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PubMed1976

Some extensions of the Keyfitz momentum relationship.

A stable population, such that the total birthrate B(t) = Boerot, is abruptly altered by modifying the age-specific birth rate, m(x). The survivor function remains unaltered. The modified population ultimately settles down to a stable behavior, such that B(t) = B1er1t. It is shown that B1/Bo = (Ro-R1)/¿e1(ro-r1)RoZ1], where Ro, R1 are the net reproduction rates before and after the change, and Za = expected age giving birth for the stable population after the change. The age structure and transients resulting from the change are also described. The effect of an abrupt change in the survivor function l(x) is also investigated for the simple case where the change is caused by altering l(x) to e-lambdaxl(x). It is shown that the above ratio becomes B1/Bo = N1/No = [1 - zota o e-kxg(x)dx]/Z1lambda, where N refers to the numbers in the population, k = ro + lambda, and g(x) = m(x)l(x), the value before the change. A measure for the reproductive worth of the population is also established.

Birth Rate↗
PubMed1998

Cost-effectiveness of oral cholera vaccine in a stable refugee population at risk for epidemic cholera and in a population with endemic cholera.

Recent large epidemics of cholera with high incidence and associated mortality among refugees have raised the question of whether oral cholera vaccines should be considered as an additional preventive measure in high-risk populations. The potential impact of oral cholera vaccines on populations prone to seasonal endemic cholera has also been questioned. This article reviews the potential cost-effectiveness of B-subunit, killed whole-cell (BS-WC) oral cholera vaccine in a stable refugee population and in a population with endemic cholera. In the population at risk for endemic cholera, mass vaccination with BS-WC vaccine is the least cost-effective intervention compared with the provision of safe drinking-water and sanitation or with treatment of the disease. In a refugee population at risk for epidemic disease, the cost-effectiveness of vaccination is similar to that of providing safe drinking-water and sanitation alone, though less cost-effective than treatment alone or treatment combined with the provision of water and sanitation. The implications of these data for public health decision-makers and programme managers are discussed. There is a need for better information on the feasibility and costs of administering oral cholera vaccine in refugee populations and populations with endemic cholera.

Administration, Oral↗
PubMed1997

Neonatal rat cardiomyocytes possess a large population of stable microtubules that is enriched in post-translationally modified subunits.

Microtubules (MTs) may be involved in several differentiation-specific cardiomyocyte functions, including myofibril organization, hypertrophic cell growth, and the negative regulation of heart cell contraction. The functional characteristics of neonatal rat heart cell MTs, which possess subsets that are enriched with either detyrosinated or acetylated tubulin subunits, were analysed. When compared with their non-myocyte neighbors, cardiomyocyte MTs were found to be more resistant to high concentrations (33 microM) of nocodazole (over 10-30 min), cold treatments (10 min to 8 h), or calcium (50-100 microM for 1-10 min, using detergent-extracted cells). These stable MTs were correlated with the modified MT population. Myocytes treated with 10 microM taxol for 4 h showed elevated levels of modified tubulin but only moderate MT bundling or rearrangement, while after an overnight treatment significant changes in intensity and distribution were noted. In contrast, non-myocytes on the same coverslip showed much greater MT rearrangements, but with only a moderate increase in the immunostaining intensity for modified MTs. Finally, hapten-labeled tubulin that was microinjected into the myocytes showed incorporation rates that were similar to those of the non-myocytes in this study as well as to fibroblast rates determined in other studies, suggesting that the modified MTs in cardiomyocytes may be only moderately stable. Thus, a significant subset of MTs in neonatal cardiomyocytes are post-translationally modified, are resistant to depolymerization by a variety of agents, but are still turning over. These MTs may help define myocyte function during heart development.

Animals↗
PubMed1999

16S rDNA sequencing of Ruminococcus albus and Ruminococcus flavefaciens: design of a signature probe and its application in adult sheep.

The ruminococci are an important group of fibrolytic bacteria inhabiting the rumen. Seventeen strains of presumptively identified Ruminococcus were evaluated by a combination of nearly complete and partial 16S rDNA sequence that identified all strains as either Ruminococcus albus or Ruminococcus flavefaciens. All sequences fell into cluster IV of the clostridia, while other species of ruminococci (e.g. Ruminococcus obeum, Ruminococcus gnavus, Ruminococcus lactaris) fall into cluster XIVa of the clostridia. Ruminococcus cluster IV sequences were used to design a 16S rRNA oligonucleotide probe to assess the relative abundance of target populations in a stable ruminal environment. A stable population (animals fed eight times per day) was established in sheep so that statistically robust comparisons could be made in the absence of variation due to diurnal rumen fluctuations. The steady state populations were sampled six times over a 24 d period and direct microscopic counts (DC), total culturable counts (TCC), and total cellulolytic counts (CEL) were determined. DC and culturable data (TCC and CEL) were compared with relative abundance estimates of Ruminococcus IV and Fibrobacter succinogenes. A combination of the Ruminococcus and F. succinogenes probes accounted for 4.0% of the bacterial population and cellulolytic bacteria (measured by most-probable numbers) were 5.2% of the total culturable count. These data suggest that a major portion of the Ruminococcus and Fibrobacter diversity has been cultured and is represented by available sequences. Steady state populations were measured over several days in three sheep and an estimate of variation in DC, TCC, CEL and 16S-based data were obtained. These variance estimates could be used to determine the theoretical sample sizes required to obtain statistically significant differences under different experimental conditions.

Animals↗
PubMed2004

The influence of settling time on the formation of aerobic granules.

Aerobic granular sludge, without the addition of carrier material, has only been reported in one suspended growth system, the Sequencing Batch Reactor (SBR) operated with short fill and settling periods. Recent studies have demonstrated that extracellular polysaccharides increased with the formation of aerobic granules, and that the shear force may stimulate production of these polysaccharides. In the study described herein, two SBRs were operated with the same shear force (air flow rate 275 L h(-1)) and two different settling times (2 and 10 min). Only the reactor with 2 min settling formed completely granular sludge, although granules were present in both reactors. Community analysis using 16S rRNA PCR products and DGGE showed that the communities diverged quickly after reactor start-up. For samples taken at steady-state, the granular population was more stable and less diverse than the flocculent reactor. EPS extraction of samples using cation exchange resin yielded similar values for aerobic granular sludge and previously reported anaerobic granules. While differences in the protein and TOC content between the flocculent and granular reactors increased appreciably as the sludge became more granular, the protein to polysaccharide ratio was relatively constant. The experiment confirmed previous theories that short settling times in SBRs select for granular sludge. The settling time results in granular sludge having a higher EPS protein content and a less diverse but more stable population.

Bacteria, Aerobic↗
PubMed1980

The sisters' riddle and the importance of variance when guessing demographic rates from kin counts.

Goldman (1978) has proposed a formula derived from stable population theory for estimating growth rates from certain counts of younger and older sisters in a population. Computer microsimulation outputs show extraordinarily large statistical errors for estimation when stable population assumptions are modified to allow for random variability such as would be encountered with field work on small populations. A derivation of Goldman's formula on more interpretable lines than that in Goldman (1978) identifies a source of variance special to this particular formula so that the large statistical errors for this example need not count in general against the feasibility of estimating demographic rates from counts of kin.

Family↗
PubMed1993

Selection bias from differential residential mobility as an explanation for associations of wire codes with childhood cancer.

Several studies of childhood cancer, especially leukemia, in residential areas have reported an association with wire configuration codes. These codes were suggested to be surrogates of electromagnetic field exposure. However, the selection criteria used in several of the studies caused the case and control populations to be non-comparable, especially with respect to residential mobility. Specifically, controls were required to be residentially stable but cases were not. Thus, an artificial association between residential mobility and cancer was created by the subject selection procedure. The present study of 5721 residences in Columbus, Ohio was conducted to learn if bias due to differences in residential mobility, rather than electromagnetic fields, could explain the reported association between wire configuration codes and childhood cancer. It was found that the proportion of homes classified as "high" wire code in the non-stable population was 31% greater than the corresponding proportion in the stable population. This finding shows that high wire codes are associated with homes in which the residents are mobile and low wire codes are associated with homes occupied by stable residents. Thus, as a consequence of this association between residential mobility and high wire codes, studies that created an artificial association between residential mobility and childhood cancer will also produce a false association between high wire codes and cancer.

Case-Control Studies↗
PubMed1994

Alterations in contact and density-dependent arrest state in senescent WI-38 cells.

Normal human WI-38 fibroblast-like cells in culture undergo a process of senescence, one feature of which is a gradual decline in proliferative capacity. As these cells reach the end of their replicative life span they exhibit decreases in the fraction of cells able to synthesize DNA, in the number of doublings per passage (constant seeding density), and in the cell harvest and saturation densities. They also display increased average cell cycle times, largely at the expense of longer G1 intervals. These alterations are accompanied by morphologic changes, including cell enlargement. Before the end of the replicative life span or phase-out, there is a highly reproducible (55/58 sublines) cell loss of approximately 50%; however, a stable population survives that can exist in a viable yet nonproliferative state for many months. This stable population maintains an extremely low saturation density, representing < 5% of that achieved by early passage cultures. Further, we show that maximum harvest densities achieved by senescent cells are lower, irrespective of seeding densities, i.e. when placed at cell densities higher than those normally achieved by senescent cultures they display a net decline in cell number. This decline continues until the cell density approximates the density that would have been achieved had the cultures been seeded at standard density (1 x 10(4) cells/cm2).(ABSTRACT TRUNCATED AT 250 WORDS)

Biomarkers↗
PubMed1999

Documenting the diet in ancient human populations through stable isotope analysis of hair.

Fundamental to the understanding of human history is the ability to make interpretations based on artefacts and other remains which are used to gather information about an ancient population. Sequestered in the organic matrices of these remains can be information, for example, concerning incidence of disease, genetic defects and diet. Stable isotopic compositions, especially those made on isolates of collagen from bones, have been used to help suggest principal dietary components. A significant problem in the use of collagen is its long-term stability, and the possibility of isotopic alteration during early diagenesis, or through contaminating condensation reactions. In this study, we suggest that a commonly overlooked material, human hair, may represent an ideal material to be used in addressing human diets of ancient civilizations. Through the analysis of the amino-acid composition of modern hair, as well as samples that were subjected to radiation (thus simulating ageing of the hair) and hair from humans that is up to 5200 years old, we have observed little in the way of chemical change. The principal amino acids observed in all of these samples are essentially identical in relative abundances and content. Dominating the compositions are serine, glutamic acid, threonine, glycine and leucine, respectively accounting for approximately 15%, 17%, 10%, 8% and 8% of the total hydrolysable amino acids. Even minor components (for example, alanine, valine, isoleucine) show similar constancy between the samples of different ages. This constancy clearly indicates minimal alteration of the amino-acid composition of the hair. Further, it would indicate that hair is well preserved and is amenable to isotopic analysis as a tool for distinguishing sources of nutrition. Based on this observation, we have isotopically characterized modern individuals for whom the diet has been documented. Both stable nitrogen and carbon isotope compositions were assessed, and together provide an indication of trophic status, and principal type (C3 or C4) of vegetation consumed. True vegans have nitrogen isotope compositions of about 7/1000 whereas humans consuming larger amounts of meat, eggs, or milk are more enriched in the heavy nitrogen isotope. We have also analysed large cross-sections of modern humans from North America and Europe to provide an indication of the variability seen in a population (the supermarket diet). There is a wide diversity in both carbon and nitrogen isotope values based at least partially on the levels of seafood, corn-fed beef and grains in the diets. Following analysis of the ancient hair, we have observed similar trends in certain ancient populations. For example, the Coptics of Egypt (1000 BP) and Chinchorro of Chile (5000-800 BP) have diets of similar diversity to those observed in the modern group but were isotopically influenced by local nutritional sources. In other ancient hair (Egyptian Late Middle Kingdom mummies, ca. 4000 BP), we have observed a much more uniform isotopic signature, indicating a more constant diet. We have also recognized a primary vegetarian component in the diet of the Neolithic Ice Man of the Oetztaler Alps (5200 BP). In certain cases, it appears that sulphur isotopes may help to further constrain dietary interpretations, owing to the good preservation and sulphur content of hair. It appears that analysis of the often-overlooked hair in archaeological sites may represent a significant new approach for understanding ancient human communities.

Amino Acids↗
PubMed1981

Survival of Rhizobium in Acid soils.

A Rhizobium strain nodulating cowpeas did not decline in abundance after it was added to sterile soils at pH 6.9 and 4.4, and the numbers fell slowly in nonsterile soils at pH 5.5 and 4.1. A strain of R. phaseoli grew when added to sterile soils at pH 6.7 and 6.9; it maintained large, stable populations in soils of pH 4.4, 5.5, and 6.0, but the numbers fell markedly and then reached a stable population size in sterile soils at pH 4.3 and 4.4. The abundance of R. phaseoli added to nonsterile soils with pH values of 4.3 to 6.7 decreased similarly with time regardless of soil acidity, and the final numbers were less than in the comparable sterile soils. The minimum pH values for the growth of strains of R. meliloti in liquid media ranged from 5.3 to 5.9. Two R. meliloti strains, which differed in acid tolerance for growth in culture, did not differ in numbers or decline when added to sterile soils at pH 4.8, 5.2, and 6.3. The population size of these two strains was reduced after they were introduced into nonsterile soils at pH 4.8, 5.4, and 6.4, and the number of survivors was related to the soil pH. The R. meliloti strain that was more acid sensitive in culture declined more readily in sterile soil at pH 4.6 than did the less sensitive strain, and only the former strain was eliminated from nonsterile soil at pH 4.8; however, the less sensitive strain also survived better in limed soil. The cell density of the two R. meliloti strains was increased in pH 6.4 soil in the presence of growing alfalfa. The decline and elimination of the tolerant, but not the sensitive, strain was delayed in soil at pH 4.6 by roots of growing alfalfa.

Journal Article↗
PubMed1985

Expected time for random genetic drift of a population between stable phenotypic states.

Natural selection and random genetic drift are modeled by using diffusion equations for the mean phenotype of a quantitative (polygenic) character in a finite population with two available adaptive zones or ecological niches. When there is appreciable selection, the population is likely to spend a very long time drifting around the peak in its original adaptive zone. With the mean phenotype initially anywhere near the local optimum, the expected time until a shift between phenotypic adaptive peaks increases approximately exponentially with the effective population size. In comparison, the expected duration of intermediate forms in the actual transition between adaptive peaks is extremely short, generally below the level of resolution in the fossil record, and increases approximately logarithmically with the effective population size. The evolutionary dynamics of this model conform to the pattern of current paleontological concepts of morphological "stasis" and "punctuated equilibria."

Biological Evolution↗
PubMed1987

Ecological compensation--a complication for testing life-history theory.

Mortality, growth and birth rates cannot vary independently in stable populations, environmental change of one variable must be accompanied by compensatory variation of another. Ecological compensation is recognized if the stable populations are genetically identical. Ecological compensation, if it operates, constrains the direction of evolutionary change, and predictions that ignore it may be in error.

Adaptation, Physiological↗
PubMed1996

Patterns of genetic polymorphism maintained by fluctuating selection with overlapping generations.

We study the form of polymorphisms that can be maintained by the joint effects of generation overlap and randomly fluctuating selection, acting on a quantitative trait affecting offspring viability. The genetic system can be single locus or multilocus, haploid or diploid. Selection is assumed to be stabilizing with a randomly fluctuating optimum, and we assume additive allelic effects without epistasis. For fluctuations above a certain threshold, nonzero genetic variance is maintained in an evolutionarily stable population. Our model allows a continuum of alleles with arbitrary effects at each locus. Nonetheless, the genotype distribution in an evolutionarily stable population is discrete--a polymorphism of a few alleles with distinctly different effects--and often involves only a pair of alleles at each locus. The form of the genotype distribution depends on the number of loci affecting the trait, and on the kurtosis of the distribution of the phenotypic optimum theta. If the trait is affected by several loci, the number of polymorphic loci increases with increased variance of fluctuations in selection. For distributions of theta with negative kurtosis (i.e., lower kurtosis than a Gaussian) the number of polymorphic loci increases gradually (0-->1-->2--> ...M), and the genetic variability is in the form of a few diallelic or triallelic loci with alleles of large effect. For distributions with positive kurtosis, the increase is abrupt (0 -> many) and involves many diallelic loci. These results do not fit the conventional multivariate Gaussian or near-Gaussian models for quantitative traits, but may partially explain recent findings that heritable variation in natural populations is often due to genes of large effect.

Diploidy↗
PubMed2005

Reproducibility of impedance cardiography parameters in outpatients with clinically stable coronary artery disease.

BACKGROUND: Impedance cardiography (ICG) is a noninvasive method of determining hemodynamic parameters. It is clinically important to determine whether any change in ICG parameters occur due to changes in disease status or therapeutic interventions, or due to normal hemodynamic and technology variability. The objective of this study was to establish the intra- and inter-day reproducibility of ICG in a stable population with coronary artery disease (CAD). METHODS: A prospective, time series design was used. The study group consisted of 96 clinically stable CAD patients in an outpatient cardiac rehabilitation program. Measurements of ICG hemodynamic parameters were obtained at four points in time: after 5 and 10 min of rest on the first day and after 5 and 10 min of rest on a second day, 1 week later. RESULTS: There were small but significant intra-day changes between the 5- and 10-min hemodynamic measures. Mean absolute percent changes in intra-day hemodynamic measures were <8%. High intra-day correlation was observed, ranging from 0.85 for mean arterial pressure to 0.99 for thoracic fluid content. There were expectedly larger inter-day hemodynamic variation of up to 18% and lower inter-day correlation for all ICG parameters ranging from 0.66 to 0.88. CONCLUSIONS: Impedance cardiography measurements demonstrate both intra- and inter-day reproducibility within clinically acceptable ranges in a clinically stable population of CAD patients. The expected ranges of variation can be used to gauge whether a patient's hemodynamic status has changed because of disease or intervention.

Cardiography, Impedance↗
PubMed1994

In vivo analysis of the stability and transport of nuclear poly(A)+ RNA.

We have studied the distribution of poly(A)+ RNA in the mammalian cell nucleus and its transport through nuclear pores by fluorescence and electron microscopic in situ hybridization. Poly(A)+ RNA was detected in the nucleus as a speckled pattern which includes interchromatin granule clusters and perichromatin fibrils. When cells are fractionated by detergent and salt extraction as well as DNase I digestion, the majority of the nuclear poly(A)+ RNA was found to remain associated with the nonchromatin RNP-enriched fraction of the nucleus. After inhibition of RNA polymerase II transcription for 5-10 h, a stable population of poly(A)+ RNA remained in the nucleus and was reorganized into fewer and larger interchromatin granule clusters along with pre-mRNA splicing factors. This stable population of nuclear RNA may play an important role in nuclear function. Furthermore, we have observed that, in actively transcribing cells, the regions of poly(A)+ RNA which reached the nuclear pore complexes appeared as narrow concentrations of RNA suggesting a limited or directed pathway of movement. All of the observed nuclear pores contained poly(A)+ RNA staining suggesting that they are all capable of exporting RNA. In addition, we have directly visualized, for the first time in mammalian cells, the transport of poly(A)+ RNA through the nuclear pore complexes.

Animals↗
PubMed1985

Calculation of age-specific fertility schedules from tabulations of parity in two censuses.

The mathematics of stable populations recently has been generalized to cover populations with time-varying fertility and mortality by a modification incorporating the sum of age-varying growth rates in place of the fixed growth rate of a stable population. Equations that characterize nonstable populations apply to any cohort-like phenomenon with a measurable property that cumulates gains or losses through time. In particular, the equations fit the relation between a population's average parity at a given age and age-specific fertility rates previously experienced at lower ages. Techniques devised to derive an intercensal life table from single-year age distributions in two censuses are adapted to estimate accurate intercensal fertility schedules from distributions of parity by age of woman in two censuses. Birth-order specific fertility schedules are also estimated.

Adolescent↗
PubMed1986

Examination of the generalized age distribution.

The formula for the age distribution and other relationships that follow from it for any (non-stable) population presented by Preston and Coale are significant contributions to demography. The formulas summarize the relationships among various demographic measures precisely, and are formally analogous to the relationships that hold for stable populations. The significance of these formulas cannot be overstated; they allow us to understand clearly the relationships among demographic measures in any arbitrary population. However, when it comes to using them for estimating demographic measures when census data are defective, the method of estimation is still affected by defective data. The reason is that the series of age-specific growth rates reflects the observed census age distributions exactly so that any defects in the census data are summarized in the growth rates. This paper begins with the formulation of the discrete version of the "new synthesis" developed by Preston and Coale. With the discrete formulation, the three kinds of errors introduced when the continuous time formulas are applied to real data can be avoided. Then it is pointed out that when two accurate census data are available, the Preston-Coale procedure of "estimating" the age distribution at the second census is equivalent to checking the identity of the age distribution formula. Also "estimating" mortality by the procedure of Preston-Coale is shown to be equivalent to obtaining mortality directly from intercensal survival rates. That the procedure which involves the age-specific growth rates is equivalent to those that involve the intercensal survival rates may have escaped notice because there are no a priori constraints for patterns of age-specific growth rates to follow. The irregularity in growth rates due to defective data are not distinguishable from true irregularity that exists in the population, contrary to the well-known regularity in the pattern of survival rates in human populations.

Actuarial Analysis↗
PubMed2006

A comparison of landscapes occupied by increasing and decreasing populations of grassland birds.

For several decades, many grassland bird species have been declining in abundance throughout the Midwest and Great Plains regions of the United States, possibly due to loss of natural grassland habitat and increasing urbanization. I used 20 years of data from the North American Breeding Bird Survey to identify increasing, decreasing, and stable populations of 36 grassland-nesting bird species. I characterized the immediate landscape (circle with radius = 30 km) surrounding each population based on data from the National Resources Inventory. For each landscape, I calculated the proportion of eight different land-cover types: restored grassland, rangeland, cultivated cropland, pasture, noncultivated cropland, forest, urban land, and water. Using a null model, I compared landscape composition of increasing, decreasing, and stable populations. As predicted on the basis of the habitat preferences of grassland birds, increasing populations inhabited landscapes that contained significantly more restored grassland and rangeland but significantly less forest land and urban land than landscapes inhabited by decreasing populations. There was no significant difference in the proportion of cropland within the landscapes of increasing and decreasing populations, although cropland composed a large proportion (>30%) of many landscapes. In contrast, restored grassland typically composed a very small proportion (<3.5%) of total land cover, yet it was significantly more common in the landscapes of increasing than decreasing populations. These results suggest that grassland birds may benefit from government initiatives, such as the Conservation Reserve Program, that promote the restoration of grassland at a landscape scale.

Agriculture↗