross mapping families. Association mapping uses populations of unknown pedigree to exploit the recombination events that have occurred over many generations, enabling a high mapping resolution. We have used a large number of genotypes to find associations between single nucleotide polymorphisms (SNPs) in two candidate genes for flowering time: the Hd1 (CONSTANS) and Hd3a (FT) rice orthologues in L. perenne. After accounting for population structure we identified highly significant statistical associations in both candidate genes with flowering time. We have validated these associations by analysing progeny from test crosses involving genotypes from the experimental population with genotypes of turf grass varieties, and by genotyping unrelated natural populations known to contrast in flowering time. Forage quality is another target in our association mapping program.
Introgression mapping exploits the ability of Lolium and Festuca to hybridise. Lolium plant lines have been established containing segments of Festuca chromosome 3 introgressed, since there is evidence to suggest that this chromosome in Festuca can provide allelic variants conferring improved drought tolerance to Lolium. We are exploiting the synteny with Chromosome 1 in rice to find likely candidate genes for further analysis and marker development.