Sequences associated with A chromosome centromeres are present throughout the maize B chromosome

Maize chromosome spreads containing the supernumerary B chromosome were hybridized with probes from various repetitive elements including CentC, CRM, and CentA, which have been localized to centromeric regions on the A chromosomes. Repetitive elements that are enriched or found exclusively near the centromeres of A chromosomes hybridized to many sites distinct from the centromere on the B chromosome. To examine whether these elements recruit kinetochore proteins at locations other than the canonical B centromere, cells were labeled with antibodies against CENH3, a key kinetochore protein. No labeling was detected outside the normal centromere and no evidence of B chromosome holocentromeric activity was observed. This finding suggests that, as in other higher eukaryotes, DNA sequence alone is insufficient to dictate kinetochore location in plants. Additionally, examination of the B centromere region in pachytene chromosomes revealed that the B-specific element ZmBs hybridizes to a much larger region than the site of hybridization of CentC, CRM, and CentA and the labeling by anti-CENH3 antibodies.This revised version was published online in December 2004 with corrections to Table 1.     

Dosage balance in gene regulation: biological implications

Classical studies in genetics involving aneuploidy and ploidy comparisons and sex-determination mechanisms indicated a balance phenomenon such that changes of individual chromosomal dosage altered the phenotype more dramatically than changes in ploidy. Recent evidence suggests that a major contributor to this balance is the behavior of molecular complexes that function in various regulatory processes affecting gene expression. In this article, we discuss the potential contribution of regulatory balance to the control of quantitative traits, hybrid vigor, genome evolution and post-zygotic speciation mechanisms.     

A genetic test of bioactive gibberellins as regulators of heterosis in maize

This study tested the hypothesis that gibberellin levels were responsible for the superior growth habit of hybrids (i.e., heterosis). If this were true, plants reduced in their capacity to produce gibberellin, such as maize plants homozygous for dwarf1 (d1), should display a lesser heterotic response. The d1 mutation was introgressed into two inbred lines of maize, B73 and Mo17, for seven generations. Plants segregating for the dwarf phenotype were produced both by self-fertilizing the introgressed inbred lines and by making reciprocal crosses between them to produce hybrids. Measurements were made of several physical traits. The results indicated that the hybrid dwarf plants experienced no loss of heterosis relative to their normal siblings. These results exclude the possibility that modulation of bioactive gibberellins is a major underlying basis of the heterotic response.     

Thesaurus-based disambiguation of gene symbols

Background  

Massive text mining of the biological literature holds great promise of relating disparate information and discovering new knowledge. However, disambiguation of gene symbols is a major bottleneck.     

Functional studies of signaling pathways in peri-implantation development of the mouse embryo by RNAi

Background

Studies of gene function in the mouse have relied mainly on gene targeting via homologous recombination. However, this approach is difficult to apply in specific windows of time, and to simultaneously knock-down multiple genes. Here we report an efficient method for dsRNA-mediated gene silencing in late cleavage-stage mouse embryos that permits examination of phenotypes at post-implantation stages.

Results

We show that introduction of Bmp4 dsRNA into intact blastocysts by electroporation recapitulates the genetic Bmp4 null phenotype at gastrulation. It also reveals a novel role for Bmp4 in the regulation the anterior visceral endoderm specific gene expression and its positioning. We also show that RNAi can be used to simultaneously target several genes. When applied to the three murine isoforms of Dishevelled, it leads to earlier defects than previously observed in double knock-outs. These include severe delays in post-implantation development and defects in the anterior midline and neural folds at headfold stages.

Conclusion

Our results indicate that the BMP4 signalling pathway contributes to the development of the anterior visceral endoderm, and reveal an early functional redundancy between the products of the murine Dishevelled genes. The proposed approach constitutes a powerful tool to screen the functions of genes that govern the development of the mouse embryo.     

Sporophytic nondisjunction of the maize B chromosome at high copy numbers

It has been known for decades that the maize B chromosome undergoes nondisjunction at the second pollen mitosis.Fluorescence in-situ hybridization (FISH) was used to undertake a quantitative study of maize plants with differing numbers of B chromosomes to observe if instability increases by increasing B dosage in root tip tissue.B chromosome nondisjunction was basically absent at low copy number,but increased at higher B numbers.Thus,B nondisjunction rates are dependent on the dosage of B's in the sporophyt...     

Interactions among modifiers of retrotransposon-induced alleles of the white locus of Drosophila melanogaster

Mutations in five loci that modify the phenotype of whiteapricot (wa), caused by the retrotransposon, copia, were examined in two-way combinations to determine whether their effects were additive or epistatic. All two-way combinations of mutations in these five loci, mottler of white (mw), suppressor of forked (su(f], suppressor of white apricot (su(wa], Enhancer of whiteapricot, (E(wa] and Darkener of apricot (Doa), are additive in their effects on wa, implying that each second-site modifier locus affects a different process. Three other copia-induced mutations, HwUa, whd81b25 and ctns were also examined for responsiveness to mutations in these modifier loci. None clearly responded. Mutations associated with B104 insertions, including Gl, vgni, ctn and wric were also examined for responsiveness to mw mutations, which have specificity for this element as well. Both vgni and wric respond to mutations in mw. The former interaction demonstrates that mw is capable of interacting with B104 elements in loci other than white. The significance of the results with respect to the nature of second-site modifier loci is discussed.     

Engineered Minichromosomes in Plants

Engineered minichromosomes provide the ability to target transgenes to a defined insertion position for predictable expression on an independent chromosome. This technology promises to provide a means to add many genes to a synthetic chromosome in sequential manner. An additional advantage is that the multiple transgenes will not be inserted into the normal chromosomes and thus will not exhibit linkage drag when converging the transgenes to different germplasm nor will they be mutagenic. Telomere truncation coupled with the introduction of site-specific recombination cassettes has proven to be an easy method to produce minichromosomes. Telomere truncation results from the transformation of plasmids carrying a block of telomere repeats at one end. Minichromosomes consisting of little more than a centromere have been produced for B chromosomes of maize. Such small chromosomes have been studied for their meiotic behavior, which differs from normal sized chromosomes in that homologue pairing is rare or nonexistent and sister chromatid cohesion fails at meiosis I. Potential modifications of the minichromosomes that can address these issues are discussed. Minichromosomes can be recovered from transformed plants that are polyploid or that carry an additional chromosome as the preferred target for truncation. Site-specific recombination has been demonstrated to operate on these terminally located sites. By introducing normal B chromosomes into lines with engineered mini-B chromosomes, the latter can be increased in copy number, which provides the potential to augment the expression of the introduced genes. Because the vast majority of plant species have the same telomere sequence, the truncating transgenes should be effective in most plants to generate engineered minichromosomes. Such chromosomes establish the means to add or subtract multiple transgenes, multigene complexes, or whole biochemical pathways to plants to change their properties for agronomic applications or to use plants as factories for the production of foreign proteins or metabolites.