Analysis and application of the En transposable element and genetic study of resistance to Bipolaris maydis in maize
Since its original isolation and molecular characterization, the En (Enhancer) transposable element has been studied extensively. It also has been used as a tool to tag and isolate genes and to elucidate mechanisms of transposition and gene regulation. The first aspect of this dissertation is on the analysis and application of En. Three En-mutable alleles at the maize A2 locus harboring an autonomous En element, but showing different somatic variegation patterns, were studied by PCR and sequence analysis. It is shown that a fine variegation type (late reversions) is caused by En insertion in the 5[superscript]' region, whereas two coarse variegation types (early reversions) are caused by En insertions in the 3[superscript]' region, in the coding sequence of the intronless A2 gene. In the second project, at least one En-mutable allele has been relocated to each of the 20 maize chromosome arms. Because En has been shown to transpose more frequently to closely linked sites, the relocation of En elements to chromosome arms (chromosome labeling) should facilitate the gene tagging process. Progress in transposon tagging of Rp1, a resistance gene to Puccinia sorghi, is reported in the third project. Instability of Rp1 in different transposable element-laden lines is compared;The second aspect of this dissertation is the genetic study of resistance to Bipolaris maydis. This resistance was determined to be monogenically controlled in the 1970's. However, non-monogenic types of resistance were also reported. Data are reported that support a two-gene model of resistance. Two transposable element lines, one chromosome labeling line, the T line, and a Cy (Cycler) line (both Rhm/Rhm) showed ~10[superscript]-5 mutation rates of Rhm to rhm when tested against the recessive rhm/rhm line. The hybrid of the two lines, however, produced ~5% resistant individuals when tested against the same rhm line. It is hypothesized that these two lines possessed two different dominant genes and recombination between the two genes yielded the ~5% resistant individuals. A genetic test showed results consistent with the two-gene model. The possibility of adjacent-1 segregation with the T line has not been excluded, which might result in viable gametes lacking Rhm and expose the effect of the tester rhm allele which will then produce resistant seedlings. A molecular marker exchange analysis is proposed to exclude this possibility.