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PubMed1996

Exploiter-mediated coexistence and frequency-dependent selection in a numerical model of biodiversity.

The key feature of the Daisyworld approach is to explicitly model feedbacks between the competitive dynamics of a planetary biota (daisies with different albedos) and the planet's climate, namely its surface temperature. Self-regulation of surface temperature and stable population dynamics are striking emergent properties of this system, despite increasing energy output from the model's sun. In this paper we attempt to perturb Daisyworld by introducing herbivores endowed with one of four strategies for feeding on daisies in a system rich in daisy diversity, and find that the herbivores only moderately diminish the temperature regulating abilities of the system. Furthermore, we find that the precise trajectory of the system's temperature regulation depends critically on which daisy species the herbivore allows to co-exist, thereby revealing that climate and patterns of biodiversity are highly interrelated in this model, and, possibly, in the real world.

Animals↗
PubMed1996

Host-parasite arms race in mutation modifications: indefinite escalation despite a heavy load?

If constantly changing genotypes are favorable in host and parasite coevolution, an indefinite escalation of mutation rates would result despite heavy mutational loads. We theoretically study this possibility by examining the mutation modifier dynamics of host and parasite that engage in genotype-specific epidemiological interaction. In the first model, we study the evolutionarily stable (ESS) mutation rate or switching rate if two alleles in a single locus are subjected to frequency-dependent selection favoring the rarer of the two. Mutation modifier locus is either tightly linked or unlinked to the selected locus. Sufficiently strong frequency-dependent selection may cause cycles in allele frequencies and a modifier with higher mutation rate enjoys the long-term advantage by randomizing the genotype of their offspring. Through the repeated events of invasion and replacement of mutation modifiers, the mutation rate continues to increase until the allele frequencies are stabilized. If some fraction of mutations are deleterious, there is no longer a pure ESS mutation rate: the evolutionarily stable population then consists of multiple strains concerning mutation modifier, typically one with a very high mutation rate and the other with a very low rate, stably coexisting and fighting off invasion by any other modifiers. These results are almost independent of the linkage between the selected and the modifier loci. In the second model, we consider the joint evolution of host and parasite mutation modifiers, assuming that a specific pair of host and parasite genotype densities change following the Nicholson-Bailey type model. If there is no cost of deleterious mutations, mutation rates of both species are escalated indefinitely by modifier evolution until they completely suppress the fluctuation of genotype densities. However, a small cost of deleterious mutation is enough to collapse this coevolutionary equilibrium of inflated mutations. Typical coevolutionary outcome is that the parasite mutation rate is accelerated to a high level; whereas the host mutation rate is driven to zero. Extension of our results to host-parasite coevolution of recombination modifier evolution is discussed.

Animals↗
PubMed1999

On sex, mate selection and the red queen.

The widespread occurrence of sexual reproduction despite the two-fold disadvantage of producing males, is still an unsolved mystery in evolutionary biology. One explanatory theory, called the "Red Queen" hypothesis, states that sex is an adaptation to escape from parasites. A more recent hypothesis, the mate selection hypothesis, assumes that non-random mating, possible only with sex, accelerates the evolution of beneficial traits. This paper tests these two hypotheses, using an agent-based or "micro-analytic" evolutionary algorithm where host-parasite interaction is simulated adhering to biological reality. While previous simpler models testing the "Red Queen" hypothesis considered mainly haploid hosts, stable population density, random mating and simplified expression of fitness, our more realistic model allows diploidy, mate selection, live history constraints and variable population densities. Results suggest that the Red Queen hypothesis is not valid for more realistic evolutionary scenarios and that each of the two hypotheses tested seem to explain partially but not exhaustively the adaptive value of sex. Based on the results we suggest that sexual populations in nature should avoid both, maximizing outbreeding or maximizing inbreeding and should acquire mate selection strategies which favour optimal ranges of genetic mixing in accordance with environmental challenges.

Animals↗
PubMed1997

Stability of a perturbed Leslie model.

A small perturbation introduced in the stochastic Leslie model leads to a long-lived stable population in a region or parameter space where the stochastic model predicts the demise of the population. This somewhat surprising result is understood by considering a related model with a density-dependent sex ratio which exhibits similar features.

Birth Rate↗
PubMed1987

Synaptic changes in the terminals of rod photoreceptors of albino mice after partial visual cell loss induced by brief exposure to constant light.

Albino mice were exposed to constant light for 7 days and were then transferred to periodic light. After initial photic damage and partial cell loss, the remaining visual cells recovered and survived as a stable population. Regions of the outer nuclear layer containing 4-6 rows of nuclei were more affected than those containing 6-10 rows. Changes in the synaptic structures in the receptor terminals of these two regions were recorded after varying survival periods. Some of the rod terminals had multiple synaptic ribbons and larger numbers of horizontal cell processes and bipolar cell dendrites. The number of terminals with multiple ribbons increased during recovery in periodic light. Morphometry demonstrated that the perimeters of horizontal and bipolar cell processes within the rod terminals were significantly larger than those in age-matched control mice, especially 4 weeks after recovery; they remained significantly larger than controls after 2 and 3 months. We suggest that partial loss of rod cells within a group of cells that are synaptically related to a common bipolar or horizontal cell results in synaptic growth inside the terminals of the surviving cells.

Albinism↗
PubMed1983

A naturalistic study of hypertension in a rural U.K. community. 2 Years results.

A naturalistic study was set up to screen, identify and treat hypertensive patients aged 20-60 years in a rural general practice. 3,222 patients (92%) of a stable population of 3,489 were screened by 2 nurse research assistants and of these 455 patients (14%) were found to be hypertensive or borderline hypertensive. After careful assessment, 192 of these patients were found suitable for treatment and subsequently 138 entered the study. Two well recognised treatment regimes were used and no significant difference between patient response resulted. 84 patients (60.9%) completed the 2 year duration of the study discussed here. The cost of the study is not feasible in an average general practice, but day to day running of such a project, run along clearly defined treatment regimes was managed easily by 2 research assistants: this reduced, therefore, the work load on individual general practitioners.

Adult↗
PubMed1988

Proliferation of rat intraspinal Schwann cells following tellurium intoxication.

A stable population of intraspinal Schwann cells, which developed following early postnatal irradiation of the spinal cord, was challenged by the addition of tellurium (Te) to the diet beginning at 30 days of age. Schwann cells incorporating [3H]thymidine were identified by 1 micron autoradiographs and by conventional electron microscopy of adjacent thin sections. Autoradiographs of areas with Schwann cell myelination showed extensive labelling of cells in the Te-fed animals. In contrast, control animals which were not fed Te showed little evidence of labelled Schwann cells. These data indicate that Schwann cells in the intraspinal environment show a proliferative response to the presence of Te in the rat's diet, as do Schwann cells in their normal extraspinal milieu.

Animals↗
PubMed1994

Evaluation of anemia caused by hemorrhoidal bleeding.

PURPOSE: Hemorrhoidal bleeding and anemia are two common entities. The purpose of this article is to review the incidence of hemorrhoidal bleeding that causes anemia in a stable population, to describe patients who have hemorrhoidal bleeding that caused anemia, and to define observations regarding the recovery of their anemia. METHOD: This was a retrospective chart review of patients seen at the Mayo Clinic and Olmsted Community Hospital in Rochester, Minnesota, from 1976 through 1990. RESULTS: Incidence of hemorrhoidal bleeding that caused anemia was found to be 0.5 patients per 100,000 population per year in Olmsted County from 1976 to 1990. The mean age of 43 patients studied was 50 (range, 25-72) years. Twenty-seven patients were male, and 16 were female. Ninety-three percent of these patients had grade 2 or 3 internal hemorrhoidal disease. Six had impaired coagulation. The mean hemoglobin concentration before treatment was 9.4 g/dl (+/- 0.97 SD). Of note was the description of hemorrhoidal bleeding that included blood squirting or clots passing in 84 percent of patients for whom a description was available. It was found that recovery from anemia after definitive treatment with hemorrhoidectomy was rapid, with a mean hemoglobin concentration of 12.3 g/dl after two months, and by six months, the mean hemoglobin concentration was 14.1 g/dl. CONCLUSION: It is imperative to evaluate other causes of anemia when presented with hemorrhoidal bleeding with anemia. If no other sources are identified, treatment of the hemorrhoid should be undertaken, and a rapid return of hemoglobin concentration can be expected within two months, and all patients should have a normal hemoglobin by six months. Failure to recover hemoglobin concentration should prompt further or repeated evaluation for other causes of bleeding.

Adult↗
PubMed1985

Decrease in alkaline triglyceride lipase in primary cultured hepatocytes from mice with sarcoma 180.

Primary cultured hepatocytes from normal mice and mice with Sarcoma 180 were characterized. The viability of freshly isolated hepatocytes from both sources was over 90% and the cells had a relatively stable population of DNA for a minimum of three days. After incubation with (3H)leucine, the syntheses and secretions of (3H)labeled trichloracetic acid-insoluble materials by hepatocytes from both normal and tumor-bearing mice increased similarly. However, the alkaline triglyceride lipase activity of a homogenate of freshly isolated hepatocytes from tumor-bearing mice was one-third that of cells from normal mice. The activity of hepatocytes from tumor-bearing mice increased less during culture than did the activity of cells isolated from normal mice.

Animals↗
PubMed1978

The regeneration of the cells of the macula densa after subtotal nephrectomy in the rat.

An autoradiographic study of the proliferative response of the cells of the Macula Densa days or months after the removal of 5/6 of the rat's kidney has shown that these cells are capable of division. The index of labelling of these cells is considerably lower than that of the other cells of the distal tubules. This applies of the kidney of normal controls and to the remnant after partial nephrectomy, shortly after operation as well as several months later. The cells of the Macula Densa therefore seem to represent a more stable population in the nephron. They therefore differ from the other cells of the distal tubules not only in appearance and function, but also in the pattern of cell proliferation.

Animals↗
PubMedDate not supplied

Decline in the incidence of late diagnosed congenital dislocation of the hip.

INTRODUCTION: We reviewed the incidence of early and late diagnosed CDH over a 13 yr period in a single maternity with a well established follow-up program and a stable population base. In March 1991, all newborn infants were nursed in the lateral or supine position, having previously been nursed prone. In addition, a dedicated hip screener was appointed in October 1992. We wished to determine whether these changes could have led to a reduction in late diagnosed CDH. METHODS: Babies with suspected CDH were seen by a single consultant orthopaedic surgeon within 1 week of birth. Splints were not applied until diagnosis was confirmed by the orthopaedic surgeon. The study period was from January 1983 to December 1995, inclusive. RESULTS: 37,383 babies were born during the study period, an average of 2,876 births per annum. The average rate of babies referred to the orthopaedic surgeon was 12.8 per 1,000 (range 5.5-28.2 per 1,000). The average incidence of babies splinted early was 7.19 per 1,000 (range 4.0-14.1 per 1,000) with no discernible increase or decrease over the yrs. In contrast, there was a clear reduction in the incidence of late diagnosed CDH; 42/24,713 births (January 1983-February 1992) compared to only 2/12,673 births (March 1992-December 1995; p < 0.0001. Twelve of the 44 infants (27 per cent) diagnosed with late diagnosed CDH were discovered by the policy of routine hip radiographs at 6 months of age of all babies born by breech presentation (2/12) or those with a positive family history of CDH in a first degree relative (10/12). Six others had a diagnosis of CDH confirmed by X-rays done at 6 months of age because of doubts about their examination in the newborn period. CONCLUSIONS: Nursing infants in the supine or lateral position and the introduction of a single hip screener were associated with a reduction in the incidence of late diagnosed CDH. Routine hip radiographs of high risk infants at 6 months of age proved to be a valuable safety net in detecting a significant proportion of previously undetected infants with CDH.

Breast Feeding↗
PubMed1993

The increasing incidence of infantile hypertrophic pyloric stenosis.

Several reports have suggested that there is an increasing incidence of infantile hypertrophic pyloric stenosis (IHPS). We examined the incidence of IHPS in a stable population in the West of Ireland over a ten year period (1981-1990). During the 10 years of the study the number of live births per year did not alter significantly. The incidence of IHPS increased significantly from 0.87/1000 live births in 1981 to 5.10/1000 in 1990 (p < 0.001, Student's test), peaking in 1989 at 6.8/1000. There was no statistical correlation between the increasing incidence and feeding habits, birth rank, family history or gender distribution. The reason(s) for this increase remain unclear.

Female↗
PubMed2006

Understanding recent trends in Swiss ambulatory care utilization when out-of-pocket payment is minimal.

OBJECTIVE: To examine trends in ambulatory care utilization when individuals face almost no financial barriers to health care. METHODS: Observational study of insurance data. Adults with minimal deductible were included. Ambulatory care visits and costs were measured from 1997 to 2002. RESULTS: Mean ambulatory care costs/insuree increased from 1292.- to 1790.- CHF, corresponding to higher increases in drug costs (+61.7 %) than services costs (+24.3 %). The proportion of visits to generalists decreased while those to hospital outpatient services increased. CONCLUSIONS: In a demographically stable population of insurees, increases in ambulatory care costs were due neither to growth in physicians' visits nor to increasing physicians' fees per act, but to what was included in or prescribed during the visits.

Adult↗
PubMed1997

Transmembrane insertion of the colicin Ia hydrophobic hairpin.

Colicin Ia is a bactericidal protein that forms voltage-dependent, ion-conducting channels, both in the inner membrane of target bacteria and in planar bilayer membranes. Its amino acid sequence is rich in charged residues, except for a hydrophobic segment of 40 residues near the carboxyl terminus. In the crystal structure of colicin Ia and related colicins, this segment forms an alpha-helical hairpin. The hydrophobic segment is thought to be involved in the initial association of the colicin with the membrane and in the formation of the channel, but various orientations of the hairpin with respect to the membrane have been proposed. To address this issue, we attached biotin to a residue at the tip of the hydrophobic hairpin, and then probed its location with the biotin-binding protein streptavidin, added to one side or the other of a planar bilayer. Streptavidin added to the same side as the colicin prevented channel opening. Prior addition of streptavidin to the opposite side protected channels from this effect, and also increased the rate of channel opening; it produced these effects even before the first opening of the channels. These results suggest a model of membrane association in which the colicin first binds with the hydrophobic hairpin parallel to the membrane; next the hairpin inserts in a transmembrane orientation; and finally the channel opens. We also used streptavidin binding to obtain a stable population of colicin molecules in the membrane, suitable for the quantitative study of voltage-dependent gating. The effective gating charge thus determined is pH-independent and relatively small, compared with previous results for wild-type colicin Ia.

Bacterial Proteins↗
PubMed2003

Male contraception: mechanical, hormonal and non-hormonal methods.

There is a definite need for an effective and reversible form of male contraception, both for maintaining a stable population in industrial countries and for diminishing population growth in developing countries. It has been agreed upon that contraception is an essential component of reproductive health for men and women (the Weimar Manifesto on Male Contraception). The development of new, effective methods of male contraception has been identified as a high priority by the WHO Task Force on methods of regulation of male fertility. Hormonal male contraception is based on suppression of gondotrophins and substitution of testosterone in order to maintain male sexual function and bone mineralisation and to prevent muscle waist. For complete interruption of spermatogenesis, an adequate suppression of intratesticular testosterone production is needed. Various contraceptive regimens have been developed and tested, including testosterone monotherapy, androgen/progestin combinations, testosterone with GnRH analogs, and selective androgen and progestin receptor modulators. The combination of testosterone with progestogen is currently the most promising approach to hormonal male contraception. Also, several non-hormonal approaches to male contraception are promising and may offer the foundation for developing new male contraceptives.

Condoms↗
PubMed1998

A new HPLC-based method for the quantitative analysis of inner stratum corneum lipids with special reference to the free fatty acid fraction.

The inner stratum corneum is likely to represent the location of the intact skin barrier, unperturbed by degradation processes. In our studies of the physical skin barrier a new high-performance liquid chromatography (HPLC)-based method was developed for the quantitative analysis of lipids of the inner stratum corneum. All main lipid classes were separated and quantitated by HPLC/light scattering detection (LSD) and the free fatty acid fraction was further analysed by gas-liquid chromatography (GLC). Mass spectrometry (MS) was used for peak identification and flame ionization detection (FID) for quantitation. Special attention was paid to the free fatty acid fraction since unsaturated free fatty acids may exert a key function in the regulation of the skin barrier properties by shifting the physical equilibrium of the multilamellar lipid bilayer system towards a noncrystalline state. Our results indicated that the endogenous free fatty acid fraction of the stratum corneum barrier lipids in essence exclusively consisted of saturated long-chain free fatty acids. This fraction was characterized as a very stable population (low interindividual peak variation) dominated by saturated lignoceric acid (C24:0, 39 molar%) and hexacosanoic acid (C26:0, 23 molar%). In addition, trace amounts of very long-chain (C32-C36) saturated and monounsaturated free fatty acids were detected in human forearm inner stratum corneum. Our analysis method gives highly accurate and precise quantitative information on the relative composition of all major lipid species present in the skin barrier. Such data will eventually permit skin barrier model systems to be created which will allow a more detailed analysis of the physical nature of the human skin barrier.

2-Propanol↗
PubMed2004

Structure and dynamics of an ancient montane forest in coastal British Columbia.

Old-growth forests are common in the snowy, montane environments of coastal western North America. To examine dynamics of a stand containing four canopy tree species (Abies amabilis, Chamaecyparis nootkatensis, Tsuga mertensiana and T. heterophylla), we used four stem-mapped, 50 m x 50 m plots. From measurements of annual rings, we obtained ages from basal discs of 1,336 live trees, developed master chronologies for each species, reconstructed early growth rates, and delineated periods of release. The stand was ancient; individuals of all four species exceeded 900 years in age, and the oldest tree exceeded 1,400 years. The four plots differed in the timing of events, and we found no evidence of major, stand-level disturbance. Instead the stand was structured by small-scale patch dynamics, resulting from events that affected one to several trees and initiated episodes of release and relatively rapid early growth. The species differed in age structure and dynamics. A. amabilis and T. heterophylla had a classical reverse-J age structure indicative of stable populations, whereas C. nootkatensis and T. mertensiana appeared to rely on local episodes of increased recruitment, which were often separated by centuries, and were probably related to multiple-tree gaps that occurred infrequently. However, such gaps could be considered normal in the long-term history of the stand, and thus these species with their long life spans can persist. Most individuals of all four species grew extremely slowly, with trees typically spending centuries in the understory before reaching the canopy, where they were able to persist for additional centuries. Thus, the key features of this forest are the very slow dynamics dominated by small-scale events, and the slow growth of stress-tolerant trees.

Age Factors↗
PubMedDate not supplied

Stable transformation of insect cells to coexpress a rapidly selectable marker gene and an inhibitor of apoptosis.

We have constructed several plasmid expression vectors to express foreign genes in stably transformed insect cells. Unlike baculovirus-based expression vectors by which genes of interest are expressed transiently before lysis of the virus-infected cells, genes can be expressed continuously over many passages in a stable cell line. Furthermore, the function of a gene or genes expressed in a stable cell line from an insect-specific promoter that is constitutively expressed can be studied in the absence of virus infection and viral gene expression. In this study, we have expressed a novel, selectable marker gene, puromycin acetyltransferase, under the control of the Drosophila melanogaster hsp 70 promoter or under the control of the AcMNPV ie-1 promoter which is active in Spodoptera frugiperda cells in the absence of virus infection. In addition, we have constructed expression vectors which coexpress two genes from separate promoters, the pac gene which confers resistance to puromycin and a baculovirus gene which inhibits apoptosis, derived from Orygia pseudotsugata nuclear polyhedrosis virus. Both genes were expressed in stable populations of S. frugiperda cells in the absence of continuous drug selection.

Acetyltransferases↗