Linkage map construction in the outcrossing species Lolium perenne

Genetic maps have proven useful tools in several fields of application. First of all, they allow to get insight into the genome organization of a species, and to compare the genome structures of different species. Genetic maps are also useful for the identification of genomic regions involved in physiological processes, and are valuable tools for the positional cloning of genes. In the framework of a project with the aim of identifying genomic regions involved in disease resistance in Lolium spp., we constructed genetic maps using different marker systems. In the ryegrass species L. perenne and L. multiflorum self-pollination is prevented by a very efficient self incompatibility system. This has clear implications in mapping studies: inbred lines and double haploids are difficult to produce, and if produced low seed yields are obtained. For this reason, we created several mapping populations by crossing two highly heterozygous unrelated plants. This implies that at any given locus, up to four different alleles might be segregating in the offspring. Different marker systems were used for the construction of the genetic maps. In first instance, AFLP was used as a high throughput marker system. This allowed to generate a high number of DNA-markers useful for map construction in a quick way. The drawback of the AFLP technique is twofold: AFLP markers are 'anonymous', and are dominant (not all alleles at a locus can be detected). Co-dominant marker systems, which allow to detect all alleles present at any locus, are much more informative and are required for the construction of detailed linkage maps in outcrossers. In this study, we used three co-dominant marker systems: SSR, STS and RFLP. The RFLP markers were generated using heterologous probes derived from other monocots, what should allow to analyze the syntenic relationships between ryegrass and other monocots. Using these markers generated with different techniques, a genetic map of ryegrass has been constructed suitable for QTL analysis and comparative genetics.