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At least 37 records · Page 2Linked to original sources
PubMed1980

Density-dependent selection in a random environment: An evolutionary process that can maintain stable population dynamics.

A theoretical analysis of natural selection is presented in which fitnesses depend on population density and randomly varying environmental processes. The theory is based on a general, heuristic analysis of a pair of coupled, nonlinear, stochastic difference equations that describe the joint dynamics of allele frequencies and population size. Four main conclusions emerge from the investigation of a particular class of models: (i) growth rates at low population densities tend to increase; (ii) individual selection, given sufficient genetic flexibility, will mold growth rates at higher densities so that in spite of i, stable deterministic population dynamics are maintained; (iii) "more fit" genotypes cannot be simply characterized-in particular, the mean population size need not be increased; and (iv) genetic polymorphisms can be maintained in both haploid and diploid organisms.

Journal Article↗
PubMed1987

Relatively stable population of c-myc RNA that lacks long poly(A).

We examined the turnover of c-myc RNA in the human promyelocytic cell line HL-60. In whole-cell RNA from rapidly growing cells we observed two major size classes of c-myc RNA, 2.4 and 2.2 kilobases (kb). When HL-60 cells were treated with actinomycin D for 30 min to inhibit transcription, the 2.4-kb c-myc RNA population was rapidly degraded, while the 2.2-kb c-myc RNA was degraded much more slowly. S1 nuclease transcript mapping and promoter-specific probes were utilized to show that both the stable 2.2-kb and the labile 2.4-kb c-myc RNA populations have 5' ends at the second promoter site (P2) and 3' ends at the second poly(A) addition site. To examine further possible structural differences between these two RNA populations, we fractionated RNA on an oligo(dT)-cellulose column to separate RNAs that contained long poly(A) tails from those which did not. We found that the labile 2.4-kb c-myc RNA population bound to oligo(dT)-cellulose, while the more stable 2.2-kb c-myc RNA population did not. Preliminary estimates of their half-lives (t1/2) showed that the poly(A)+ c-myc RNA had a t1/2 of 12 min, while the c-myc RNA that did not bind to oligo(dT)-cellulose had a t1/2 of greater than 1 h. Several other cell types contain both poly(A)+ and nonpoly(A)+ c-myc RNAs including HeLa cells, normal human bone marrow cells, and normal mouse fetal liver cells. In agreement with the results in HL-60 cell, HeLa cell poly(A)+ c-myc RNA was more labile than c-myc RNA that lacked poly(A). The stable, nonpoly(A)+ c-myc RNA population may be important in the posttranscriptional regulation of c-myc expression.

Cell Line↗
PubMed1991

Epidemiologic trends in multiple sclerosis in Møre and Romsdal, Norway: a prevalence/incidence study in a stable population.

The western part of Norway has been a low-to medium-frequency area for multiple sclerosis (MS). The prevalence of definite/probable MS on January 1, 1961, was 24.3/100,000 in the county of Møre and Romsdal, western Norway. Based on the same diagnostic criteria, the prevalence of definite/probable MS increased to 75.4/100,000 on January 1, 1985. The average annual incidence rate increased from 1.94/100,000 in the period 1950-1954 to 3.78/100,000 from 1975-1979. Remitting MS in the younger age groups of both sexes increased the most. We consider this increase of MS to be due to alteration in exogenous factors as variation in genetic susceptibility cannot account for the increase in the stable western Norwegian population. The rise in prevalence/incidence over the last 20 to 25 years in western Norway supports the theory that MS is a disease influenced by exogenous factors that show variation over time.

Adolescent↗
PubMed1987

Dynamic and stable populations of microtubules in cells.

Using a new immunocytochemical technique, we have visualized the spatial arrangement of those microtubules in cells that are stable to biotin-tubulin incorporation after microinjection. Cells fixed at various periods of time after injection were exposed to antibody to biotinylated tubulin and several layers of secondary antibodies; these layers prevented reaction of biotin-containing microtubules with antitubulin antibodies. The microtubules that had not incorporated biotin-tubulin could then be stained with anti-tubulin and a fluorescent secondary antibody. In BSC1 cells, most microtubules in the cell exchange with a half-time of 10 min. A separate population of microtubules can be detected, using the above techniques, that are stable to exchange for 1 h or more; these have a characteristic pericentrosomal spatial arrangement as compared to the majority of dynamic microtubules. Unlike the dynamic microtubules, most of the stable microtubules are nongrowing. The average BSC-1 cell contains approximately 700 microtubules: approximately 500 growing at 4 micron min-1, 100 shrinking at approximately 20 micron min-1, and approximately 100 that are relatively more stable to exchange. The potential significance of these stable microtubules is discussed.

Animals↗
PubMed1996

Comparison of mortality from acute myocardial infarction between 1979 and 1992 in a geographically defined stable population.

This study documents mortality from acute myocardial infarction (AMI), in hospital and at 1 year, for each of 3 selected 1-year periods in a stable community over a 13-year period beginning in 1979 and continuing into the thrombolytic era, to detect any changes occurring in conjunction with the introduction of new therapies. Every patient with AMI occurring in a geographically defined stable community (Hamilton, Ontario, Canada) in 3 1-year periods (1979 to 1980 [n = 816], 1986 to 1987 [n = 816], and 1991 to 1992 [n = 831]) was identified and clinically characterized by standardized criteria. Subsequent in-hospital and 1-year survival were ascertained prospectively. The 3 cohorts were similar in prognostic factors. Mean age was progressively greater over the study period from 63 years in 1979 to 1980, to 67 years in 1991 to 1992 (p = 0.02). There was no change in in-hospital mortality rates from 1979 to 1980 (17%) and 1986 to 1987 (16%). However, from 1986 to 1987 and 1991 to 1992, in-hospital mortality decreased from 16% to 9% (p < 0.001) and 1-year mortality decreased from 26% to 19% (p < 0.001). For patients who survived the hospital phase of AMI, 1-year mortality did not change and was between 11% and 12% in each of the 3 study periods. From 1986 to 1987 and 1991 to 1992, there was an increase in the use of thrombolytic therapy from 5% to 44% of patients. The acute use of aspirin increased from 30% to 88% and the acute use of beta blockers increased from 19% to 48% of patients. The observed increase in use of these agents could account for half of the actual mortality reduction observed. This prospective population-based survey demonstrates improved in-hospital survival after AMI associated with increased use of established effective therapies between 1987 and 1992. The 1-year mortality of hospital survivors of AMI was unchanged throughout the period of study, remaining at 11% to 12%.

Age Factors↗
PubMed2006

Stochastic stable population growth in integral projection models: theory and application.

Stochastic matrix projection models are widely used to model age- or stage-structured populations with vital rates that fluctuate randomly over time. Practical applications of these models rest on qualitative properties such as the existence of a long term population growth rate, asymptotic log-normality of total population size, and weak ergodicity of population structure. We show here that these properties are shared by a general stochastic integral projection model, by using results in (Eveson in D. Phil. Thesis, University of Sussex, 1991, Eveson in Proc. Lond. Math. Soc. 70, 411-440, 1993) to extend the approach in (Lange and Holmes in J. Appl. Prob. 18, 325-344, 1981). Integral projection models allow individuals to be cross-classified by multiple attributes, either discrete or continuous, and allow the classification to change during the life cycle. These features are present in plant populations with size and age as important predictors of individual fate, populations with a persistent bank of dormant seeds or eggs, and animal species with complex life cycles. We also present a case-study based on a 6-year field study of the Illyrian thistle, Onopordum illyricum, to demonstrate how easily a stochastic integral model can be parameterized from field data and then applied using familiar matrix software and methods. Thistle demography is affected by multiple traits (size, age and a latent "quality" variable), which would be difficult to accommodate in a classical matrix model. We use the model to explore the evolution of size- and age-dependent flowering using an evolutionarily stable strategy (ESS) approach. We find close agreement between the observed flowering behavior and the predicted ESS from the stochastic model, whereas the ESS predicted from a deterministic version of the model is very different from observed flowering behavior. These results strongly suggest that the flowering strategy in O. illyricum is an adaptation to random between-year variation in vital rates.

Ecosystem↗
PubMed1997

Is there a benefit to episiotomy at operative vaginal delivery? Observations over ten years in a stable population.

OBJECTIVE: Our purpose was to examine the association between maternal vaginal and perineal morbidity and episiotomy performed at operative vaginal delivery. STUDY DESIGN: We obtained data from 2041 consecutive operative vaginal deliveries and compared yearly rates of episiotomy, lacerations, and potential confounders with linear regression and stratified analyses. RESULTS: Between 1984 and 1994 the use of episiotomy for operative vaginal deliveries fell significantly (93.4% to 35.7%, R2 = 0.85, p = 0.0001). This change was associated with a rise in the rate of vaginal lacerations (16.1% to 40.0%, R2 = 0.80, p = 0.0002), a decrease in the rate of fourth-degree lacerations (12.2% to 5.4%, R2 = 0.62, p = 0.004), but no significant change in the rate of third-degree lacerations. These associations held in separate analyses stratified by parity and type of instrument used for delivery. The prevalence of other previously reported risks for perineal morbidity did not change during the study period. CONCLUSION: At our institution a statistically and clinically significant reduction in the use of episiotomy for operative vaginal deliveries was not associated with a change in the rate of third-degree lacerations but was associated with an increase in the rate of vaginal lacerations and a decrease in the rate of fourth-degree lacerations.

Confounding Factors, Epidemiologic↗
PubMed2001

The generation and characterization of a cell line derived from a sporadic renal angiomyolipoma: use of telomerase to obtain stable populations of cells from benign neoplasms.

Angiomyolipomas are benign tumors of the kidney derived from putative perivascular epithelioid cells, that may undergo differentiation into cells with features of melanocytes, smooth muscle, and fat. To gain further insight into angiomyolipomas, we have generated the first human angiomyolipoma cell line by sequential introduction of SV40 large T antigen and human telomerase into human angiomyolipoma cells. These cells show phenotypic characteristics of angiomyolipomas, namely differentiation markers of smooth muscle (smooth muscle actin), adipose tissue (peroxisome proliferator-activator receptor gamma, PPARgamma), and melanocytes (microophthalmia, MITF), thus demonstrating that a single cell type can exhibit all of these phenotypes. These cells should serve as a valuable tool to elucidate signal transduction pathways underlying renal angiomyolipomas.

Actins↗
PubMed1972

Group selection on the boundary of a stable population.

A model of group selection is constructed for the case of differential extinction acting on small boundary populations of a large, fixed population. Consideration is restricted to extinction operators acting at or near to carrying capacity. Under the assumption that the extinction rate is large relative to individual genetic parameters affecting gene frequencies in boundary populations, we discuss the conditions under which differential extinction is most likely to produce a significant effect. In particular, a condition for bimodality in the distribution of gene frequencies in boundary populations (population polymorphism) is that there be some critical allele frequency at which the extinction rate jumps from high to low. An extinction operator linear in allele frequency produces no qualitative effect of this kind. In consequence, we are able to make precise rather limited circumstances under which group selection is likely to have a significant effect.

Journal Article↗