Project Leader and Principal UC Investigators
David J. Mackill, research geneticist, USDA-ARS
|Scientists working on this project are employing advanced
techniques in molecular genetics to introduce important agronomic traits into California
rice. Their work currently centers on three main areas:
Maintaining and evaluating a diverse set of rice varieties and wild species, importing new germplasm and introducing useful traits.
Identification of useful genes with DNA markers to show the location of genes associated with seedling vigor, submergence tolerance, stem rot resistance, cold tolerance and water weevil tolerance.
Development of the genetic mechanisms necessary for commercial hybrid rice production.
Thirty-seven new crosses, 10 with tropical indica types, were made from exotic lines to introduce genetic variability into California rice. An additional 10 crosses were made with exotic japonica cultivars. During 1994 geneticists began making backcrosses of a first generation cross between M-202 and a wild species, Oryza nivara. Additional backcrosses are being made so that scientists can study the effect that genes from this wild species have on agronomic characteristics important to California rice.
Geneticists completed a survey of U.S. rice cultivars with an advanced molecular technique utilizing RAPD (random amplified polymorphic DNA). The results of survey and a subsequent study showed that the popular California cultivars could be differentiated by several identifiable bands produced by these markers.
Scientists reported some variability within samples obtained from breeders' nurseries and samples from seed growers but are unsure whether this variability is due to the method of DNA extraction or actual genetic differences.
A new type of genetic marker, termed AFLP (amplified fragment length polymorphism) was used to help identify differences between cultivars. The marker has shown some difficulty for DNA "fingerprinting" but appears to be useful for genetic mapping in crosses involving japonica cultivars.
Identification of Useful Genes
Seedling vigor - Markers were used to identify gene locations for this trait in a cross of Black Gora (indica, high vigor) and Labelle (japonica, low vigor). Useful I genes in Black- Gora have e been ident ified for root length.
Submergence tolerance - Sources of tolerance have been identified and transferred into improved indica lines. but the genetics are not well understood. Researchers are "tagging" genes associated with this trait to assist them in further efforts to incorporate submergence tolerance into California japonica cultivars.
Stem rot resistance - Resistant and susceptible lines were identified for four crosses. Researchers are now surveying the parents for genetic differences.
Cold tolerance - Studies of the genetics of cold tolerance were begun in a cross between M-202 and an indica cultivar. Researchers are mapping genes controlling this trait at the seedling and booting stages.
Water weevil tolerance - Based on genetic differences established with molecular markers, two crosses were made for studying the genetics of rice water weevil tolerance. These will be advanced to a later generation before analysis will begin.
Five backcrosses have been made to transfer cytoplasmic male sterility (cms) into California rice cultivars. Initial results indicate these cms lines show stable sterility. Restorer genes, which are necessary to take advantage of the cms system of seed production, are also being transferred. The wide compatibility gene, which may be useful for development of indica-japonica hybrid rice crosses, is being transferred into California medium grain cultivars with the aid of a closely linked molecular marker.
Researchers are also examining a method of hybrid seed production that relies on plants that are sterile under longer day lengths but produce self pollinated seed under shorter day length. These plants are called photoperiod sensitive genetic male sterile (PGMS) mutants. Researchers identified 800 specimens that appear to be male sterile.
The mutants were identified from populations of irradiated rice grown at the Rice Experiment Station, as well as spontaneous mutants in growers' fields of M-202 and M-204. Fertility rates ranged from close to zero to more than 60 percent. Specimens obtained from the irradiated rice showed a higher percentage of sterility. Researchers took plant samples from most of the mutants and transplanted them to the greenhouse for further observation and analysis.