Association mapping is a powerful strategy for identifying genes underlying quantitative traits in plants. We have assembled and characterized genetic and phenotypic diversity of a sorghum [Sorghum bicolor (L.) Moench] panel suitable for association mapping, comprised of 377 accessions representing all major cultivated races (tropical lines from diverse geographic and climatic regions), and important U.S. breeding lines and their progenitors. Accessions were phenotyped for eight traits, and levels of population structure and familial relatedness were assessed with 47 simple sequence repeat (SSR) loci. The panel exhibited substantial morphological variation and little genotypic differentiation was observed between the converted tropical and breeding lines. The phenotypic and genotypic data were used to evaluate the performance of several association models in controlling for spurious associations. Our analysis indicated that association models that accounted for both population structure and kinship performed better than those that did not. In addition, we found that the optimal number of subpopulations used to correct for population structure was trait dependent. Although augmentation of the genotypic data with additional SSR loci may be necessary, the association models, genotypic data, and germplasm panel described here provide a starting point for sorghum researchers to begin association studies of traits and markers or candidate genes of interest.

Transposable elements are ubiquitous in plant genomes, where they frequently comprise the majority of genomic DNA. The maize genome, which is believed to be structurally representative of large plant genomes, contains single genes or small gene islands interspersed with much longer blocks of retrotransposons. Given this organization, it would be desirable to identify molecular markers preferentially located in genic regions. In this report, the features of a newly described family of miniature inverted repeat transposable elements (MITEs) (called Heartbreaker), including high copy number and polymorphism, stability, and preference for genic regions, have been exploited in the development of a class of molecular markers for maize. To this end, a modification of the AFLP procedure called transposon display was used to generate and display hundreds of genomic fragments anchored in Hbr elements. An average of 52 markers were amplified for each primer combination tested. In all, 213 polymorphic fragments were reliably scored and mapped in 100 recombinant inbred lines derived from a cross between the maize inbreds B73 ؋ Mo17. In this mapping population, Hbr markers are distributed evenly across the 10 maize chromosomes. This procedure should be of general use in the development of markers for other MITE families in maize and in other plant and animal species where MITEs have been identified.

Cultivated peanut (Arachis hypogaea L), an important agronomic crop, exhibits a considerable amount of variability for morphological traits and for resistance'to diseases and pests. In contrast, molecular marker assays have detected little variation at the nucleic acid level. Identification of molecular markers would be of great help to peanut breeders, geneticists, and taxonomists. The objectives of this work were to identify simple sequence repeat (SSR) markers in cultivated peanut and to test these markers for their ability to discriminate among accessions. Peanut total genomic DNA libraries were constructed and screened with 3~p-labeled dinucleotide repeats, (GT)I0 and (CTh0. DiNA sequences were obtained from the SSR-containing clones and, when possible, primer pairs were designed on the basis of DNA sequences flanking the repeat motif. Primer pairs were tested in polymerase chain reaction (PCR) assays using a collection of 22 peanut DNAs, representing both cultivated peanut and wild species. In all, six SSR markers, five from the library screening procedure and one additional marker obtained from a search of publicly available DNA sequences, detected polymorphisms among the peanut DNAs. Discrimination power was high among the cultivated peanuts, with 17 unique genotypes represented among the 19 accessions tested. From two to 14 DNA fragments were amplified per SSR marker, and as a group, the six markers may amplify up to 10 putive SSR loci. The SSR markers identified in this study were more effective in detecting molecular variation in cultivated peanut than all other DNA-based markers evaluated to date. C ULTIVATED PEANUT is an important crop internationally for both direct human consumption and as an oilseed crop. Peanuts were domesticated in South America, probably in Southern Bolivia or Northern Argentina (Gregory et al., 1980; Kochert et al., 1996). Peanut culture was subsequently dispersed to many areas of South America, Mexico, and the Caribbean islands. The crop was unknown to the Europeans when they arrived in the New World early in the 16th century. The value of peanuts was quickly recognized, however, and the crop was later distributed to Africa and Asia by the Portuguese and Spanish. Peanuts are now widely grown in both the New and the Old World. China and India are the largest producers. The genus Arachis contains about 70 taxa, all of which are native to South America (Krapovickas and Gregory, 1994). Nearly all Arachis species are diploid, but cultivated peanut is an allotetraploid. Evidence from molecular marker studies of genetic variation suggests that