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Biomedical subjects

R Bürger

Publications and source records attributed to R Bürger.

12 recordsLinked to original sources
PubMed1992

Survival of small populations under demographic stochasticity.

We estimate the mean time to extinction of small populations in an environment with constant carrying capacity but under stochastic demography. In particular, we investigate the interaction of stochastic variation in fecundity and sex ratio under several different schemes of density dependent population growth regimes. The methods used include Markov chain theory, Monte Carlo simulations, and numerical simulations based on Markov chain theory. We find a strongly enhanced extinction risk if stochasticity in sex ratio and fluctuating population size act simultaneously as compared to the case where each mechanism acts alone. The distribution of extinction times deviates slightly from a geometric one, in particular for short extinction times. We also find that whether maximization of intrinsic growth rate decreases the risk of extinction or not depends strongly on the population regulation mechanism. If the population growth regime reduces populations above the carrying capacity to a size below the carrying capacity for large r (overshooting) then the extinction risk increases if the growth rate deviates from an optimal r-value.

Animals↗
PubMed1992

[Cystic space-occupying lesions of the seminal vesicles. Their demonstration with MRT].

Between October 1990 and March 1991, 8 patients with cystic lesions in the region of the posterior bladder wall, the seminal vesicles and the prostate were examined by MRI. In all cases there was accurate characterisation of the lesion and of its anatomy. In 3 patients with abnormalities in the retrovesical space there was very good agreement with the operative findings. 4 patients were treated conservatively. In these patients there was good agreement with the findings on cystoscopy, CT and sonography.

Adult↗
PubMed1992

The appendix as a continence mechanism.

Although we have progressed very well in creating large capacity, low pressure reservoirs, the construction of a simple and reliable continent outlet still remains a problem. The appendix vermiformis serves well as a continence mechanism for either the bladder or intestinal reservoirs for urine. The different surgical techniques described in the literature are reviewed and discussed in this context. Moreover, we report on our clinical and experimental results of using the appendix during the Mainz pouch procedure for continent urinary diversion.

Adolescent↗
PubMed1992

[Kinetic study of the lumbar vertebrae and the lumbosacral passage in German shepherd dogs. 1. Functional anatomy and kinetic foundation].

In the dog congenital or acquired stenosis of the lumbosacral region is commonly encountered. In humans the lumbosacral junction is the most often affected part of the vertebral column. Lumbosacral instability, one possible etiology for stenosis of the vertebral canal plays an important role in man. This study summarizes the functional anatomy and some important geometric and kinematic considerations necessary for the understanding of the motion (flexion-extension) between two adjacent vertebra.

Animals↗
PubMed1991

Moments, cumulants, and polygenic dynamics.

A new approach for describing the evolution of polygenic traits subject to selection and mutation is presented. Differential equations for the change of cumulants of the allelic frequency distribution at a particular locus and for the cumulants of the distributions of genotypic and phenotypic values are derived. The derivation is based on the assumptions of random mating, no sex differences, absence of random drift, additive gene action, linkage equilibrium, and Hardy-Weinberg proportions. Cumulants are a set of parameters that, like moments, describe the shape of a probability density. Compared with moments, however, they have properties that make them a much more convenient tool for investigating polygenic traits. Applications to directional and stabilizing selection are given.

Alleles↗
PubMed1991

Surgical aspects of urolithiasis in children.

Operative management of renal calculi has changed radically in the last decade. New operative techniques, the use of intra-operative ultrasound and Doppler sonography have even made possible the removal of complete staghorn calculi in short and kidney-preserving procedures. The introduction of percutaneous nephrolithotomy and stone extraction using a specially designed ureteroscope and, above all, the clinical application of extracorporeal shock wave lithotripsy have made stone surgery less invasive and have provided a higher rate of preservation of kidney function.

Endoscopy↗
PubMed1990

Continent appendix stoma: a modification of the Mainz pouch technique.

The submucosally embedded in situ appendix guarantees an ideal continence mechanism in patients with ileocecal urinary reservoirs. To date this modification of the Mainz pouch technique has been performed successfully in 13 patients. The appendicocutaneous stoma was placed at the lower right abdominal quadrant in 12 patients and into the umbilical funnel in 1. Urodynamic investigations of the submucosally embedded appendix showed a maximum closure pressure of more than 80 cm. water in pressure profiles. Only 1 case of stomal stenosis required reoperation.

Adult↗
PubMed1989

Linkage and the maintenance of heritable variation by mutation-selection balance.

The role of linkage in influencing heritable variation maintained through a balance between mutation and stabilizing selection is investigated for two different models. In both cases one trait is considered and the interactions within and between loci are assumed to be additive. Contrary to most earlier investigations of this problem no a priori assumptions on the distribution of genotypic values are imposed. For a deterministic two-locus two-allele model with recombination and mutation, related to the symmetric viability model, a complete nonlinear analysis is performed. It is shown that, depending on the recombination rate, multiple stable equilibria may coexist. The equilibrium genetic and genic variances are calculated. For a polygenic trait in a finite population with a possible continuum of allelic effects a simulation study is performed. In both models the equilibrium genetic and genic variances are roughly equal to the house-of-cards prediction or its finite population counterpart as long as the recombination rate is not extremely low. However, negative linkage disequilibrium builds up. If the loci are very closely linked the equilibrium additive genetic variance is slightly lower than the house-of-cards prediction, but the genic variance is much higher. Depending on whether the parameters are in favor of the house-of-cards or the Gaussian approximation, different behavior of the genetic system occurs with respect to linkage.

Alleles↗
PubMed1986

On the maintenance of genetic variation: global analysis of Kimura's continuum-of-alleles model.

Methods of functional analysis are applied to provide an exact mathematical analysis of Kimura's continuum-of-alleles model. By an approximate analysis, Kimura obtained the result that the equilibrium distribution of allelic effects determining a quantitative character is Gaussian if fitness decreases quadratically from the optimum and if production of new mutants follows a Gaussian density. Lande extended this model considerably and proposed that high levels of genetic variation can be maintained by mutation even when there is strong stabilizing selection. This hypothesis has been questioned recently by Turelli, who published analyses and computer simulations of some multiallele models, approximating the continuum-of-alleles model, and reviewed relevant data. He found that the Kimura and Lande predictions overestimate the amount of equilibrium variance considerably if selection is not extremely weak or mutation rate not extremely high. The present analysis provides the first proof that in Kimura's model an equilibrium in fact exists and, moreover, that it is globally stable. Finally, using methods from quantum mechanics, estimates of the exact equilibrium variance are derived which are in best accordance with Turelli's results. This shows that continuum-of-alleles models may be excellent approximations to multiallele models, if analysed appropriately.

Alleles↗
PubMed1986

Evolutionary dynamics of functionally constrained phenotypic characters.

A nonlinear analysis is performed, employing the theory of Lyapunov functions, to examine the relative importance of genetic and phenotypic covariance matrices for the evolution of functionally coupled quantitative traits in an adaptive topography with several directions of increasing fitness. The analysis is based on Lande's evolution equations for phenotypic characters. It is supposed that evolution of a set of functionally constrained characters far from equilibrium corresponds to evolution along a ridge in the fitness landscape. It is shown that the pattern of variation and covariation restricts the possible directions of evolutionary change in the following sense. Any population starting sufficiently near the ridge will evolve along it, provided that one eigenvector of the genetic covariance matrix and one eigenvector of the phenotypic covariance matrix point into the direction of the ridge. Otherwise, the set of initial positions of a population enabling evolution along the ridge is more or less restricted, depending on the degree of deviation of the eigenvectors from the direction of the ridge. Moreover, too much phenotypic variance of the characters under stabilizing selection may inhibit any evolution along the ridge. Thus, the present analysis establishes population-genetic prerequisites and constraints for the evolution of functionally constrained phenotypic traits.

Biological Evolution↗
PubMed1985

On the evolution of dominance modifiers II: a non-equilibrium approach to the evolution of genetic systems.

The evolution of dominance is both the simplest and best investigated example of the evolution of genetic systems. Nevertheless, there exists striking empirical material, e.g. industrial melanism, for which no satisfactory explanation could so far be provided. In this paper we take an approach to this classical problem based on a global analysis together with computer simulations. It reveals that during the evolution of dominance one has to distinguish a "nonequilibrium phase" and a "Fisherian phase". The non-equilibrium phase appears to be characterized by the fact that in general the selection intensity at the primary locus does not affect the degree of modifier selection but only the time necessary for passing through this phase. A further essential conclusion is that modifier evolution only obtains a reasonable amount of efficiency if the population reaches the Fisherian phase already with a high modifier frequency. Using these results, predictions on the population genetic prerequisites for the evolution of dominance are derived. From these we conclude that even in populations in which dominance evolution has occurred it cannot be expected that back-crosses into relics of the ancestral population lead to a breakdown of dominance within a few generations. These predictions are in accordance with empirical data on Biston betularia and Odontopera bidentata.

Animals↗