AFLP) markers, to measure genetic diversity in perennial ryegrass (Huff, 1997; Roldan-Ruiz et al., 2000; An essential prerequisite to cultivar identification is to determine Sweeney and Danneberger, 1994, 1997, 2000. Both whether cultivars are differentiated genetically. We investigated genetic diversity among and within seven perennial ryegrass (Lolium RAPDs and AFLPs detect substantial genetic variation perenne L.) cultivars (Loretta, Linn, Manhattan II, Affinity, Jet, Penn-within perennial ryegrass cultivars and generally demfine, and Palmer III) using simple sequence repeat (SSR) markers, onstrate that cultivars can be discriminated on the basis with the goal of determining whether cultivars could be differentiated of genetic characteristics. However, it appears that some on the basis of genetic data. In each cultivar we genotyped 30 individuclosely related cultivars lack distinct genetic boundaries als with 22 SSR markers, 18 of which had not been reported previously. (Huff, 1997), and this can complicate cultivar identifica-Our results indicated that each of the seven cultivars contained high tion. The comingling of genetic boundaries has been but similar levels of genetic diversity. Within cultivar heterozygosity examined in detail only with RAPD data, and the extent ranged from 0.589 to 0.643. The cultivars could be distinguished by of this problem may vary among marker systems. a number of statistical criteria, including: (i) a small but significant An additional promising marker system is simple seproportion (14.6%) of among-cultivar genetic variation, based on analysis of molecular variance (AMOVA); (ii) significant between quence repeats (SSRs). SSRs are short stretches of tancultivar F ST values that ranged from 0.065 to 0.197; (iii) separation of demly repeated di-, tri-, or tetranucleotide motifs (Weindividuals in principal component analysis (PCA); and (iv) correct ber, 1990) that have become the marker of choice for identification of individuals by the genotype assignment test, which is many genetic analyses. SSRs are broadly used for four related to discriminant analysis. The genotype assignment test worked reasons. First, each SSR locus is genetically well defined particularly well; it correctly assigned all 210 individuals to their culti-

No abstract

Simple sequence repeats (SSRs) have proven to be useful genetic markers in a wide variety of plants, but have yet to be widely applied to turfgrasses. Here we describe a study of SSRs in perennial ryegrass (Lolium perenne L.). A library of perennial ryegrass genomic DNA was screened with (GA). and (GT). probes, and SSR-containing clones were isolated and sequenced. On the basis of this screen, we estimated that there are roughly 5800 (GA), and (GT). SSRs in haploid perennial ryegrass genome. Polymerase chain reaction (PCR) primers were designed to amplify the isolated SSRs, and six polymorphic $SRs were identified. Polymorphism in the these six SSRs was sufficient to discriminate among 18 individuals representing I1 perennial ryegrass clones and seven other Lolium species. Half-sibs could be distinguished with data from as few as three SSRs. The SSR genotype data was also used to infer genetic relationships among the individuals of our sample. The relationships were in broad agreement with those established by previous analyses, suggesting that SSR data will be usefni for exploring relationships among perennial ryegrass cultivars. In total, this study indicates that SSRs are sufficiently abundant and sufficiently polymorphic to be useful genetic markers in perennial ryegrass.

genic races; thus, host resistance genes remain effective for only a few years before the pathogen population Gray leaf spot, caused by the fungus Pyricularia grisea (Cooke) shifts to new virulent races (Bonman et al., 1992; Zhu Sacc. [teleomorph Magnaporthe grisea (T.T. Herbert) Yaegashi & Udagawa], can be a devastating disease on perennial ryegrass (Lolium et al. , 2000). This has led researchers to identify durable perenne L.). The identification and utilization of perennial ryegrass (quantitative) resistance mechanisms to more successcultivars with improved resistance to gray leaf spot would reduce the fully prevent disease outbreaks in rice (Ahn and Ou, need for fungicide applications. The objectives of this study were to: 1982; Cho et al., 1998). Chen et al. (2003) have studied (i) evaluate cultivars, experimental selections, and single-plot progequantitative disease resistance to rice blast using a renies of perennial ryegrass for resistance to gray leaf spot, (ii) develop combinant inbred line population in rice and a barley populations from selected resistant parents to determine improve-(Hordeum vulgare L.) double haploid population. They ments in the next generation, and (iii) determine heritability and the found quantitative trait loci associated with resistance response to selection for gray leaf spot resistance in perennial ryegrass in both populations and suggested that there may be populations. Two perennial ryegrass field experiments consisting of synteny in the quantitative trait loci (QTL) of blast commercial cultivars and experimental selections, were established in 2000 and 2001 at Adelphia, NJ. Susceptibility of germplasm to gray resistance between barley and rice genomes. The simileaf spot was evaluated following ephemeral natural outbreaks of the larity in QTL between rice and barley indicates that disease that occurred three to four weeks post seeding. Parents with similar resistance mechanisms may be employed by difimproved resistance to gray leaf spot were selected based on progeny ferent grass species, including perennial ryegrass. Returf plot evaluations in 2000, inter-pollinated and seeded into turf sults of Curley et al. (2003) support this hypothesis. plots in the 2001 experiment. Most cultivars and selections evaluatedSince the first report of gray leaf spot disease on in both experiments had greater than 50% gray leaf spot disease. The perennial ryegrass turf in 1991, the disease has spread high broad-sense heritability estimate (0.92) and similar response of from the transition zone, through the Mid-Atlantic states progeny compared to selected parents indicated that parent selection and north to New England (Schumann, 1999), as well as based on progeny tests was a good selection method to predict the west through Indiana (Harmon et al., 2000) and Illinois combining ability of the parents. It also proved successful in improving gray leaf spot resistance in the next generation, which will be important

No abstract