bluuebunny/crossref_metadata_2025_split huggingface dataset:
Photosynthetic responses to a climate‐warming manipulation for contrasting meadow species in the Rocky Mountains, Colorado, USA
Authors: Loik M. E. , Redar S. P. , Harte J. 
Published: March 2003

Abstract1. Microclimate was measured and photosynthetic responses to a climate warming manipulation were compared for the evergreen shrub Artemisia tridentata and the herbaceous forb Erigeron speciosus in the Rocky Mountains, Colorado, USA.2. Soil was warmer and drier under infra‐red heaters compared with control plots.3. Midday xylem pressure potential did not differ for A. tridentata on heated vs control plots but was lower for E. speciosus on heated plots compared with controls. Leaf temperatures did not vary for the two species on heated or control plots.4. There were no significant treatment or species differences in the diurnal patterns of CO2 assimilation or stomatal conductance to water vapour. Also, there were no differences in responses to leaf temperature.5. The quantum yield for CO2 assimilation over a range of PPFD was lower for plants on heated plots. There was a marked difference between species in the pattern of stomatal conductance to water vapour over a range of PPFD, but no differences as a result of the heating treatment.6. The quantum efficiency of PSII electron transport was significantly affected by heating. Non‐radiative energy dissipation was greater for A. tridentata compared with E. speciosus. There was recovery of FV/FM for A. tridentata but not for E. speciosus.7. Heating appears to affect plants via changes in soil water content rather than by increasing leaf temperature. The deciduous species E. speciosus appears to undergo some permanent closure of PSII on heated plots, in contrast to the evergreen shrub A. tridentata. Such differences may help explain the increase in above‐ground biomass accumulation in response to heating for shrubs, compared with the decrease observed for deciduous herbaceous species.
Experimental assessment of the functional morphology of inflorescences of Daucus carota (Apiaceae): testing the ‘fly catcher effect’
Authors: Lamborn E. , Ollerton J. 
Published: March 2003

Abstract1. The functional significance of inflorescence morphology of Daucus carota L. (Apiaceae) was tested using an inflorescence manipulation experiment. Specifically, we sought to explain the role (if any) in pollination of the dark central floret of this species.2. In central England, D. carota was found to be pollinated by a taxonomically wide range of insects that varied in their inflorescence visitation rates and in their pollen loads. There was also variation in abundance of some taxa between the 2 years of this study.3. By removing the dark central floret and adding false florets we sought to affect pollinator visitation rates and seed set in a manner consistent with the ‘fly catcher effect’ hypothesis of Eisikowitch (1980). Taxa responded to these manipulations but not in ways that shed light on the adaptive role of the dark central floret. Seed set was likewise unaffected in any consistent way.4. The role of the dark central floret remains an enigma. We speculate, following Darwin (1888), that the dark central floret may now be functionless and possibly represents a trait that has persisted long after its original function has been lost.
Interference competition and sexual selection promote polymorphism in Colias (Lepidoptera, Pieridae)
Authors: Nielsen M. G. , Watt W. B. 
Published: March 2003

Abstract

‘Alba’ is a genetic polymorphism in butterflies of the genus Colias (the sulphurs) which confers on its female carriers white (A–) rather than yellow (aa) wings. Previous studies of ‘alba’s’ resource‐based effects in Colias alexandra and Colias scudderi suggested the rare morph should be eliminated in each species. Here the selective regime acting on ‘alba’s’ wing colour effect is examined to determine if it can maintain polymorphism.
Field experiments and observations reveal species‐specific differences in morph fecundity resulting from selection on wing colour. ‘Alba’ female and male Colias lose flight time, an index of Colias fitness, to unproductive courtship with pierines (white butterflies in the subfamily Pieridae), a form of interference competition. Flight time losses are greater in C. alexandra, which co‐occur more extensively with pierines than C. scudderi. ‘Alba’ females also spent less time being harassed by courting male Colias than yellow females; this form of sexual selection on morph wing colour is particularly strong in C. alexandra.
Loss of male and ‘alba’ female Colias flight time and the degree of male Colias preference for the yellow morph correlate with the extent each Colias species co‐occurs with pierines, suggesting interference competition influences Colias courtship decisions and ‘alba’ frequency.
Both interference competition and sexual selection are density dependent in their effects; these effects were modelled using Holling’s equation. Modelling results show temporal variation in Colias and pierine densities over the past 20 years caused morph fitness to vary such that in at least one year each morph in each species had greater relative fitness with respect to colour‐based selective pressures. Although this selective regime does not maintain polymorphism under temporal models of fitness variation, trade‐offs between these colour‐based effects and ‘alba’s’ temperature and resource‐based effects in a spatially heterogeneous environment could maintain polymorphism.
Phosphorus acquisition by Bromus madritensis ssp. rubens from soil interspaces shared with Mojave Desert shrubs
Authors: Yoder C. K. , Nowak R. S. 
Published: March 2003

Abstract

Bromus madritensis ssp. rubens (L.) Husnot (Red Brome) is an invasive annual grass that is associated with increases in fire frequency and decreases in perennial plant diversity. The success of Red Brome in the Mojave Desert has been attributed to its competitive ability, but competition between Red Brome and native shrubs for below‐ground resources has not been investigated previously. In this study we present a modification of previous dual‐isotope methods that assesses competitive interactions and responses among plants under field conditions. We then use this method to (i) determine if direct competition for phosphorus (P) occurs between Red Brome and native shrubs, and (ii) evaluate the effectiveness of phosphate acquisition by Red Brome from soil interspaces shared with different Mojave Desert shrub species.
Clipping Red Brome to remove ≈85% of its foliage on the day prior to labelling soil interspaces with P isotopes did not have the desired effect of inhibiting phosphorus uptake by Red Brome. Therefore we were unable to verify that direct competition for P occurred between Red Brome and native shrubs.
Nonetheless, by sampling plant tissue from unclipped strips of Red Brome that were centred between two shrubs, we were able to evaluate the effectiveness of phosphate acquisition by Red Brome from interspaces shared with different shrubs. There were no differences in the amount of phosphorus acquired by Red Brome from soil interspaces shared with the evergreen shrub Larreatridentata (D.C.) Cov. versus soil interspaces shared with the drought deciduous shrub Lycium pallidum Miers. However, Red Brome acquired more than six times as much P from soil interspaces shared with Larrea than from soil interspaces shared with the drought deciduous shrub Ambrosia dumosa Payne. Less P uptake by Red Brome from soil interspaces shared with Ambrosia suggests greater effectiveness of Ambrosia compared with Larrea to deplete soil P.
Energy expenditure and water flux of free‐living Dune Larks in the Namib: a test of the reallocation hypothesis on a desert bird
Authors: Williams J. B. 
Published: March 2003

Abstract1. Dune Larks (Mirafra erythroclamys) are the only avian species that has evolved the capability of surviving in the Namib Sand Sea, one of the driest regions in the world. Their field metabolic rate (FMR) and water flux, as measured by doubly labelled water, during the breeding and non‐breeding season are reported here.2. FMR is relatively constant over the annual cycle, which conforms to the ‘reallocation hypothesis’. With data pooled for both seasons, males expended energy at a rate of 97·5 ± 20·4 kJ day−1 (±1 SD) and females at 87·5 ± 13·0 kJ day−1. The data supported the hypothesis that Dune Larks do not possess a reduced FMR, a result attributable to their elevated basal metabolism. In a previous laboratory investigation, Dune Larks had a low rate of evaporative water loss despite their elevated basal metabolic rate (BMR). The present study demonstrated that this low evaporative water loss translates into a low water influx in the field.3. The FMR of Dune Lark females equalled 88·1 ± 6·9 kJ day−1 during incubation, and 88·5 ± 9·9 kJ day−1 when they were feeding 8–10‐day‐old nestlings. These data do not support the idea that the incubation period is a time of low energy demand.4. Dune Larks had a relatively constant water influx throughout the year. For sexes combined, mass‐adjusted values of water influx averaged 0·59 ml H2O/(day mass0·682).5. Calculations showed that neonates received 10 kJ day−1 whereas towards the end of the nestling period, chicks were fed 75 kJ day−1, the maximum gross energy intake for nestlings. During chick rearing, males increased the amount of food that they found by 57·7%, females by 64·3%, late in the nestling period.6. Dune Larks foraged in the morning and late afternoon, but were constrained by their environment to seek shade at midday during the breeding season.7. Metabolic water production plays a significant role in the water economy of Dune Larks. Assuming a mixed diet, 47% of their water intake comes from the oxidation of food stuffs.
The resolvase encoded by Xanthomonas campestris transposable element ISXc5 constitutes a new subfamily closely related to DNA invertases
Authors: Liu Chia‐Chen , Hühne Rolf , Tu Jenn , Lorbach Elke , Dröge Peter 
Published: March 2003

BackgroundConservative site‐specific recombination is responsible for the resolution of cointegrates which result during the transposition of class II transposable elements. Resolution is catalysed by a transposon‐encoded recombinase, resolvase, that belongs to a large family of recombinases, including DNA invertases. Resolvases and the related invertases are likely to employ similar reaction mechanisms during recombination. There are important differences, however. Resolvases require two accessory DNA binding sites within each of the two directly repeated recombination sites. Invertases instead need a host factor, Fis, and an enhancer type DNA sequence, in addition to two inversely orientated recombination sites.ResultsThe resolvase encoded by transposable element ISXc5 from the Gram‐negative phytopathogen Xanthomonas campestris shows two features which distinguish it from other known resolvases. First, it is more closely phylogenetically related to invertases than other resolvases. In particular, two functionally important regions seem highly conserved between this resolvase and members of the invertase subfamily. Second, the enzyme exhibits a large extension of its carboxy‐terminal domain with unknown function. We purified ISXc5 resolvase and analysed its resolution reaction in vitro. Our biochemical and DNA topological analysis reveals that critical features of resolution are similar, if not identical, to that carried out by γδ resolvase. However, despite its apparent similarity to invertases, we were unable to detect recombination on standard substrates for DNA inversion, in either the presence or absence of Fis.ConclusionsISXc5 resolvase employs a reaction mechanism which is common to members of the resolvase family. Its position near the evolutionary borderline to invertases and its high degree of identity within two functionally important regions with members of the DNA invertase subfamily suggest that only a few replacements of critical residues may suffice to convert this resolvase into a functional, possibly Fis‐dependent invertase.
Epigenetic reprogramming of the human H19 gene in mouse embryonic cells does not erase the primary parental imprint
Authors: Mitsuya Kohzoh , Meguro Makiko , Sui Hajime , Schulz Thomas C. , Kugoh Hiroyuki , Hamada Hiroshi , Oshimura Mitsuo 
Published: March 2003

BackgroundGenomic imprinting in mammals is thought to result from epigenetic modifications to chromosomes during gametogenesis, which leads to differential allelic expression during development. There is a requirement for an appropriate experimental system to enable the analysis of the mechanisms of genomic imprinting during embryogenesis.ResultsTo develop a novel in vitro system for studying the molecular basis of genomic imprinting, we constructed mouse cell lines containing either a paternal or maternal human chromosome 11, by microcell‐mediated chromosome transfer. Allele‐specific expression and DNA methylation studies revealed that the imprinting status of the human H19 gene was maintained in mouse A9 mono‐chromosomal hybrids. Each parental human chromosome was introduced independently into mouse near‐diploid immortal fibroblasts (m5S) and two embryonal carcinoma (EC) cell lines (OTF9‐63 and P19). The paternal allele of human H19 remained in a repressed state in m5S cells, but was de‐repressed in both EC cells. The paternal H19 allele was demethylated extensively in OTF9‐63 cells, whereas the only alteration in P19 hybrids was de novo methylation on both alleles in the 3′ region. Following in vitro differentiation, the expressed paternal H19 allele was selectively repressed in differentiated derivatives of EC hybrids.ConclusionThese results indicated that human imprint marks could function effectively in mouse cells, and that the imprinting process was epigenetically reprogrammed in embryonal carcinoma cells, without erasure of the primary imprint that marked the parental origin. Therefore, these mono‐chromosomal hybrids could provide a valuable in vitro system to study the mechanisms involved in the regulation of imprinted gene expression.
Individual LCR hypersensitive sites cooperate to generate an open chromatin domain spanning the human β‐globin locus
Authors: Li Guolong , Lim Kim‐Chew , Douglas Engel James , Bungert Jörg 
Published: March 2003

BackgroundThe human β‐globin locus control region (LCR) is composed of five DNase I hypersensitive (HS) sites located 5′ to the multiple genes it regulates. The LCR has been shown to comprise, among other essential properties, an activity that is required for generating a chromatin structure which renders the entire β‐globin gene locus accessible to exogenous nucleases. This nuclease‐sensitive state is generally believed to be reflective of the chromatin environment that is permissive for transcriptional activation of the globin genes.ResultsHere we show, in mice bearing intact YAC transgenes that encompass the whole human β‐globin locus, that the deletion of individual core LCR HS sites negatively affects the ability of the LCR to confer this open chromatin conformation throughout the locus, and when analysed in concert with the effect that these same mutations have on transcription, the data show that the chromatin opening activity is a necessary, but not sufficient, prerequisite for globin gene expression. The results also show that after deletion of individual hypersensitive sites, the mutated LCR is no longer able to provide an accessible chromatin environment that is independent from the site of YAC transgene integration.ConclusionsThese experiments provide further evidence for the hypothesis that the HS sites must act cooperatively to fulfil the multiple functions that are attributable to the LCR.
Hierarchical and co‐operative binding of OmpR to a fusion construct containing the ompC and ompF upstream regulatory sequences of Escherichia coli
Authors: Bergstrom Lisa C. , Qin Ling , Harlocker Susan L. , Egger Linda A. , Inouye Masayori 
Published: March 2003

Background: OmpR is a transcription factor that regulates the expression of the porin genes ompF and ompC in Escherichia coli. The phosphorylation state of OmpR, directed by the osmosensor EnvZ, determines its ability to bind to the upstream regulatory regions of these genes, a total of 14 phospho‐OmpR binding sites. While it has been possible to study the stoichiometry and hierarchy of the OmpR–DNA interaction in the upstream regions of ompF and ompC, their disunited location on the bacterial chromosome has made it difficult to compare the individual binding affinities of respective sites. Results: Using 1,10‐phenanthroline‐Cu+ footprinting on a fused construct containing both the ompF and ompC upstream regulatory sequences, and gel shift experiments on oligomers corresponding to individual sites, we have established a comparative hierarchy for OmpR binding, as F1, C1 > F2, F3 > C2 > C3. In addition, the binding patterns reveal an apparent co‐operative relationship between OmpR molecules bound at several upstream motifs. Densitometric analyses of the footprinted regions provide support for these observations. Mutational analysis of this construct reveals that the alteration of a conserved cytidine in the F1 motif (–86) causes a loss of OmpR affinity and disrupts hierarchical OmpR‐binding in the entire ompF region. Conclusions: The present results provide a unique view of the OmpR interaction with the two respective promoters, ompF and ompC, and an insight into the question of how the expression of ompF and ompC are reciprocally regulated by medium osmolarity.
Transmembrane tyrosine phosphatase LAR induces apoptosis by dephosphorylating and destabilizing p130Cas
Authors: Weng Liang‐Ping , Wang Xin , Yu Qiang 
Published: March 2003

Background:LAR is a transmembrane receptor‐like protein tyrosine phosphatase (PTP). Genetic studies of Drosophila LAR suggest that LAR may function to regulate cell adhesions or adhesion‐mediated signal transduction. The over‐expression of LAR in mammalian tissue culture cells does not affect cell adhesion but induces caspase‐dependent apoptosis. This study investigates molecular mechanisms of LAR‐induced apoptosis by searching for in vivo substrates of LAR which are responsible for LAR‐induced apoptosis.Results:The over‐expression of LAR in tissue culture cells specifically decreased the steady state protein level of p130Cas, a multifunctional signal assembly protein in signal transduction, by reducing the tyrosine phosphorylation and protein stability of p130Cas. The reduction of p130Cas protein level could be inhibited by tyrosine phosphatase inhibitors. Phosphatase domain‐deleted mutant LARs had no effect on p130Cas. LAR also preferentially dephosphorylated p130Casin vitro. Subcellularly, LAR and p130Cas were co‐localized along stress fibres and at focal adhesions. LAR over‐expression eliminated p130Cas from focal adhesions without affecting focal adhesion assembly. Restoring the level of p130Cas alleviated LAR‐induced apoptosis.Conclusions:p130Cas is an in vivo substrate of LAR. LAR specifically dephosphorylates and destabilizes p130Cas and may play a role in regulating cell adhesion‐mediated cell survival. The function of p130Cas in focal adhesions may not be to regulate focal adhesion assembly and cell adhesion but rather to transduce the cell adhesion‐generated signals which are essential for cell survival.
High copy number suppression of the meiotic arrest caused by a dmc1 mutation: REC114 imposes an early recombination block and RAD54 promotes a DMC1‐independent DSB repair pathway
Authors: Bishop Douglas K. , Nikolski Yuri , Oshiro June , Chon Jason , Shinohara Miki , Chen Xi 
Published: March 2003

Background DMC1, the meiosis‐specific eukaryotic homologue of bacterial recA, is required for completion of meiotic recombination and cell cycle progression past prophase. In a dmc1 mutant, double strand break recombination intermediates accumulate and cells arrest in prophase. We isolated genes which, when present at high copy numbers, suppress the meiotic arrest phenotype conferred by dmc1 mutations.ResultsAmong the genes isolated were two which suppress arrest by altering the recombination process. REC114 suppresses formation of double strand break (DSB) recombination intermediates. The low viability of spores produced by dmc1 mutants carrying high copy numbers of REC114 is rescued when reductional segregation is bypassed by mutation of spo13. High copy numbers of RAD54 suppress dmc1 arrest, promote DSB repair, and allow formation of viable spores following reductional segregation. Analysis of the combined effects of a null mutation in RED1, a gene required for meiotic chromosome structure, with null mutations in RAD54 and DMC1 shows that RAD54, while not normally important for repair of DSBs during meiosis, is required for efficient repair of breaks by the intersister recombination pathway that operates in red1 dmc1 double mutants.ConclusionsOver‐expression of REC114 suppresses meiotic arrest by preventing formation of DSBs. High copy numbers of RAD54 activate a DMC1‐independent mechanism that promotes repair of DSBs by homology‐mediated recombination. The ability of RAD54 to promote DMC1‐independent recombination is proposed to involve suppression of a constraint that normally promotes recombination between homologous chromatids rather than sisters.
Control of genetic stability in Escherichia coli: the SbcB 3′‐5′ exonuclease suppresses illegitimate recombination promoted by the RecE 5′‐3′ exonuclease
Authors: Yamaguchi Hirotaka , Hanada Katsuhiro , Asami Yasuo , Kato Jun‐ichi , Ikeda Hideo 
Published: March 2003

BackgroundThe EscherichiacolisbcB gene, which codes for a 3′‐5′ exonuclease, ExoI, is known to suppress illegitimate recombination. In contrast, the recE gene, which codes for a 5′‐3′ exonuclease, Exo VIII promotes joining between DNA ends having short stretches of homology. Therefore, it seems likely that the 3′‐5′ and 5′‐3′ exonucleases regulate genetic instability that is mediated by illegitimate recombination. However, there has been little evidence to substantiate the involvement of exonuclease activity in the promotion and suppression of illegitimate recombination.ResultsUsing a plasmid system for the analysis of deletion formation, we first demonstrated that deletion formation is increased by the sbcA mutation, which activates the expression of RecE 5′‐3′ exonuclease. It is thought that DNA ends having 3′‐single stranded overhangs are important for illegitimate recombination. Next, we found that a large supply of SbcB 3′‐5′ exonuclease suppresses the deletion formation enhanced by the RecE exonuclease. Moreover, the SbcB exonuclease even suppressed deletion formation in cells not expressing RecE exonuclease.ConclusionWe conclude that DNA ends with 3′‐overhangs produced by 5′‐3′ dsDNA exonuclease activity are proficient for illegitimate recombination, while blunt DNA ends produced by 3′‐5′ ssDNA exonuclease activity are deficient for illegitimate recombination. Therefore, both exonucleases may play important roles in genetic stability by controlling end‐joining between DNA molecules.