Cassava {Manihot esculenta Crantz) staroh is traded in international markets more than starch from any other source. The starch industry requires cassava cultivars with novel starch characteristics for commercial exploitation. A natural source of waxy (amylose-free) cassava starch, clone AM 206-5, was identified at the International Center for Tropical Agriculture (CIAT). The granule-bound starch synthase I {GBSSl) gene is related to the waxy starch trait. The objectives of this study were to introgress the waxy starch mutation into Thailand's cassava breeding populations, to analyze the inheritance of waxy starch in cassava, and to develop molecular markers for this trait. AM 206-5 {wxwx) was crossed with several elite oultivars {WxWx) to transfer the wx alíele. Unrelated "F/' {Wxwx) genotypes were crossed among themselves to produce a "Fg" segregating population with negligible levels of inbreeding depression. The waxy starch phenotype was recovered in 25.7% ofthe 11,192 genotypes tested. This result provides further evidence that cassava is a functional diploid and that only one copy of GBSSl is present in cassava. A full-length genomic DNA sequence of GBSSl was isolated and characterized from C8, a waxy starch "f^" clone derived from AM 206-5. We exploited these sequence differences to develop two diagnostic single-nucleotideamplified polymorphism (SNAP) markers to differentiate homozygous waxy {wxwx) from the heterozygous {Wxwx) and homozygous {WxWx) nonwaxy genotypes.

The genus Malus has anywhere between 25 and 33 species along with several subspecies. Malus species as well as clones within the same species have varying ploidy levels, as these are more than likely collected from different trees and (or) from different locations. In recent years, large numbers of Malus germplasm accessions have been collected and maintained at the United States National Germplasm Clonal Repository; however, genome sizes of this material have not yet been determined. In this study, leaf tissues from young grafted trees of 100 Malus species and hybrids growing in a nursery at the University of Illinois were collected and immediately used for extracting nuclei. Leaf tissues from apple and maize line W-22, used as an internal standard, were co-chopped and prepared for flow cytometric analysis. Apple nuclei were stained with propidium iodide, an intercalating dye, and a minimum of 8000 nuclei per sample were analyzed. Mean fluorescence of apple nuclei was then determined. A total of four replications per sample was used. Among 100 Malus accessions analyzed, one tetraploid, three triploid, and 96 diploid genotypes were identified. Significant differences in genome size were identified among the three ploidy types observed and also within diploid genotypes. The 2C mean value for tetraploids was 3.13 pg and ranged from 2.27 to 2.41 pg for triploids, whereas 2C values for diploids ranged between 1.44 and 1.72 pg. In addition, leaf impressions of young, fully expanded leaves were collected from young trees of 10 selected genotypes based on their ploidy and flow cytometric analysis and used to measure the nucleotypic parameter stomatal length. Ten stomata were measured per slide, three slides were analyzed per leaf, and three leaves were analyzed per accession. Overall, mean length of stomata ranged between 19.47 microm (diploid) and 27.6 microm (tetraploid), indicating that stomatal length in a tetraploid Malus genotype was 1.4-fold higher than that of a diploid genotype. A positive correlation between genome size and the nucleotypic parameter stomatal length was observed.

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Bacillus subtilis can acquire a higher tolerance to tetracycline by increasing the gene dosage of its resistance gene tetB. In this study, we estimated the multiplication effect of tetB on tetracycline tolerance. Cells harbouring multiple copies of tetB were found to comprise approximately 30 % of the total tetracycline-resistant cell population when selected on medium containing 10 mg tetracycline ml "1. Disruption of recA resulted in a significant decrease in the frequency of tetB amplification. Although four direct repeats exist around tetB, the majority of tetB amplicons were found to be flanked by non-homologous sequences, indicating that the initial duplication of tetB can occur largely through RecA-independent recombination. The correlation between the tetB copy number and the MIC values for tetracycline indicated that more than three copies of tetB were required for tolerance to 10 mg tetracycline ml "1 . Thus, the RecA-dependent expansion step appears to be necessary for developing significant tetracycline tolerance mediated by tetB amplification.