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S H Hulbert

Publications and source records attributed to S H Hulbert.

8 recordsLinked to original sources
PubMed1995

Mu1-related transposable elements of maize preferentially insert into low copy number DNA.

The Mutator transposable element system of maize was originally identified through its induction of mutations at an exceptionally high frequency and at a wide variety of loci. The Mu1 subfamily of transposable elements within this system are responsible for the majority of Mutator-induced mutations. Mu 1-related elements were isolated from active Mutator plants and their flanking DNA was characterized. Sequence analyses revealed perfect nine base target duplications directly flanking the insert for 13 of the 14 elements studied. Hybridizational studies indicated that Mu1-like elements insert primarily into regions of the maize genome that are of low copy number. This preferential selection of low copy number DNA as targets for Mu element insertion was not directed by any specific secondary structure(s) that could be detected in this study, but the 9-bp target duplications exhibited a discernibly higher than random match with the consensus sequence 5'-G-T-T-G-G/C-A-G-G/A-G-3'.

Chromosome Mapping↗
PubMed1995

New rust resistance specificities associated with recombination in the Rp1 complex in maize.

We address the question of whether genetic reassortment events, including unequal crossing over and gene conversion, at the Rp1 complex are capable of generating novel resistance specificities that were not present in the parents. Some 176 events involving genetic reassortment within the Rp1 complex were screened for novel resistance specificities with a set of 11 different rust biotypes. Most (150/176) of the events were susceptible to all tested rust biotypes, providing no evidence for new specificities. Eleven events selected as double-resistant recombinants, when screened with the 11 test biotypes, showed the combined resistance of the two parental types consistent with a simple recombination and pyramiding of the parental resistances. Nine events selected either as having partial resistance or complete susceptibility to a single biotype possessed resistance to a subset of the biotypes that the parents were resistant to, suggesting segregation of resistance genes present in the parental Rp1 complex. Four events gave rise to novel specificities being resistant to at least one rust biotype to which both parents were susceptible. All four had flanking marker exchange, demonstrating that crossing over within the Rp1 complex is associated with the appearance of new rust resistance specificities.

Basidiomycota↗
PubMed1993

Complex duplications in maize lines.

Rp1 is a disease resistance complex and is the terminal morphological marker on the short arm of maize chromosome 10. Several restriction fragment length polymorphisms (RFLPs), which map within 5 map units of Rp1, were examined to determine if they are also complex in structure. Two RFLP loci, which mapped distally to Rp1, BNL3.04 and PIO200075, existed in a single copy in all maize lines examined. These two loci cosegregated perfectly in 130 test cross progeny. Two RFLP loci that map proximally to Rp1 had unusual structures, which have not yet been reported for maize RFLPs; the loci were complex, with variable numbers of copies in different maize lines. One of the loci, NP1285, occasionally recombined in meiosis to yield changes in the number of copies of sequences homologous to the probe. The other proximal locus, detected by the probes NPI422, KSU3, and KSU4, was relatively stable in meiosis and no changes in the number of restriction fragments were observed. The similarity in map position between Rp1 and the complex RFLP loci indicate there may be genomic areas where variable numbers of repeated sequences are common. The structure of these complex loci may provide insight into the structure and evolution of Rp1.

Chromosome Mapping↗
PubMed1993

Unequal exchange and meiotic instability of disease-resistance genes in the Rp1 region of maize.

The Rp1 region of maize was originally characterized as a complex locus which conditions resistance to the fungus Puccinia sorghi, the causal organism in the common rust disease. Some alleles of Rp1 are meiotically unstable, but the mechanism of instability is not known. We have studied the role of recombination in meiotic instability in maize lines homozygous for either Rp1-J or Rp1-G. Test cross progenies derived from a line that was homozygous for Rp1-J, but heterozygous at flanking markers, were screened for susceptible individuals. Five susceptible individuals were derived from 9772 progeny. All five had nonparental combinations of flanking markers; three had one combination of recombinant flanking markers while the other two had the opposite pair. In an identical study with Rp1-G, 20 susceptible seedlings were detected out of 5874 test cross progeny. Nineteen of these were associated with flanking marker exchange, 11 and 8 of each recombinant marker combination. Our results indicate that unequal exchange is the primary mechanism of meiotic instability of Rp1-J and Rp1-G.

Basidiomycota↗
PubMed1991

Recombination at the Rp1 locus of maize.

The Rp1 locus of maize determines resistance to races of the maize rust fungus (Puccinia sorghi). Restriction fragment length polymorphism markers that closely flank Rp1 were mapped and used to study the genetic fine structure and role of recombination in the instability of this locus. Susceptible progeny, lacking the resistance of either parent, were obtained from test cross progeny of several Rp1 heterozygotes. These susceptible progeny usually had non-parental genotypes at flanking marker loci, thereby verifying their recombinational origin. Seven of eight Rp1 alleles (or genes) studied were clustered within about 0.2 map units of each other. Rp1G, however, mapped from 1-3 map units distal to other Rp1 alleles. Rp5 also mapped distally to most Rp1 alleles. Other aspects of recombination at Rp1 suggested that some alleles carry duplicated sequences, that mispairing can occur, and that unequal crossing-over may be a common phenomenon in this region; susceptible progeny from an Rp1A homozygote had recombinant flanking marker genotypes, and susceptible progeny from an Rp1D/Rp1F heterozygote showed both possible nonparental flanking marker genotypes.

Alleles↗
PubMed1990

Genetic mapping and characterization of sorghum and related crops by means of maize DNA probes.

Cloned DNA fragments from 14 characterized maize genes and 91 random fragments used for genetic mapping in maize were tested for their ability to hybridize and detect restriction fragment length polymorphisms in sorghum and other related crop species. Most DNA fragments tested hybridized strongly to DNA from sorghum, foxtail millet, Johnsongrass, and sugarcane. Hybridization to pearl millet DNA was generally weaker, and only a few probes hybridized to barley DNA under the conditions used. Patterns of hybridization of low-copy sequences to maize and sorghum DNA indicated that the two genomes are very similar. Most probes detected two loci in maize; these usually detected two loci in sorghum. Probes that detected one locus in maize generally detected a single locus in sorghum. However, maize repetitive DNA sequences present on some of the genomic clones did not hybridize to sorghum DNA. Most of the probes tested detected polymorphisms among a group of seven diverse sorghum lines tested; over one-third of the probes detected polymorphism in a single F2 population from two of these lines. Cosegregation analysis of 55 F2 individuals enabled several linkage groups to be constructed and compared with the linkage relationships of the same loci in maize. The linkage relationships of the polymorphic loci in the two species were usually conserved, but several rearrangements were detected.

Blotting, Southern↗
PubMed1988

Genetic analysis of the fungus, Bremia lactucae, using restriction fragment length polymorphisms.

Restriction fragment length polymorphisms (RFLPs) were developed as genetic markers for Bremia lactucae, the biotrophic Oomycete fungus which causes lettuce downy mildew. By using 55 genomic and cDNA probes, a total of 61 RFLP loci were identified among three heterothallic isolates of B. lactucae. Of these 61 RFLP loci, 53 were heterozygous in at least one of the three strains and thus were informative for linkage analysis in at least one of two F1 crosses that were performed. Analysis of the cosegregation of these 53 RFLPs, eight avirulence loci and the mating type locus allowed the construction of a preliminary genetic linkage map consisting of 13 small linkage groups. Based on the extent of linkage detected among probes, the genome of B. lactucae can be estimated to be approximately 2000 cM. Linkage was detected between a RFLP locus and an avirulence gene, providing a potential starting point for chromosome walking to clone an avirulence gene. The high frequency of DNA polymorphism in naturally occurring isolates and the proper Mendelian segregation of loci detected by low copy number probes indicates that it will be possible to construct a detailed genetic map of B. lactucae using RFLPs as markers. The method of analysis employed here should be applicable to many other outbreeding, heterozygous species for which defined inbred lines are not available.

Blotting, Southern↗