Molecular Marker Technology for the Study of Genetic Variation and Comparative Genetics in Pasture Grasses

Molecular genetic marker systems and associated linkage maps were developed for the most important temperate pasture grasses. SSR and AFLP systems are of particularly high value for genetic analysis of perennial ryegrass (Lolium perenne L.). Due to the obligate outbreeding reproductive system of this 120 species, natural and synthetic populations are genetically heterogeneous. AFLP-based DNA profiling has been used to relate genetic variation within and between populations to their breeding histories, and parallel analysis with SSRs has produced largely congruent results. The merits of different analytical techniques for SSR-based diversity analysis are discussed. SSRs provide a more readily implementable high throughput method for large-scale characterization of perennial ryegrass germplasm collections, with high potential value for population-based marker-trait linkage detection. AFLPs have also been used to detect genetic variation between different taxa in the Lolium genus, permitting identification of species-specific diagnostic markers. Molecular markers are also suitable for comparative genetic analysis. This is generally achieved with RFLPs, but the close phylogenetic relationship between perennial ryegrass and annual ryegrass (L rigidum Gaud.) suggests that genomic DNA-derived SSRs may also prove useful. This marker system has been used to study the genetic control of resistance to the herbicide glyphosate in crosses based on resistant and susceptible Australian ecotypes, providing support for the hypothesis of simple genetic control, but also identifying possible divergences of genome structure between Lolium species.