Showing 2461 - 2470 of 5714 Items
Stress alters rates and types of loss of heterozygosity in candida albicans
Date: 2011-01-01
Creator: A. Forche, D. Abbey, T. Pisithkul, M. A. Weinzierl, T., Ringstrom, D. Bruck, K. Petersen, J. Berman
Access: Open access
- Genetic diversity is often generated during adaptation to stress, and in eukaryotes some of this diversity is thought to arise via recombination and reassortment of alleles during meiosis. Candida albicans, the most prevalent pathogen of humans, has no known meiotic cycle, and yet it is a heterozygous diploid that undergoes mitotic recombination during somatic growth. It has been shown that clinical isolates as well as strains passaged once through a mammalian host undergo increased levels of recombination. Here, we tested the hypothesis that stress conditions increase rates of mitotic recombination in C. albicans, which is measured as loss of heterozygosity (LOH) at specific loci. We show that LOH rates are elevated during in vitro exposure to oxidative stress, heat stress, and antifungal drugs. In addition, an increase in stress severity correlated well with increased LOH rates. LOH events can arise through local recombination, through homozygosis of longer tracts of chromosome arms, or by whole-chromosome homozygosis. Chromosome arm homozygosis was most prevalent in cultures grown under conventional lab conditions. Importantly, exposure to different stress conditions affected the levels of different types of LOH events, with oxidative stress causing increased recombination, while fluconazole and high temperature caused increases in events involving whole chromosomes. Thus, C. albicans generates increased amounts and different types of genetic diversity in response to a range of stress conditions, a process that we term "stress-induced LOH" that arises either by elevating rates of recombination and/or by increasing rates of chromosome missegregation. IMPORTANCE Stress-induced mutagenesis fuels the evolution of bacterial pathogens and is mainly driven by genetic changes via mitotic recombination. Little is known about this process in other organisms. Candida albicans, an opportunistic fungal pathogen, causes infections that require adaptation to different host environmental niches. We measured the rates of LOH and the types of LOH events that appeared in the absence and in the presence of physiologically relevant stresses and found that stress causes a significant increase in the rates of LOH and that this increase is proportional to the degree of stress. Furthermore, the types of LOH events that arose differed in a stress-dependent manner, indicating that eukaryotic cells generate increased genetic diversity in response to a range of stress conditions. We propose that this "stress-induced LOH" facilitates the rapid adaptation of C. albicans, which does not undergo meiosis, to changing environments within the host. © 2011 Forche et al.
Levels of DNA polymorphism vary with mating system in the nematode genus Caenorhabditis
Date: 2002-01-01
Creator: Andrew Graustein, John M. Caspar, James R. Walters, Michael F. Palopoli
Access: Open access
- Self-fertilizing species often harbor less genetic variation than cross-fertilizing species, and at least four different models have been proposed to explain this trend. To investigate further the relationship between mating system and genetic variation, levels of DNA sequence polymorphism were compared among three closely related species in the genus Caenorhabditis: two self-fertilizing species, Caenorhabditis elegans and C. briggsae, and one cross-fertilizing species, C. remanei. As expected, estimates of silent site nucleotide diversity were lower in the two self-fertilizing species. For the mitochondrial genome, diversity in the selfing species averaged 42% of diversity in C. remanei. Interestingly, the reduction in genetic variation was much greater for the nuclear than for the mitochondrial genome. For two nuclear genes, diversity in the selfing species averaged 6 and 13% of diversity in C. remanei. We argue that either population bottlenecks or the repeated action of natural selection, coupled with high levels of selfing, are likely to explain the observed reductions in species-wide genetic diversity.
A database and synthesis of northern peatland soil properties and Holocene carbon and nitrogen accumulation
Date: 2014-09-01
Creator: Julie Loisel, Zicheng Yu, David W. Beilman, Philip Camill, Jukka, Alm, Matthew J. Amesbury, David Anderson, Sofia Andersson, Christopher Bochicchio, Keith Barber, Lisa R. Belyea, Joan Bunbury, Frank M. Chambers, Daniel J. Charman, François De Vleeschouwer, Barbara Fiałkiewicz-Kozieł
Access: Open access
- Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 ± 3% (standard deviation) for Sphagnum peat, 51 ± 2% for non-Sphagnum peat, and at 49 ± 2% overall. Dry bulk density averaged 0.12 ± 0.07 g/cm3, organic matter bulk density averaged 0.11 ± 0.05 g/cm3, and total carbon content in peat averaged 47 ± 6%. In general, large differences were found between Sphagnum and non-Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m2/yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25–28 g C/m2/yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore, we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons, respectively. The database is publicly available at https://peatlands.lehigh.edu.
Scattering equations and BCJ relations for gauge and gravitational amplitudes with massive scalar particles
Date: 2014-09-01
Creator: Stephen G. Naculich
Access: Open access
- Abstract: We generalize the scattering equations to include both massless and massive particles. We construct an expression for the tree-level n-point amplitude with n − 2 gluons or gravitons and a pair of massive scalars in arbitrary spacetime dimension as a sum over the (n − 3)! solutions of the scattering equations, à la Cachazo, He, and Yuan. We derive the BCJ relations obeyed by these massive amplitudes.
Genotypic evolution of azole resistance mechanisms in sequential Candida albicans isolates
Date: 2007-10-01
Creator: Alix Coste, Anna Selmecki, Anja Forche, Dorothée Diogo, Marie Elisabeth, Bougnoux, Christophe D'Enfert, Judith Berman, Dominique Sanglard
Access: Open access
- TAC1 (for transcriptional activator of CDR genes) is critical for the upregulation of the ABC transporters CDR1 and CDR2, which mediate azole resistance in Candida albicans. While a wild-type TAC1 allele drives high expression of CDR1/2 in response to inducers, we showed previously that TAC1 can be hyperactive by a gain-of-function (GOF) point mutation responsible for constitutive high expression of CDR1/2. High azole resistance levels are achieved when C. albicans carries hyperactive alleles only as a consequence of loss of heterozygosity (LOH) at the TAC1 locus on chromosome 5 (Chr 5), which is linked to the mating-type-like (MTL) locus. Both are located on the Chr 5 left arm along with ERG11 (target of azoles). In this work, five groups of related isolates containing azole-susceptible and -resistant strains were analyzed for the TAC1 and ERG11 alleles and for Chr 5 alterations. While recovered ERG11 alleles contained known mutations, 17 new TAC1 alleles were isolated, including 7 hyperactive alleles with five separate new GOF mutations. Single-nucleotide- polymorphism analysis of Chr 5 revealed that azole-resistant strains acquired TAC1 hyperactive alleles and, in most cases, ERG11 mutant alleles by LOH events not systematically including the MTL locus. TAC1 LOH resulted from mitotic recombination of the left arm of Chr 5, gene conversion within the TAC1 locus, or the loss and reduplication of the entire Chr 5. In one case, two independent TAC1 hyperactive alleles were acquired. Comparative genome hybridization and karyotype analysis revealed the presence of isochromosome 5L [i(5L)] in two azole-resistant strains. i(5L) leads to increased copy numbers of azole resistance genes present on the left arm of Chr 5, among them TAC1 and ERG11. Our work shows that azole resistance was due not only to the presence of specific mutations in azole resistance genes (at least ERG11 and TAC1) but also to their increase in copy number by LOH and to the addition of extra Chr 5 copies. With the combination of these different modifications, sophisticated genotypes were obtained. The development of azole resistance in C. albicans is therefore a powerful instrument for generating genetic diversity. Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Multiproxy lake sediment records at the northern and southern boundaries of the Aspen Parkland region of Manitoba, Canada
Date: 2009-09-01
Creator: Rebecca Teed, Charles Umbanhower, Philip Camill
Access: Open access
- Aspen parkland in central Canada may change substantially with increased warming and aridity as prairies replace forests, fire return intervals decrease and lake levels decline. We examined the relationships among vegetation, climate, fire and lake-ecosystem properties using lake sediment cores from the current northern and southern boundaries of the aspen parkland in southwestern Manitoba. We analyzed pollen, charcoal, sediment magnetics, biogenic silica, phosphorus, grain size and LOI, and dated the cores using Pb and C (AMS, calibrated). The Jones Lake record, from the southern edge of the parkland, began considerably earlier (~11 000 cal. BP) than the Mallard Pond record at the northern edge (~8600 cal. BP). These sites were characterized as prairie communities with low fire severity and relatively low lake productivity during the warm, dry period from 9000 to 6000 cal. BP. Beginning around 6500 cal. BP at Jones Lake and 3400 cal. BP at Mallard Pond, conditions appeared to get wetter as indicated by arboreal pollen percentage increases from ~30% to 40-60%, concurrent with a rise in charcoal and proxies for lake productivity (biogenic silica and percent organic phosphorus). Similar to previous studies along the prairie-forest border, we found that charcoal increased during warmer, wetter periods with increased forest cover and fuel loading rather than during warmer, drier periods of prairie dominance. Our results underscore the importance of regional changes in moisture, and its effects on lake levels and forest biomass, as a dominant control of the aspen parkland dynamics. © The Author(s), 2009. 210 14
Flow around phoronids: Consequnces of a neighbor to suspension feeders
Date: 1990-01-01
Creator: Amy S. Johnson
Access: Open access
- This article is in Free Access Publication and may be downloaded using the “Download Full Text PDF” link at right. © 1990, by the Association for the Sciences of Limnology and Oceanography, Inc.
Vital Statistics: American Folk Drawing and Watercolors from a Private Collection
Date: 1986-01-01
Creator: Philip M. Isaacson
Access: Open access
- Catalog of an exhibition held at the Bowdoin College Museum of Art from Sept. 11 to Nov. 9, 1986
Long Day with The Office: An Analysis of College Student Demand for Streaming Video on Netflix Access to this record is restricted to members of the Bowdoin community. Log in here to view.
Date: 2020-01-01
Creator: Summers Askew
Access: Access restricted to the Bowdoin Community