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R S Hawley

Publications and source records attributed to R S Hawley.

At least 37 records · Page 2Linked to original sources
PubMed1998

Unaltered meiotic chromosome segregation in Drosophila melanogaster raised on a 5% quercetin diet.

Flavonoid plant pigments are an integral part of the human diet. Although potentially negative mitotic effects of flavonoids have been observed in model organisms, investigation into meiotic effects of flavonoids has been neglected. As flavonoids affect cell signalling and DNA replication, and because the flavonoid content of the human food supply is being increased, determining the effects of flavonoids on meiotic fidelity is important. Here, the effect of the human food supply's most prevalent flavonoid, quercetin, on the level of meiotic recombination and the amount of X and 4th chromosome non-disjunction in Drosophila melanogaster females was determined. This model organism was chosen since Drosophila melanogaster and Homo sapiens share a remarkable number of commonalities in the meiotic processes of oogenesis and because genetic techniques allow a detailed analysis of meiotic processes in Drosophila. No significant effect on either non-disjunction levels or the percentage distribution of exchange bivalents was observed. A significant effect was observed on the number of offspring; F1 and F2 generations of flies raised on a quercetin diet produced over 10% more progeny than flies raised on a control diet. In this investigation, high quercetin consumption by Drosophila melanogaster females did not pose a threat to meiotic fidelity.

Animals↗
PubMed1997

The developmental genetics of hybrid inviability: a mitotic defect in Drosophila hybrids.

We report studies of the developmental basis of hybrid inviability in the Drosophila melanogaster complex. The pathology of these hybrids closely resembles that of mitotic mutants in D. melanogaster. We use mosaic and cytological analyses to show that hybrid male inviability is associated with, and probably caused by, a defect in mitotic cell division. In the mosaic study, we find that male clones produced in otherwise female hybrids are not cell lethal but are very small, probably reflecting defects in mitotic proliferation. Cytological inspection of larval neuroblasts reveals a profound mitotic defect in hybrids: chromosomes show a near-complete failure to condense even after 2 hr of incubation in colchicine. Both the defect in clonal proliferation and in chromatin condensation are rescued by mutations known to rescue normally inviable hybrid males. We present a simple model in which hybrid inviability is partly or entirely caused by a mitotic defect; this defect is, in turn, caused by an interaction between the Hybrid male rescue (Hmr) locus of D. melanogaster and autosomal gene(s) from D. melanogaster's sister species.

Animals↗
PubMed1996

Direct evidence of a role for heterochromatin in meiotic chromosome segregation.

We have investigated the mechanism that enables achiasmate chromosomes to segregate from each other at meiosis I in D. melanogaster oocytes. Using novel cytological methods, we asked whether nonexchange chromosomes are paired prior to disjunction. Our results show that the heterochromatin of homologous chromosomes remains associated throughout prophase until metaphase I regardless of whether they undergo exchange, suggesting that homologous recognition can lead to segregation even in the absence of chiasmata. However, partner chromosomes lacking homology do not pair prior to disjunction. Furthermore, euchromatic synapsis is not maintained throughout prophase. These observations provide a physical demonstration that homologous and heterologous achiasmate segregations occur by different mechanisms and establish a role for heterochromatin in maintaining the alignment of chromosomes during meiosis.

Animals↗
PubMed1996

Spontaneous X chromosome MI and MII nondisjunction events in Drosophila melanogaster oocytes have different recombinational histories.

Recent studies of human oocytes have demonstrated an enrichment for distal exchanges among meiosis I (MI) nondisjunction events and for proximal exchanges among meiosis II (MII) events. Our characterization of 103 cases of spontaneous X chromosome nondisjunction in Drosophila oocytes strongly parallels these observations. The recombinational histories of MI (97/103) and MII (6/103) nondisjunctional ova were strikingly different. MI nondisjunction occurred primarily in oocytes with non-exchange X chromosomes; of the new nondisjoining exchange bivalents, most carried distal crossovers. Thus, spontaneous MI nondisjunction reflects the failure of the achiasmate segregation systems. MII nondisjunction occurred only in oocytes with proximal exchanges. We propose several models to explain how very proximal exchanges might impair proper segregation.

Animals↗
PubMed1996

Cloning of the Drosophila melanogaster meiotic recombination gene mei-218: a genetic and molecular analysis of interval 15E.

The mei-218 gene product is required for both meiotic crossing over and for the production of recombination modules, suggesting that these organelles are required for meiotic exchange. In this study the null phenotype of mei-218 was defined through the analysis of three preexisting and five new alleles. Consistent with previous studies, in homozygous mei-218 mutants meiotic crossing over is reduced to < 10% of normal levels. A molecular analysis of mei-218 was initiated with the isolation and mapping of lethal mutations and genome rearrangements in the region containing mei-218, polytene interval 15E on the X chromosome. This high resolution genetic map was aligned with a physical map constructed from cosmid and P1 clones by genetically mapping restriction fragment length polymorphisms and localizing rearrangement breakpoints. Within a region of 65 kb, we have identified seven transcription units, including mei-218 and the Minute(1)15D gene, which encodes ribosomal protein S5. The mei-218 mutant phenotype has been rescued by germline transformation with both a genomic fragment and a cDNA under the control of the hsp83 promoter. The mei-218 gene is predicted to produce an 1186-amino acid protein that has no significant similarities to any known proteins.

Alleles↗
PubMed1996

Recombination and nondisjunction in humans and flies.

Recent studies of Drosophila and humans indicate that aberrant genetic recombination is an important component of nondisjunction in both species. In both, a proportion of nondisjunction is associated with failure to pair and/or recombine and in both, exchanges which are either too distal or too proximal increase the likelihood of malsegregation. In this review we provide two perspectives on these observations: first, a review of exchange and chromosome segregation in model organisms, focusing on Drosophila, and secondly an overview of nondisjunction in humans. This format allows us to describe the paradigms developed from studies of model organisms and to ask whether these paradigms apply to the human situation.

Animals↗
PubMed1995

Chromosomal control of meiotic cell division.

Chromosomes have multiple roles both in controlling the cell assembly and structure of the spindle and in determining chromosomal position on the spindle in many meiotic cells and in some types of mitotic cells. Moreover, functionally significant chromosome-microtubule interactions are not limited to the kinetochore but are also mediated by proteins localized along the arms of chromosomes. Finally, chromosomes also play a crucial role in control of the cell cycle.

Anaphase↗
PubMed1995

The mei-41 gene of D. melanogaster is a structural and functional homolog of the human ataxia telangiectasia gene.

The D. melanogaster mei-41 gene is required for DNA repair, mitotic chromosome stability, and normal levels of meiotic recombination in oocytes. Here we show that the predicted mei-41 protein is similar in sequence to the ATM (ataxia telangiectasia) protein from humans and to the yeast rad3 and Mec1p proteins. There is also extensive functional overlap between mei-41 and ATM. Like ATM-deficient cells, mei-41 cells are exquisitely sensitive to ionizing radiation and display high levels of mitotic chromosome instability. We also demonstrate that mei-41 cells, like ATM-deficient cells, fail to show an irradiation-induced delay in the entry into mitosis that is characteristic of normal cells. Thus, the mei-41 gene of Drosophila may be considered to be a functional homolog of the human ATM gene.

Animals↗
PubMed1995

Induction of metaphase arrest in Drosophila oocytes by chiasma-based kinetochore tension.

In normal Drosophila melanogaster oocytes, meiosis arrests at metaphase I and resumes after oocyte passage through the oviduct. Thus, metaphase arrest defines a control point in the meiotic cell cycle. Metaphase arrest only occurs in oocytes that have undergone at least one meiotic exchange. Here it is shown that crossovers between homologs attached to the same centromere do not induce metaphase arrest. Hence, exchanges induce metaphase arrest only when they physically conjoin two separate kinetochores. Thus, the signal that mediates metaphase arrest is not the exchange event per se but the resulting tension on homologous kinetochores.

Animals↗
PubMed1995

DNA binding and meiotic chromosomal localization of the Drosophila nod kinesin-like protein.

The Drosophila no distributive disjunction (nod) gene encodes a kinesin-like protein that has been proposed to push chromosomes toward the metaphase plate during female meiosis. We report that the nonmotor domain of the nod protein can mediate direct binding to DNA. Using an antiserum prepared against bacterially expressed nod protein, we show that during prometaphase nod protein is localized on oocyte chromosomes and is not restricted to either specific chromosomal regions or to the kinetochore. Thus, motor-based chromosome-microtubule interactions are not limited to the centromere, but extend along the chromosome arms, providing a molecular explanation for the polar ejection force.

Amino Acid Sequence↗
PubMed1995

Identification of the chromosome localization domain of the Drosophila nod kinesin-like protein.

The nod kinesin-like protein is localized along the arms of meiotic chromosomes and is required to maintain the position of achiasmate chromosomes on the developing meiotic spindle. Here we show that the localization of ectopically expressed nod protein on mitotic chromosomes precisely parallels that observed for wild-type nod protein on meiotic chromosomes. Moreover, the carboxyl-terminal half of the nod protein also binds to chromosomes when overexpressed in mitotic cells, whereas the overexpressed amino-terminal motor domain binds only to microtubules. Chromosome localization of the carboxyl-terminal domain of nod depends upon an 82-amino acid region comprised of three copies of a sequence homologous to the DNA-binding domain of HMG 14/17 proteins. These data map the two primary functional domains of the nod protein in vivo and provide a molecular explanation for the directing of the nod protein to a specific subcellular component, the chromosome.

Animals↗
PubMed1995

The Drosophila meiotic recombination gene mei-9 encodes a homologue of the yeast excision repair protein Rad1.

Meiotic recombination and DNA repair are mediated by overlapping sets of genes. In the yeast Saccharomyces cerevisiae, many genes required to repair DNA double-strand breaks are also required for meiotic recombination. In contrast, mutations in genes required for nucleotide excision repair (NER) have no detectable effects on meiotic recombination in S. cerevisiae. The Drosophila melanogaster mei-9 gene is unique among known recombination genes in that it is required for both meiotic recombination and NER. We have analyzed the mei-9 gene at the molecular level and found that it encodes a homologue of the S. cerevisiae excision repair protein Rad1, the probable homologue of mammalian XPF/ERCC4. Hence, the predominant process of meiotic recombination in Drosophila proceeds through a pathway that is at least partially distinct from that of S. cerevisiae, in that it requires an NER protein. The biochemical properties of the Rad1 protein allow us to explain the observation that mei-9 mutants suppress reciprocal exchange without suppressing the frequency of gene conversion.

Amino Acid Sequence↗
PubMed1994

A structure-function analysis of NOD, a kinesin-like protein from Drosophila melanogaster.

We have analyzed a collection of 12 mutations in the Drosophila melanogaster nod locus, which encodes a kinesin-like protein involved in female meiotic chromosome segregation. The kinesin-like domain is at the N-terminus of the protein, while the C-terminal portion of the protein is unique. Four of the mutations are missense and affect highly conserved domains of the kinesin-like portion of the predicted protein, and thus demonstrate that the sequence conservation is biologically relevant. Surprisingly, two other mutations, which behave genetically as null alleles, are the result of mutations in the last exon of the nod gene. Thus, these two mutations affect the most C-terminal residues in the unique portion of the predicted protein. Based on these mutations, we suggest that this part of the protein may also be essential for wild-type function. The mutations were induced by either gamma-rays or ethyl methanesulfonate (EMS). All of the gamma-ray induced mutations were small or large chromosomal rearrangements, while all of the EMS mutations were G-->A transitions. These findings are consistent with the biochemical basis of the mode of action of each mutagen.

Alleles↗