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The success of a breeding program depends on having adequate diversity in the germplasm. However, as advanced breeding stocks and materials are generated, one casualty is the diversity itself. As a result, breeding programs in many crop species have reached a point of diminishing returns and it is feared that unless new diversity is infused into the breeding germplasm, we face catastrophic reductions in crop productivity if the climate turns adverse. Although some scientists favor transgenic approaches, a 'back to nature' approach to genetic diversity may prove faster and more effective. Wild and exotic relatives of crop plants hold a wealth of alleles that, if we can find them, can help break yield barriers and enhance tolerance to stresses. Many approaches, based largely on quantitative trait loci genetics, have been proposed and used for this purpose, but most are either highly laborious or discover relevant variation inefficiently. Here, we propose a gene-centered approach, dubbed MAGIC (mutant-assisted gene identification and characterization), that uses Mendelian mutants or other genetic variants in a trait of interest as reporters to identify novel genes and variants for that trait. MAGIC is similar to enhancer--suppressor screens, but rather than relying on variation created in the laboratory, it reveals variation created and refined by nature over millions of years of evolution. This approach could be an effective tool for exploring novel variation and a valuable means to harness natural diversity and define genetic networks.

Point mutations are important tools for understanding gene functions and genetic interactions, as well as for identifying neomorphs. The Maize Targeting Induced Local Lesions IN Genomes (TILLING) Project has been established to provide reverse genetics resources that can screen ethyl methonyl sulfonate (EMS)--mutagenized populations of maize (Zea mays L.) for individuals carrying point mutations in virtually any gene in the genome. In addition, a variation on TILLING, EcoTILLING, con be used on Maize Diversity Lines to gouge how much genetic diversity is present in maize gerrnplasm at any given locus. The maize populations developed for TILLING, in the B73 and W22 inbred lines, also serve as an excellent and publicly available forward genetics resource. Abbreviations: EMS, ethyl methonyl sulfonate; TILLING, Maize Targeting Induced Local Lesions IN Genomes.

Transposon footprints are rearrangements of DNA sequences caused by the flanking of mobile elements. An analysis of a corn gene responsible for expressing waxiness revealed that the development of transposon footprints followed a definite arrangement within the sites where they excise. It is suggested that the definitive arrangement of transposon footprints result from the flanking sequences of transposons during DNA repair processes.

The maize waxy (Wx)-m5 allele was isolated and characterized. In the absence of an Ac insertion, the Wx-m5 allele is phenotypically wild-type, although the level of enzyme activity is reduced relative to the wild-type. Sequence analysis showed that Wx-m5 contains either one or two Ds inserts, with the insertion at -470 relative to the transcriptional start site constituting a part of the Wx gene. In the presence of Ac, Wx-m5 exhibits mutant wx sectors and gives rise to wx-m and wx derivatives. The clonal sectors of Wx-m5 represents transposon insertion with the concommitant loss of gene expression.

The Tc1/mariner transposable element superfamily is widely distributed in animal and plant genomes. However, no active plant element has been previously identified. Nearly identical copies of a rice (Oryza sativa) Tc1/mariner element called Osmar5 in the genome suggested potential activity. Previous studies revealed that Osmar5 encoded a protein that bound specifically to its own ends. In this report, we show that Osmar5 is an active transposable element by demonstrating that expression of its coding sequence in yeast promotes the excision of a nonautonomous Osmar5 element located in a reporter construct. Element excision produces transposon footprints, whereas element reinsertion occurs at TA dinucleotides that were either tightly linked or unlinked to the excision site. Several site-directed mutations in the transposase abolished activity, whereas mutations in the transposase binding site prevented transposition of the nonautonomous element from the reporter construct. This report of an active plant Tc1/mariner in yeast will provide a foundation for future comparative analyses of animal and plant elements in addition to making a new wide host range transposable element available for plant gene tagging. [ABSTRACT FROM AUTHOR]