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W R Engels

Publications and source records attributed to W R Engels.

42 records · Page 3Linked to original sources
PubMed1980

Site-specific X-chromosome rearrangements from hybrid dysgenesis in Drosophila melanogaster.

When the germ line of Drosophila males is destabilized by a syndrome known as hybrid dysgenesis, X-chromosome rearrangements are found in up to 10 percent of the gametes produced. Some of these aberrations are simple inversions, but many are complex multibreak rearrangements. Furthermore, most of the breakpoints fall into a few highly localized positions on the chromosome. These positions are mostly at points of intercalary heterochromatin and may vary from one strain to the next. the results suggest that they may represent points of insertion of mobile DNA sequences.

Animals↗
PubMed1979

The estimation of mutation rates when premeiotic events are involved.

When mutation or recombination events occur premeiotically, the distribution of exceptional individuals among the offspring will be "clustered" as opposed to binomial. Even though the exact nature of the clustering is usually unknown, unbiased methods for measuring mutation rate and determining the precision of these measurements are given to replace a biased method now frequently used. When clustering is pronounced, the unweighted average mutation rate is found to be a more efficient estimator than the usual average weighted by family size. Methods of statistical inference and optimal experimental design in the absence of specific knowledge of the mechanism of clustering are also discussed.

Animals↗
PubMed1979

Extrachromosomal control of mutability in Drosophila melanogaster.

Hybrid dysgenesis is a syndrome of germ-line aberrations including, e.g., sterility and mutation, found in certain interstrain hybrids of Drosophila melanogaster. Previous studies of sterility have shown that elements responsible for dysgenesis may reside on all major chromosomes, but that their dysgenesis-causing ability is controlled by an unknown extrachromosomal factor. Dysgenic hybrids also give rise to unstable visible mutations thought to be DNA insertions at certain sensitive loci. One such unstable allele at the singed bristle locus, designated snw, was found to mutate at extraordinary rates exceeding 50%. This instability was shown to be under the same extrachromosomal control as hybrid dysgenesis itself. That is, the mutability of snw was reversibly suppressed when placed in the background cytotype known to prevent sterility and other characteristics of hybrid dysgenesis. These results suggest that snw may represent an insertion at the singed locus of a hypothetical gene responsible for hybrid dysgenesis.

Animals↗
PubMed1979

Hybrid dysgenesis in Drosophila melanogaster: the biology of female and male sterility.

High levels of female and male sterility were observed among the hybrids from one of the two reciprocal crosses between a wild strain of D. melanogaster known as pi2 and laboratory strains. The sterility, which is part of a common syndrome called hybrid dysgenesis, was found to be associated with the rudimentary condition of one or both of the ovaries or testes. All other tissues, including those of the reproductive system were normal, as were longevity and mating behavior. The morphological details of the sterility closely mimic the agametic condition occurring when germ cells are destroyed by irradiation or by the maternal-effect mutation, grandchildless. We suggest that sterility in hybrid dysgenesis is also caused by failure in the early development of germ cells. There is a thermo-sensitive period beginning at approximately the time of initiation of mitosis among primordial germ cells a few hours before the egg hatches and ending during the early larval stages. Our results suggest that hybrid dysgenesis, which also includes male recombination, mutation and other traits, may be limited to the germ line, and that each of the primordial germ cells develops, or fails to develop, independently of the others. This hypothesis is consistent with the observed frequencies of unilateral and bilateral sterility, with the shape of the thermosensitivity curves and with the fact that males are less often sterile than females. The features of this intraspecific hybrid sterility are found to resemble those seen in some interspecific Drosophila hybrids, especially those from the cross D. melanogaster X D. simulans.

Animals↗
PubMed1978

Base ratio, DNA content, and quinacrine-brightness of human chromosomes.

Human chromosomes were labeled with base-specific radioactive DNA precursors and examined autoradiographically to measure their DNA content and base ratio (percentage A-T base pairs). The requirement that incorporation of labeled bases be uniform during DNA synthesis was met by the use of inhibitors of de novo synthesis of DNA precursors. The genome was subdivided into 75 segments based on quinacrine banding, and the base ratio of each was calculated by a method that corrects for bias due to the scatter of grains about their source. Estimates of base ratio are shown to be sufficiently precise to detect variability among chromosomes and among segments within a chromosome. Analysis of these data and of measurements of the quinacrine fluorescence intensity of segments leads to the following conclusions. Base ratio is positively correlated with brightness, as predicted from independent in vitro studies. Larger chromosomes tend to have higher base ratios and to be brighter than smaller ones. The best prediction of the brightness of a segment must take into account not only its base ratio but also its DNA content. To explain these results, we suggest an evolutionary model in which chromosomes containing repeated sequences of A-T-rich DNA tend to grow by means of unequal sister chromatid and meiotic exchanges.

Base Sequence↗