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排序方式: 共有111条查询结果,搜索用时 15 毫秒
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Detecting non-neutral heterogeneity across a region of DNA sequence in the ratio of polymorphism to divergence 总被引:11,自引:4,他引:7
Natural selection, in the form of balancing selection or selective sweeps,
can result in a decoupling of the amounts of molecular polymorphism and
divergence. Thus natural selection can cause some areas of DNA sequence to
have greater silent polymorphism, relative to divergence between species,
than other areas. It would be useful to have a statistical test for
heterogeneity in the polymorphism to divergence ratio across a region of
DNA sequence, one that could identify heterogeneity greater than that
expected from the neutral processes of mutation, drift, and recombination.
The only currently available test requires that a region be arbitrarily
divided into sections that are compared with each other, and the
subjectivity of this division could be problematic. Here a test is proposed
in which runs of polymorphic and fixed sites are counted, where a "run" is
a set of one or more sites of one type preceded and followed by the other
type. The number of runs is smaller than otherwise expected if
polymorphisms are clumped together. By simulating neutral evolution and
comparing the observed number of runs to the simulations, a statistical
test is possible which does not require any a priori decisions about
subdivision.
相似文献
3.
Summary Suspensions of collagen fibrils obtained from derma of Elasmobranchia and Actinopterygia of different body sizes and developmental stages were examined by transmission electron microscopy. Fibril diameters were measured and classified into groups comprising a 20 nm diameter interval. Diagrams showing fibril populations of each fish were made. The measurements were averaged and their confidence intervals and standard errors determined. For each species other diagrams were plotted in which the mean diameters were correlated to the body length of each sample. The results show that: 1) a correlation exists between an increase in diameter of collagen fibrils and somatic growth until sexual maturity is reached; 2) fibril populations are subsequently spread over a wider range due to the presence in the derma of classes of newly formed and therefore thinner fibrils. The deposition of new fibrils is possibly influenced by individual factors; 3) no relationship exists between mean fibril diameter and body size; 4) no relationship exists between phylogenetic position and pattern of diameter distribution.Research supported by a grant from C.N.R. Roma (69.02087.0115.1150) 相似文献
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Janine JH Oosterhof G Jolanda Elving Ietse Stokroos Arie van nieuw Amerongen Henny C van der Mei Henk J Busscher 《Biofouling》2013,29(6):347-353
The integrity of biofilms on voice prostheses used to rehabilitate speech in laryngectomized patients causes unwanted increases in airflow resistance, impeding speech. Biofilm integrity is ensured by extracellular polymeric substances (EPS). This study aimed to determine whether synthetic salivary peptides or mucolytics, including N-acetylcysteine and ascorbic acid, influence the integrity of voice prosthetic biofilms. Biofilms were grown on voice prostheses in an artificial throat model and exposed to synthetic salivary peptides, mucolytics and two different antiseptics (chlorhexidine and Triclosan). Synthetic salivary peptides did not reduce the air flow resistance of voice prostheses after biofilm formation. Although both chlorhexidine and Triclosan reduced microbial numbers on the prostheses, only the Triclosan-containing positive control reduced the air flow resistance. Unlike ascorbic acid, the mucolytic N-acetylcysteine removed most EPS from the biofilms and induced a decrease in air flow resistance. 相似文献
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Improved tests for heterogeneity across a region of DNA sequence in the ratio of polymorphism to divergence 总被引:15,自引:9,他引:6
The neutral theory of molecular evolution predicts that the ratio of
polymorphisms to fixed differences should be fairly uniform across a region
of DNA sequence. Significant heterogeneity in this ratio can indicate the
effects of balancing selection, selective sweeps, mildly deleterious
mutations, or background selection. Comparing an observed heterogeneity
statistic with simulations of the heterogeneity resulting from random
phylogenetic and sampling variation provides a test of the statistical
significance of the observed pattern. When simulated data sets containing
heterogeneity in the polymorphism-to-divergence ratio are examined,
different statistics are most powerful for detecting different patterns of
heterogeneity. The number of runs is most powerful for detecting patterns
containing several peaks of polymorphism; the Kolmogorov-Smirnov statistic
is most powerful for detecting patterns in which one end of the gene has
high polymorphism and the other end has low polymorphism; and a newly
developed statistic, the mean sliding G statistic, is most powerful for
detecting patterns containing one or two peaks of polymorphism with reduced
polymorphism on either side. Nine out of 27 genes from the Drosophila
melanogaster subgroup exhibit heterogeneity that is significant under at
least one of these three tests, with five of the nine remaining significant
after a correction for multiple comparisons, suggesting that detectable
evidence for the effects of some kind of selection is fairly common.
相似文献
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Bongertz V Ouverney EP Fernandez SC Grinsztejn B Veloso V Couto-Fernandez JC Pilotto JH Morgado MG 《Memórias do Instituto Oswaldo Cruz》2007,102(7):817-825
Highly active antiretroviral treatment (HAART) of human immunodeficiency type 1 (HIV-1) infection is very effective in controlling infection, but elimination of viral infection has not been achieved as yet, and upon treatment interruption an immediate rebound of viremia is observed. A combination of HAART with an immune stimulation might allow treatment interruption without this rebounding viremia, as the very low viremias observed with successful HAART may be insufficient to permit maintenance of a specific anti-HIV-1 immune response. The objective of this study was to compare the humoral immune response of individuals undergoing successful HAART (NF=no failure) with that of individuals with evidence of failure of therapy (FT) and to verify if the viremia peaks observed in individuals with therapy failure would act as a specific stimulus for the humoral anti-HIV-1 immune response. Antibodies binding to gp120 V3 genotype consensus peptides were more frequently observed for FT, mainly against peptides corresponding to sequences of genotypes prevalent in the Rio de Janeiro city area, B and F. HIV-1 neutralization of HIV-1 IIIB and of four primary isolates from Rio de Janeiro was less frequently observed for plasma from the NF than the FT group, but this difference was more expressive when plasma from individuals with detectable viremia were compared to that of individuals with undetectable viral loads in the year before sample collection. Although statistically significant differences were observed only in some specific comparisons, the study indicates that presence of detectable viremia may contribute to the maintenance of a specific anti-HIV-1 humoral immune response. 相似文献