Location: Southern Horticultural Research2013 Annual Report
1a. Objectives (from AD-416):
The objective of this cooperative research project is to enhance crape myrtle breeding for the southern U.S. by expanding the use of molecular genetic tools in ARS and Cooperator’s breeding programs, identify the genetic basis for host plant insect resistance, and expand the genetic base for future breeding by overcoming fertility barriers to wide hybridization and acquiring, genetically assessing, and evaluating new germplasm not currently available in the United States.
1b. Approach (from AD-416):
Conventional breeding programs for crape myrtle are well-established at the Texas AgriLife Research and Extension Center at Dallas and Thad Cochran USDA-ARS Southern Horticultural Laboratory. Simple Sequence Repeats (SSRs) have been developed by ARS but are not widely disseminated. ARS and Cooperator intend to maximize the usefulness of this and other molecular technology by using it to assess genetic diversity among existing cultivars and germplasm, assess genetic diversity among new and jointly-collected germplasm, determine the genetic basis for insect pest resistance, verify intra- and interspecific hybrids, and overcome barriers to wide hybridization. SSR markers from ARS will be used in collaboration with AgriLife-generated mapping populations to produce the first genetic linkage map for crape meyrtle using a F1 pseudo-testcross strategy. Amplied fragment length polymorphism (AFLP) or DNA amplification fingerprinting (DAF) markers will be developed by ARS as needed to complete the map. Additional populations will be created at the Texas AgriLife Research and Extension Center at Dallas as necessary to complete the map. Genetic resistance to insect pests including, but not limited to metallic flea beetle, Japanese beetle and soft scale will be explored by screening parent stocks using field and laboratory feeding trials. Inheritance of resistance will be evaluated in progeny from two or more generations of controlled crosses between resistant and susceptible plants. Molecular markers will be used to verify inheritance and search for genes linked to resistance, which will be appended to the linkage map if/when discovered. DNA fingerprints for cultivar identification and intraspecific hybrid verification will be published using existing SSR markers, existing germplasm, and established SSR analyses including heterozygosity, genetic distance, and cluster analysis. If it is possible to jointly collect new germplasm, it will be assessed in the same manner and appended to previously published studies. Both parties are expected to share horticultural expertise and cross-evaluate previously-selected material being prepared for release. Cooperator’s expertise in Lagerstroemia ovule culture (i.e. embryo rescue) will be used in an attempt to restore fertility in ASR-generated intraspecific hybridizations. Both parties will share expertise, germplasm, and molecular biotechnology, but neither party will participate in the selection of material from each other’s breeding programs, nor will the studies outlined in this approach directly generate material for release.
3. Progress Report:
More than 100 molecular markers were developed to genotype and DNA fingerprint crape myrtle cultivars. Results were published previously and used to start a linkage mapping experiment with the segregating population between two crape myrtle cultivars, ‘Carolina Beauty’ and ‘Natchez’. In the last year ARS scientists grew out more than 6,000 intra- and interspecific hybrids from the breeding program. These plants are maintained at the greenhouse in pots and are ready for advanced selection and use in the genome mapping experiment. ARS scientists also applied mutagen treatments on major crape myrtle cultivars and the resulting seedlings are under evaluation for unique flower traits. ARS scientists also applied colchicine to double chromosomes in order to restore fertility to a hybrid between L. india and L. speciosa that is male sterile. Finally, in FY 2013 we obtained 2.2 gigabases of transcriptome sequences using next generation sequencing from ‘Carolina Beauty’ and ‘Natchez’ cultivars. At least 475 candidate genes that were found, were differentially expressed in ‘Natchez’, which is resistant to powdery mildew.