|Host Plant Resistance to Insects Laboratory|
For the past forty years, the Hessian fly, a major insect pest of wheat world wide, has been controlled by the widespread use of resistant wheat cultivars. This type of host-plant resistance is achieved by identifying resistance genes in common wheat, its wild species relatives, durum wheat (pasta wheat) or rye and incorporating these resistance genes into new cultivated varieties through classical plant breeding. However, within 8-10 years of the release of a new wheat cultivar, the Hessian fly is able to overcome the resistance gene and can survive on cultivars that were formerly resistant. Thus, we must constantly continue our search for new sources of resistance genes to replace ones that are no longer effective. Since it takes roughly 8-10 years to identify a new source of resistance and incorporate it into a cultivar, the wheat crop is constantly under threat.
USDA-ARS scientists, in conjunction with scientists at Purdue, are devising deployment strategies that will lead to more durable resistance. This collaboration integrates entomologists, who identify new sources of resistance and determine which genes will be effective in various parts of the U.S., molecular biologists and geneticists, who determine the chromosomal locations of genes and their mechanisms of resistance, and plant breeders, who incorporate the genes into new resistant wheat varieties.
I. Gene Pyramiding.
One strategy for increasing the durability of resistance is to incorporate multiple resistance genes into a single wheat variety. This gene pyramiding strategy will force the Hessian fly to overcome all of the resistance genes simultaneously in order to survive on the new variety. If the pyramided genes have never been deployed as single genes, then it is estimated that the durability of resistance could be extended for 50 years. However, during the process of constructing a cultivar, it is difficult to verify the number of resistance genes that have been successfully pyramided. Plants with only one resistance gene are just as resistant as plants that contain three resistance genes, although the resistance of the latter is probably more durable. The goals of this project are to generate information that will facilitate gene pyramiding.
Goal 1) Identification of DNA markers that are coinherited with individual resistance genes. The presence of multiple markers in a single plant will indicate that multiple resistance genes are also present.
Goal 2) Determination of the chromosomal locations of resistance genes. If multiple genes are to be pyramided, it is essential to verify that genes from different sources are not actually the same gene.
Goal 3) Incorporate several new genes that confer Hessian fly resistance into single wheat cultivars. Through marker-assisted breeding, future cultivars will have important traits that previously were difficult to manipulate.
II. Subgenomic characterization.
The molecular cloning of genes conferring resistance to the Hessian fly will allow direct transfer of resistance to previously developed wheat cultivars. In addition, information gained from molecular analysis of these genes will deepen our understanding of their modes of action and will lead to new strategies for insect resistance in wheat as well as other crop plants. However, wheat is a difficult plant in which to undertake targeted gene cloning projects, due to its large genome resulting from hexaploidy and a high fraction of repetitive DNA. Rice, on the other hand, has a small diploid genome containing little repetitive DNA and subgenomic segments in which the gene order is similar to that of wheat. Molecular studies that compare wheat segments containing Hessian fly resistance genes to evolutionarily related rice segments will lead to new cloning strategies. The goals of this project are to generate information that will facilitate targeted cloning in plants with large genomes.
Goal 1) Identification of DNA markers that delineate related segments in wheat and rice genomes. Markers that are coinherited with Hessian fly resistance genes and whose chromosomal location has been identified in both wheat and rice will delineate segments of the rice genome that may contain genes related to Hessian fly resistance genes.
Goal 2) Identify many markers within rice segment to serve as first step for targeted cloning in wheat.
The P.I.'s publications from March 1995 through the present are listed. The P.I.'s previous work was on rice.
Williams, C. E., B. Wang, T. E. Holsten, J. Scambray, F. de Assis Goes da Silva and P. C. Ronald, 1996. Markers for selection of the rice Xa21 disease resistance gene. Theor. Appl. Genet. 93:1119-1122.
Williams, C. E. , S. Yanagihara, S. McCouch, D. Mackill, P. Ronald. Predicting success of wide crosses in rice, based on a PCR marker for the S-5n allele at a hybrid-sterility locus. Submitted to Crop Science 2-26-97.
Collier, C.C., H.W. Ohm, M.J. Jones amd C. E. Williams, 1997. Molecular markers to track the pyramiding of several Hessian fly resistance genes into a single line of common wheat. Plant and Animal Genome V, pp 92.