RAPD characterization of heterogeneous perennial ryegrass cultivars

The number of cultivars of perennial ryegrass has steadily increased since the early 1960s. Perennial ryegrass has a self-incompatible, cross-pollinated breeding system and thus, each cultivar is a heterogeneous population of individual genotypes. The ability to accurately distinguish large numbers of genetically heterogeneous populations from one another, based solely on morphological data and agronomic performance, is limited. Random amplified polymorphic DNA (RAPD) markers offer a nearly unlimited supply of molecular traits on which to base distinctive characterizations of plant populations. Analysis of molecular variance (AMOVA) provides a statistical tool which partitions molecular marker variation within and among populations and performs significance testing of differences. The objective of this research was to survey RAPD marker variation within and among populations of perennial ryegrass to characterize breeding germplasm and commercial cultivars using AMOVA. A range of genetic diversity was represented by choosing 18 grass populations that included related crosses, derived selections, and unrelated ecotypes. For each population, 10 individuals were examined for 33 RAPD markers produced with two primers. Markers present in all individuals of a particular population but not present in any other individual of another population (fixed marker difference) were not observed among the 18 grass populations, so characterizations were based on marker frequency differences among populations. RAPD markers analyzed with AMOVA closely reflected the breeding histories of the perennial ryegrass germplasm or commercial cultivars. This was particularly true for relationships structured on a population basis. Results indicated that high selection intensity substantially altered the distribution of RAPD markers between parent and progeny populations thereby obscuring measurements of genetic relatedness. Furthermore, this study observed a limited ability to separate closely related populations. Evidence for the narrow germplasm base of turfgrass cultivars was observed; although, many perennial ryegrass populations appeared to retain levels of within population variability comparable with that of land race ecotypes. The methods of analysis presented should provide a supplement to traditional morphological and agronomic data for plant variety protection and for measuring genetic diversity within breeding programs.