Stability of deletion,insertion and point mutations at the bronze locus in maize

Summary Phenotypic revertants from several kinds of mutations, including deletions, have been detected by pollen analysis at the wx and Adh loci in maize. Mutations in these genes give phenotypic revertants with median frequencies of 0.7 and 0.5×10^–5, respectively. However, the nature of such revertants can only be analyzed following their recovery from conventional matings. In the current study large seed populations derived from crosses involving several bz (bronze) locus mutations in maize were examined for reversion to a Bz (purple) expression. Deletion, insertion and point mutations were included in the study. Principally, over 2 million gametes of the bz-R mutation, which is shown here to be associated with a 340 base pair deletion within the transcribed region of the gene, have been screened for reversion. No revertants from it or any of the other bz mutations have been recovered, even though a total of almost 5 million gametes from homoallelic crosses have been examined to date. Results from seed analysis are discussed in reference to those from pollen analysis in maize.     

Genetic and molecular analysis of the Spm-dependent a-m2 alleles of the maize a locus

The Suppressor-mutator (Spm) transposable element family of maize consists of the fully functional standard Spm (Spm-s) and many mutant elements. Insertion of an Spm element in or near a gene can markedly alter its expression, in some cases bringing the gene under the control of the mechanisms that regulate expression of the element. To gain insight into such mechanisms, as well as to enlarge our understanding of the Spm element's genetic organization, we have analyzed derivatives of a unique Spm insertion at the maize a locus in which the gene is co-expressed and co-regulated with the element. We describe the genetic properties and the structure of the a locus and Spm element in 9 strains (collectively designated the a-m2 alleles) selected by McClintock from the original a-m2 allele for heritable changes affecting either the Spm element or expression of the a gene. Most of the mutations are intra-element deletions within the 8.3-kb Spm element; many alter both Spm function and expression of the gene. Spm controls a gene expression in alleles with internally deleted, transposition-defective Spm elements and element ends contain the target sequences that mediate Spm's ability to activate expression of the gene. We argue that the properties of the a-m2 alleles reflect the operation of an element-encoded positive regulatory mechanism, as well as a negative regulatory mechanism that affects expression of the element, but appears not to be mediated by an element-encoded gene product.     

Identification of genetic factors for alachlor tolerance in maize by molecular markers analysis

Genetic factors controlling tolerance to the herbicide Alachlor in maize were localised by means of two different strategies. In the first approach, backcross (BC) plants, derived from pollen which had been subjected to selective pressure for resistance to the herbicide, were analysed for segregation distortion at 47 RFLP loci and compared to BC plants obtained from non-selected pollen. Preferential transmission of five chromosomal regions where putative QTLs (Quantitative Trait Loci) are localised was revealed in the BC plants from selected pollen. A second approach was based on a classical linkage analysis for segregation of the same set of RFLPs and factors controlling the trait, in a BC population of 210 individuals, by means of regression analysis. This study detected seven significant loci in four genomic regions. Overall, two loci revealed both segregation distortion and association with the expression of the trait, indicating linkage to genes expressed in both gametophytic and sporophytic phase. Three chromosomal regions appeared to carry factors involved in plant tolerance to Alachlor which are not expressed in pollen. Conversely, three loci were linked to factors selectable in pollen, but did not reveal significant association with tolerance in the plant in the segregating populations.     

细菌素编码基因的定位分析

细菌素生物合成相关的基因经常成簇出现:结构基因、对自身产生免疫的基因及产生辅助蛋白质的基因组成操纵子结构,其中结构基因是细菌素编码基因,它可能在质粒上也可能在染色体上,为了初步定位细菌素编码基因是在质粒上还是染色体上,综述细菌素编码基因的初步定位方法,为深入研究细菌素提供依据。     

A genetic and molecular analysis of P-induced mutations at the glucose-6-phosphate dehydrogenase locus in Drosophila melanogaster

Summary We examined P factor induced mutations of the Zw gene of Drosophila melanogaster in order to learn more about the site specificity of such mutations. Approximately 70000 chromosomes were screened using a powerful positive selection scheme. As only two mutants were discovered, Zw is a cold spot for transposable element insertion. One mutation involved a complex P element associated chromosomal rearrangement which was used to define the orientation of the gene with respect to the centromere of the X chromosome. The second mutation was either a simple, non-dysgenically induced point mutation or a very unstable insertion.     

Genetic and molecular characterization of the maize rp3 rust resistance locus

In maize, the Rp3 gene confers resistance to common rust caused by Puccinia sorghi. Flanking marker analysis of rust-susceptible rp3 variants suggested that most of them arose via unequal crossing over, indicating that rp3 is a complex locus like rp1. The PIC13 probe identifies a nucleotide binding site-leucine-rich repeat (NBS-LRR) gene family that maps to the complex. Rp3 variants show losses of PIC13 family members relative to the resistant parents when probed with PIC13, indicating that the Rp3 gene is a member of this family. Gel blots and sequence analysis suggest that at least 9 family members are at the locus in most Rp3-carrying lines and that at least 5 of these are transcribed in the Rp3-A haplotype. The coding regions of 14 family members, isolated from three different Rp3-carrying haplotypes, had DNA sequence identities from 93 to 99%. Partial sequencing of clones of a BAC contig spanning the rp3 locus in the maize inbred line B73 identified five different PIC13 paralogues in a region of approximately 140 kb.     

Analysis of genetic recombination in maize populations using molecular markers

Summary Variation in recombination rate is important to plant breeders since a major objective is to obtain favorable recombinants of linked genes. The ability to increase recombination (R) in circumstances in which favorable and unvavorable genes are linked (Corn Belt x exotic populations) and to decrease recombination when many favorable genes are linked (narrow-based, elite populations) would be of immense value. However, the concept of variation in recombination frequencies between linked genes has received limited attention despite its implications in breeding and genetic linkage studies. Molecular techniques have allowed better estimations of this variation. In this study, attempts were made to characterize: (1) the R values in the Pgm1-Adh1 and Adh1-Phi1 adjacent regions of chromosome 1 and the Idh2-Mdh2 region of chromosome 6 in F₂ families of three maize (Zea mays L.) populations; (2) the environmental effect on R values of F₂s from two populations. One population, NSO, was a Corn Belt synthetic, and the other two populations, CBMEX3 and CBCAR5, were composites from crosses between Corn Belt and exotic germ-plams.Wide ranges of estimated recombination ( ) values were observed among families in each population for all three chromsomal regions. The distribution of values for the Pgm1-Adh1 region showed that the F₂ families of each population fell into two broad categories: 0.30–0.50 and 0.02–0.20. No intermediates (0.21–0.29) were found. The distributions were almost normal for the Adh1-Phi1 and the Idh2-Mdh2 regions. It would appear that the major dispersion in the Pgm1-Adh1 region was controlled by the effects of a single gene, while the Adh1-Phi1 and Idh2-Mdh2 regions were only affected by polygenes. No correlation was found between recombination values of the two adjacent regions, indicating that the genes affecting recombination for the Pgm1-Adh1 region may be specific for that region.For the Pgm1-Adh1 region, no differences in values were found among the three populations. For the Adh1-Phi1 region, frequencies of CBMEX3 and NSO were not significantly different, but both had significantly greater values than CBCAR5. For the Idh2-Mdh2 region, CBMEX3 was significantly different from NSO. There were significant differences between some paired F₂ families within each population for each chromosome region.No significant differences in response to the two environments were detected in CBMEX3 and NSO for either region in chromosome 1.Published as Journal Paper No. 9498 of the Nebraska Agric Res Div, University of Nebraska, Lincoln, Neb. Research supported in part by USDA Competitive Grant 87-CRCR-2359     

A genetic and developmental analysis of mutations in the Deformed locus in Drosophila melanogaster

Individuals expressing recessive mutations in the Deformed (Dfd) locus of Drosophila melanogaster were examined for embryonic and adult defects. Mutant embryos were examined in both scanning electron microscope and light microscope preparations. The adult Dfd recessive mutant phenotype was assessed in somatic clones and in survivors homozygous for hypomorphic alleles of the gene. The time of Dfd+ action was determined by studying a temperature conditional allele. Dfd+ is required in three embryonic cephalic segments to form a normal head. Mutant embryos of Dfd display defects in derivatives of the maxillary segment, of the mandibular segment, and of some more anterior segments. In the adult fly, defects are seen in the posterior aspect of the head when the gene is mutant. A transformation from head to thoracic-like tissue is seen dorsally and a deletion of structures is seen ventrally. Shift studies utilizing a temperature conditional allele have shown that the gene product is necessary during at least two periods of development, during embryonic segmentation and head involution and during the late larval and pupal stages. From these studies we conclude that Dfd is a homeotic gene necessary for proper specification of both the embryonic and the adult head.     

Characterization of the molecular defect in a null allele of the opaque-2 locus of maize

The molecular defect in an opaque-2 (o2) mutant, previously characterized as a null allele, has been identified as containing an insertion of the transposable element of the Bergamo (Bg) family. Restriction mapping and partial sequence analysis of the Bg in the o2 null allele indicates that this element is distinct from the previously described Bg as well as the defective Bg (rbg) of the o2m(r) allele. It is, however, inserted at the same site in O2 as the rbg of 62m(r) and can transpose when Bg is present. This study shows that, depending on genetic background, this allele may not behave as a stable null which could dramatically influence the conclusions drawn from experiments based on this particular mutant.     

Quantitative trait locus analysis of fatty acid concentrations in maize.

A study was conducted to determine the number and chromosomal location of quantitative trait loci (QTL) influencing the concentration of five fatty acids in 200 F2S1 lines derived from an Illinois High Oil (IHO) by Illinois Low Oil (Early Maturity) (ILO(EM)) cross. Restriction fragment length polymorphism (RFLP) analysis was performed on the 200 S1 lines and concentrations of palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), and linolenic (18:3) acids were determined in self-pollinated kernels harvested from plants grown in replicated field trials during 1992 and 1993. A series of 74 cDNA and genomic clones were used and these revealed 80 polymorphic loci spaced, on average, 24 cM apart throughout the maize genome. Analysis of variance detected significant (p < 0.05) associations between several RFLP loci and the concentration of each fatty acid. A total of 15 RFLP loci clustered in 12 chromosomal regions were associated with the concentration of 16:0, 17 loci clustered in 10 regions were associated with the concentration of 18:0, 12 loci clustered in eight regions were associated with the concentration of 18:1 and 18:2, and 17 loci clustered in eight regions were associated with the concentration of 18:3. Multiple linear regression models consisting of four RFLP loci explained 24 and 62% of the total phenotypic and genotypic variation (R2) among the 200 F2S1 lines for 16:0, five loci explained 51 and 71% of the variation for 18:0, three loci explained 67 and 79% of the variation for 18:1, two loci explained 67 and 81% of the variation for 18:2, and seven loci explained 52 and 78% of the variation for 18:3 in these 200 F2S1 lines. The ratio of 18:1 to 18:2 was tightly interrelated as the same QTL were associated with the concentrations of 18:1 and 18:2. A quantitative trait locus that explained 63% of the phenotypic variation in the ratio of 18:1 to 18:2 is tightly linked to umc65 on chromosome 6 in the region of the linoleic acid1 locus.     

Structural and genetic analysis of the bvg locus in Bordetella species

The bvg locus contains two genes, bvgA and bvgS, which control the expression of the virulence-associated genes in Bordetella species by a system similar to the two-component systems used by a variety of bacterial species to respond to environmental stimuli. We determined the nucleotide sequence of the bvg loci of Bordetella parapertussis and Bordetella bronchiseptica and compared them with the previously determined sequence of Bordetella pertussis. The nucleotide and amino acid sequences of the bvg loci of these species are well conserved in those regions coding for the protein domains which have putative kinase and DNA-binding activities. In marked contrast, the region of BvgS that codes for the protein domain with putative sensor activity shows a high degree of variability. In total, we find 198 base-pair changes in the bvg loci of B. parapertussis and B. bronchiseptica relative to the bvg locus of B. pertussis. One hundred and seventy-three of these base-pair changes are identical in B. parapertussis and B. bronchiseptica. This confirms our previous observation that B. parapertussis and B. bronchiseptica are more related to each other than to B. pertussis. We have mapped the mutations that cause phase changes in B. bronchiseptica and we have found that in three cases these are due to spontaneous deletions in the bvgS gene. The wild-type bvg locus present on a multicopy plasmid cannot complement avirulent derivatives of B. bronchiseptica to wild-type levels, but it can do so when the bvgA gene on the plasmid is inactivated. This suggests that hyperexpression of bvgA down-regulates the bvg system.     

The genetic analysis of the flp locus of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae, one of the most important porcine respiratory pathogens, exhibits tight adherence to cell surfaces. The Flp pilus, which is assembled by the proteins encoded by the flp (fimbrial low-molecular-weight protein) operon, may play an important role in the bacterial adherence. In this study, the flp operons of twelve A. pleuropneumoniae serotype reference strains were sequenced and analyzed. The phenotypic diversity of fimbriae was observed using transmission electron microscopy, and the adherence ability was tested against a porcine lung epithelial cell line. The complete flp operon was identified in the reference strains of serotypes 1, 4, 5, 7, 12, and 13, consisting of 14 genes (flp1-flp2-tadV-rcpCAB-tadZABCDEFG). Fimbriae were observed protruding from the bacterial cell surfaces of these strains. In contrast, the flp promoter was absent in serotypes 2, 3, 6, 9, and 11, and the flp1 gene was truncated in serotypes 10 and 15. No pilus was observed on the surfaces of these strains. The piliated strains have higher efficiency of adhesion than the pilus-negative strains. Our data demonstrated that the Flp pili are involved in A. pleuropneumoniae adherence. The genetic diversity of the flp operons among different strains may contribute, at least in part, to the variation in virulence of Actinobacillus pleuropneumoniae.