Comparison of population genetic patterns in two widespread freshwater mussels with contrasting life histories in western North America.

We investigate population genetic structuring in Margaritifera falcata, a freshwater mussel native to western North America, across the majority of its geographical range. We find shallow rangewide genetic structure, strong population-level structuring and very low population diversity in this species, using both mitochondrial sequence and nuclear microsatellite data. We contrast these patterns with previous findings in another freshwater mussel species group (Anodonta californiensis/A. nuttalliana) occupying the same continental region and many of the same watersheds. We conclude that differences are likely caused by contrasting life history attributes between genera, particularly host fish requirements and hermaphroditism. Further, we demonstrate the occurrence of a 'hotspot' for genetic diversity in both groups of mussels, occurring in the vicinity of the lower Columbia River drainage. We suggest that stream hierarchy may be responsible for this pattern and may produce similar patterns in other widespread freshwater species.

Characterization of Metarhizium species and varieties based on molecular analysis, heat tolerance and cold activity.

AIMS: The genetic relationships and conidial tolerances to high and low temperatures were determined for isolates of several Metarhizium species and varieties. METHODS AND RESULTS: Molecular-based techniques [AFLP and rDNA (ITS1, ITS2 and 5.8S) gene sequencing] were used to characterize morphologically identified Metarhizium spp. isolates from a wide range of sources. Conidial suspensions of isolates were exposed to wet heat (45 + or - 0.2 degrees C) and plated on potato dextrose agar plus yeast extract (PDAY) medium. After 8-h exposure, the isolates divided clearly into two groups: (i) all isolates of Metarhizium anisopliae var. anisopliae (Ma-an) and Metarhizium from the flavoviride complex (Mf) had virtually zero conidial relative germination (RG), (ii) Metarhizium anisopliae var. acridum (Ma-ac) isolates demonstrated high heat tolerance (c. 70-100% RG). Conidial suspensions also were plated on PDAY and incubated at 5 degrees C for 15 days, during which time RGs for Ma-an and Ma-ac isolates were virtually zero, whereas the two Mf were highly cold active (100% RG). CONCLUSIONS: Heat and cold exposures can be used as rapid tools to tentatively identify some important Metarhizium species and varieties. SIGNIFICANCE AND IMPACT OF THE STUDY: Identification of Metarhizium spp. currently relies primarily on DNA-based methods; we suggest a simple temperature-based screen to quickly obtain tentative identification of isolates as to species or species complexes.

Cell cycle regulation of the replication licensing system: involvement of a Cdk-dependent inhibitor.

The replication licensing factor (RLF) is an essential initiation factor that is involved in preventing re-replication of chromosomal DNA in a single cell cycle. In Xenopus egg extracts, it can be separated into two components: RLF-M, a complex of MCM/P1 polypeptides, and RLF-B, which is currently unpurified. In this paper we investigate variations in RLF activity throughout the cell cycle. Total RLF activity is low in metaphase, due to a lack of RLF-B activity and the presence of an RLF inhibitor. RLF-B is rapidly activated on exit from metaphase, and then declines during interphase. The RLF inhibitor present in metaphase extracts is dependent on the activity of cyclin-dependent kinases (Cdks). Affinity depletion of Cdks from metaphase extracts removed the RLF inhibitor, while Cdc2/cyclin B directly inhibited RLF activity. In metaphase extracts treated with the protein kinase inhibitor 6-dimethylaminopurine (6-DMAP), both cyclin B and the RLF inhibitor were stabilized although the extracts morphologically entered interphase. These results are consistent with studies in other organisms that invoke a key role for Cdks in preventing re-replication of DNA in a single cell cycle.

The role of MCM/P1 proteins in the licensing of DNA replication.

The DNA replication licensing system ensures that eukaryotic chromosomes replicate precisely once per cell cycle. A central component of the licensing system, RLF-M, has recently been shown to consist of a complex of Mcm/P1 proteins. This result allows us to integrate data about the MCM/P1 family obtained in different eukaryotes, ranging from yeast to man, into a general picture of the way that chromosome replication is controlled.

The RLF-B component of the replication licensing system is distinct from Cdc6 and functions after Cdc6 binds to chromatin.

Replication licensing factor (RLF) is an essential initiation factor that can prevent re-replication of DNA in a single cell cycle [1] [2]. It is required for the initiation of DNA replication, binds to chromatin early in the cell cycle, is removed from chromatin as DNA replicates and is unable to re-bind replicated chromatin until the following mitosis. Chromatography of RLF from Xenopus extracts has shown that it consists of two components termed RLF-B and RLF-M [3]. The RLF-M component consists of complexes of all six Xenopus minichromosome maintenance (MCM/P1) proteins (XMcm2-7), which bind to chromatin in late mitosis and are removed as replication occurs [3] [4] [5] [6] [7]. The identity of RLF-B is currently unknown. At least two factors must be present on chromatin before licensing can occur: the Xenopus origin recognition complex (XORC) [8] [9] and Xenopus Cdc6 (XCdc6) [10]. XORC saturates Xenopus sperm chromatin at approximately one copy per replication origin whereas XCdc6 binds to chromatin only if XORC is bound first [9] [10] [11]. Although XORC has been shown to be a distinct activity from RLF-B [9], the relationship between XCdc6 and RLF-B is currently unclear. Here, we show that active XCdc6 is loaded onto chromatin in extracts with defective RLF, and that both RLF-M and RLF-B are still required for the licensing of XCdc6-containing chromatin. Furthermore, RLF-B can be separated from XCdc6 by immunoprecipitation and standard chromatography. These experiments demonstrate that RLF-B is both functionally and physically distinct from XCdc6, and that XCdc6 is loaded onto chromatin before RLF-B function is executed.

Microbial metabolism directly affects trace gases in (sub) polar snowpacks.

Concentrations of trace gases trapped in ice are considered to develop uniquely from direct snow/atmosphere interactions at the time of contact. This assumption relies upon limited or no biological, chemical or physical transformations occurring during transition from snow to firn to ice; a process that can take decades to complete. Here, we present the first evidence of environmental alteration due to in situ microbial metabolism of trace gases (methyl halides and dimethyl sulfide) in polar snow. We collected evidence for ongoing microbial metabolism from an Arctic and an Antarctic location during different years. Methyl iodide production in the snowpack decreased significantly after exposure to enhanced UV radiation. Our results also show large variations in the production and consumption of other methyl halides, including methyl bromide and methyl chloride, used in climate interpretations. These results suggest that this long-neglected microbial activity could constitute a potential source of error in climate history interpretations, by introducing a so far unappreciated source of bias in the quantification of atmospheric-derived trace gases trapped within the polar ice caps.

Peripheral neurological assessment methods for workers exposed to hand-arm vibration. An appraisal.

Peripheral neurological assessment methods for workers exposed to hand-arm vibration have included vibration perception and esthesiometric threshold testing, electroneurography, handgrip force, and manipulative dexterity. For epidemiologic investigations with the purpose of detecting vibration effects anticipated as moderate to large in size in occupational populations, these methods have demonstrated their usefulness. Concerning their value in the assessment of individual workers, there is little quantitative information, as there have been no studies which have conducted rigorous "gold standard" neurological evaluation with which the results of independently performed diagnostic tests can be compared. However, results from four papers which used depth-sense (or ridge) and two-point discrimination esthesiometry were available for an analysis of the sensitivity of these tests in the detection of Taylor-Pelmear stages 2 and 3 of the hand-arm vibration syndrome. With specificity set at 90%, sensitivity ranged from 45 to 96% for depth-sense esthesiometry and from 19 to 75% for two-point discrimination. In addition, likelihood ratios were determined, as a measure of the capacity of the tests to alter pretest probability of disease. Because of their direct clinical interpretation and application, the use of likelihood ratios is suggested for future research on diagnostic methods used for vibration-exposed workers.

Ranking clinical problems and ocular diseases in ophthalmology: an innovative approach to curriculum design.

The MD program of the Faculty of Health Sciences, McMaster University, Hamilton, Ont., has used a problem-based, self-directed, small-group learning approach to medical education since 1969. Substantial curriculum revision was begun in 1983 as part of a process of institutional renewal. A faculty survey of all academic clinicians in the Division of Ophthalmology, Department of Surgery, was carried out in 1984 to determine which problems and diseases the teaching faculty thought had the highest priority for student learning. The results have been used by educational planners in revising the curriculum. They have also served to clarify faculty members' expectations of students within an ophthalmology rotation.

Landscape-scale genetic variation in a forest outbreak species, the mountain pine beetle (Dendroctonus ponderosae).

The mountain pine beetle Dendroctonus ponderosae is a native species currently experiencing large-scale outbreaks in western North American pine forests. We sought to describe the pattern of genetic variation across the range of this species, to determine whether there were detectable genetic differences between D. ponderosae occupying different host trees in common localities, and to determine whether there was molecular evidence for a past demographic expansion. Using a combination of amplified fragment length polymorphism (AFLP) and mitochondrial sequencing analyses, we found evidence of genetic structuring among populations that followed a broad isolation-by-distance pattern. Our results suggest that the geographical pattern of gene flow follows the core distribution of the principal D. ponderosae host species, around rather than across the Great Basin and Mojave Deserts. Patterns of haplotype diversity and divergence were consistent with a range-wide population expansion. This signal was particularly pronounced in the northern part of the species' range, where outbreak activity is currently increasing. Using AFLP markers, we were unable to detect significant differences among groups of insects sampled from different host trees in common locations. Incidentally, we found that a large proportion of the polymorphic AFLP markers were gender-specific, occurring only in males. While we did not include these markers in our analyses, this finding warrants further investigation.

DNA replication licensing factor.

DNA Replication Licensing Factor (RLF) is an essential activity required to restrict the duplication of genomic DNA to precisely once per cell cycle. Recent fractionation of RLF activity from Xenopus egg extracts has resulted in the identification of two essential components, RLF-B and RLF-M. RLF-M has been purified to homogeneity and has been shown to consist of a complex of proteins in the MCM/P1 family. RLF-B is still unidentified, but possible candidates for this activity have been identified in yeast. Elucidation of the RLF mechanism will provide important insights into the way that chromosome replication is controlled.

A pragmatic approach to standard setting--the example of coal tar products and asphalt.

This article will outline a pragmatic approach directed to incorporating key elements of a scientific review of the literature and derive a proposal for an occupational exposure standard for coal tar, coal tar pitch, creosote, petroleum pitch, bitumen and asphalt, six substances which contain polynuclear aromatic hydrocarbons. Five approaches to the standard setting process are reviewed and their strengths and weaknesses discussed. Unfortunately there does not exist an acceptable epidemiological data set, other than the coke oven emission studies, on which to base a valid and reliable risk assessment model. Based on comparative potency experiments of complex mixtures, consideration of the state-of-the-art sampling and analytical methods, prevention of acute human health effects, and current existing standards for these substances throughout the world, a set of recommended exposure standards are derived for health policy makers.

Initiation of archaeal DNA replication.

The identification of DNA as the genetic material and the elucidation of its structure by Watson and Crick [Watson and Crick, (1953) Nature (London) 171, 737-738], which has its 50th anniversary this year, first suggested the simple elegance with which the problem of passing on precise genetic information from one generation to the next could be solved. Semi-conservative replication is perhaps one of the simplest biological concepts to explain and understand. However, despite an enormous amount of effort in the intervening years, details of the way in which this process is regulated and performed are still unclear in many organisms. Recent work suggests that, due to their simplicity, the Archaea may make a good model for understanding some of the aspects of eukaryotic replication that still elude us.

A double-hexamer archaeal minichromosome maintenance protein is an ATP-dependent DNA helicase.

The minichromosome maintenance (MCM) proteins are essential for DNA replication in eukaryotes. Thus far, all eukaryotes have been shown to contain six highly related MCMs that apparently function together in DNA replication. Sequencing of the entire genome of the thermophilic archaeon Methanobacterium thermoautotrophicum has allowed us to identify only a single MCM-like gene (ORF Mt1770). This gene is most similar to MCM4 in eukaryotic cells. Here we have expressed and purified the M. thermoautotrophicum MCM protein. The purified protein forms a complex that has a molecular mass of approximately 850 kDa, consistent with formation of a double hexamer. The protein has an ATP-independent DNA-binding activity, a DNA-stimulated ATPase activity that discriminates between single- and double-stranded DNA, and a strand-displacement (helicase) activity that can unwind up to 500 base pairs. The 3' to 5' helicase activity requires both ATP hydrolysis and a functional nucleotide-binding site. Moreover, the double hexamer form is the active helicase. It is therefore likely that an MCM complex acts as the replicative DNA helicase in eukaryotes and archaea. The simplified replication machinery in archaea may provide a simplified model for assembly of the machinery required for initiation of eukaryotic DNA replication.

DNA replication in thermophiles.

DNA replication enzymes in the thermophilic Archaea have previously attracted attention due to their obvious use in methods such as PCR. The proofreading ability of the Pyrococcus furiosus DNA polymerase has resulted in a commercially successful product (Pfu polymerase). One of the many notable features of the Archaea is the fact that their DNA processing enzymes appear on the whole to be more like those found in eukaryotes than bacteria. These proteins also appear to be simpler versions of those found in eukaryotes. For these reasons, archaeal organisms make potentially interesting model systems to explore the molecular mechanisms of processes such as DNA replication, repair and recombination. Why archaeal DNA-manipulation systems were adopted over bacterial systems by eukaryotic cells remains a most interesting question that we suggest may be linked to thermophily.

Purification of an MCM-containing complex as a component of the DNA replication licensing system.

Replication licensing factor (RLF) ensures that eukaryotic chromosomal DNA is replicated exactly once in each cell cycle. On exit from metaphase, RLF is activated and binds to or modifies chromatin. This modification (the 'licence') is required for subsequent DNA replication; the licence is also inactivated in the process of replication. Active RLF is not imported into the nucleus, so further DNA replication cannot occur until the DNA is relicensed by passage throught mitosis. We have developed an assay to purify RLF from Xenopus eggs. Activity resolves into two components, RLF-M and RLF-B, both of which are required for licensing. RLF-M has been purified to apparent homogeneity: it consists of three polypeptides, one of which is a Xenopus homologue of the yeast MCM3 protein. Xenopus Mcm3 associates with chomatin in G1 and is removed during replication, consistent with its being a component of the RLF system.

Interaction between the origin recognition complex and the replication licensing system in Xenopus.

The origin recognition complex (ORC) binds to origins of replication in budding yeast. We have cloned a Xenopus homolog of the largest ORC polypeptide (XORC1). Immunodepletion of XOrc1 from Xenopus egg extracts blocks the initiation of DNA replication. We have purified Xenopus ORC, consisting of a protein complex similar to yeast ORC. In Xenopus egg extracts, ORC associates with chromatin throughout G1 and S phases. RLF-M, a component of the replication licensing system, also associates with chromatin early in the cell cycle but dissociates during S phase. We show that the assembly of RLF-M onto chromatin is dependent on the presence of chromatin-bound ORC, leading to sequential assembly of initiation proteins onto replication origins during the cell cycle.

Biochemical and cellular effects of roscovitine, a potent and selective inhibitor of the cyclin-dependent kinases cdc2, cdk2 and cdk5.

Cyclin-dependent kinases (cdk) play an essential role in the intracellular control of the cell division cycle (cdc). These kinases and their regulators are frequently deregulated in human tumours. Enzymatic screening has recently led to the discovery of specific inhibitors of cyclin-dependent kinases, such as butyrolactone I, flavopiridol and the purine olomoucine. Among a series of C2, N6, N9-substituted adenines tested on purified cdc2/cyclin B, 2-(1-ethyl-2-hydroxyethylamino)-6-benzylamino-9-isopropylpurine (roscovitine) displays high efficiency and high selectivity towards some cyclin-dependent kinases. The kinase specificity of roscovitine was investigated with 25 highly purified kinases (including protein kinase A, G and C isoforms, myosin light-chain kinase, casein kinase 2, insulin receptor tyrosine kinase, c-src, v-abl). Most kinases are not significantly inhibited by roscovitine. cdc2/cyclin B, cdk2/cyclin A, cdk2/cyclin E and cdk5/p35 only are substantially inhibited (IC50 values of 0.65, 0.7, 0.7 and 0.2 microM, respectively). cdk4/cyclin D1 and cdk6/cyclin D2 are very poorly inhibited by roscovitine (IC50 > 100 microM). Extracellular regulated kinases erk1 and erk2 are inhibited with an IC50 of 34 microM and 14 microM, respectively. Roscovitine reversibly arrests starfish oocytes and sea urchin embryos in late prophase. Roscovitine inhibits in vitro M-phase-promoting factor activity and in vitro DNA synthesis in Xenopus egg extracts. It blocks progesterone-induced oocyte maturation of Xenopus oocytes and in vivo phosphorylation of the elongation factor eEF-1. Roscovitine inhibits the proliferation of mammalian cell lines with an average IC50 of 16 microM. In the presence of roscovitine L1210 cells arrest in G1 and accumulate in G2. In vivo phosphorylation of vimentin on Ser55 by cdc2/cyclin B is inhibited by roscovitine. Through its unique selectivity for some cyclin-dependent kinases, roscovitine provides a useful antimitotic reagent for cell cycle studies and may prove interesting to control cells with deregulated cdc2, cdk2 or cdk5 kinase activities.

Concordance of occupational and environmental exposure information elicited from patients with Alzheimer's disease and surrogate respondents.

Identification of risk factors for Alzheimer's disease through the use of well designed case-control studies has been described as a research priority. Increasing recognition of the neurotoxic potential of many industrial chemicals such as organic solvents raises the question of the occupational and environmental contribution to the etiology of this high-priority health problem. The intention of this study was to develop and evaluate a methodology that could be used in a large scale case-control study of the occupational and environmental risk factors for dementia or a population-based surveillance system for neurotoxic disorders. The specific objectives of this study were to investigate: 1) the reliability of exposure-eliciting, interviewer-administered questionnaires given to patients with Alzheimer's disease (SDAT); 2) the reliability of exposure-eliciting interviewer-administered questionnaires given to the family of patients with SDAT and the agreement with the responses of the patient or surrogate respondents; 3) the reliability and agreement of responses of age- and sex-matched control patients and their families selected from geriatric care institutions and the community, with respect to the same exposure-eliciting and interviewer-administered questionnaire; and 4) the reliability of agent-based exposure ascertainment by a single, trained rater. The results of the study demonstrate that occupational and environmental histories from which exposure information can be derived is most reliably elicited from job descriptions of cases and control subjects rather than job titles alone or detailed probes for potential neurotoxic exposures. This will necessitate the use of standardized interviewer-administered instruments to derive this information in case-control studies of Alzheimer's disease or population-based surveillance systems for occupational and environmental neurotoxicity.

Setting educational priorities for learning the concepts of population health.

Following the World Health Organization's policy of 'Health for All by the Year 2000', doctors are increasingly being seen as health care providers to populations of patients, in addition to their more traditional role as doctors to individuals in a one-to-one encounter. In order for doctors to take on this expanded role, they must learn the knowledge and skills appropriate to population health. In this paper, we propose a method of educational priority-setting which allows educational planners to identify those diseases and adverse health conditions most appropriate for studying the concepts of population health. Using the Measurement Iterative Loop of Tugwell and colleagues as a framework, a table of Priority Illness Conditions was developed and compared with a previous priority list developed from a survey of clinical teachers at the McMaster University Medical School. Discussion of the implications for this approach in setting educational priorities at undergraduate, postgraduate and continuing medical education levels is presented, along with a review of possible shortcomings and caveats in using this approach.