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991.
Stephen R. Palumbi 《Journal of molecular evolution》1989,29(2):180-187
Summary Selective constraints on DNA sequence change were incorporated into a model of DNA divergence by restricting substitutions to a subset of nucleotide positions. A simple model showed that both mutation rate and the fraction of nucleotide positions free to vary are strong determinants of DNA divergence over time.When divergence between two species approaches the fraction of positions free to vary, standard methods that correct for multiple mutations yield severe underestimates of the number of substitutions per site. A modified method appropriate for use with DNA sequence, restriction site, or thermal renaturation data is derived taking this fraction into account. The model also showed that the ratio of divergence in two gene classes (e.g., nuclear and mitochondrial) may vary widely over time even if the ratio of mutation rates remains constant.DNA sequence divergence data are used increasingly to detect differences in rates of molecular evolution. Often, variation in divergence rate is assumed to represent variation in mutation rate. The present model suggests that differing divergence rates among comparisons (either among gene classes or taxa) should be interpreted cautiously. Differences in the fraction of nucleotide positions free to vary can serve as an important alternative hypothesis to explain differences in DNA divergence rates. 相似文献
992.
Robert L. Charlebois Ronald E. Subden Kenneth Carey 《Journal of industrial microbiology & biotechnology》1987,2(3):167-174
Summary The fitness distribution of new mutations inSaccharomyces cerevisiae strain Montrachet was determined for cells on agar irradiated for four periods of time with ultraviolet light. The fitness distributions were obtained by converting a large number of colony diameters into relative fitnesses. The distributions were then used to perform a computer simulation with the purpose of predicting the potential of a stock culture to increase in general fitness through selection, given a frequency and magnitude of mutations. 相似文献
993.
K. C. Engvild 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,69(5-6):457-461
Summary Progeny from crosses of Nicotiana langsdorffii with gamma irradiated pollen of Nicotiana alata Crimson Bedder showed skewed segregation in the F2 favoring the maternal parent. This is probably not gene transfer in a strict sense, rather just an extreme case of reduced transmission of irradiated chromosomes, leading to massive overrepresentation of maternal genes. Gene transfer or mutational loss may explain some anomalous F1 plants. Segregation in the F2 progeny showed the presence of several genes from the irradiated pollen. Crosses of Nicotiana sylvestris, N. plumbaginifolia N. paniculata, and Petunia parodii with irradiated pollen from N. alata and Petunia hybrida showed no evidence of gene transfer, nor did experiments with irradiated mentor pollen. This indicates that gene transfer with irradiated pollen between non-crossing species or between species giving sterile hybrids is probably a rare phenomenon. 相似文献
994.
Summary
Escherichia coli K803 cells were mutagenized and screened for the presence of clones sensitive to -rays but not to ultraviolet light. One new mutant of this type, named rorB, was isolated. This mutant is both cross-sensitive to mitomycin C and shows reduced conjugal recombination frequencies, but to a lesser extent than the phenotypically similar mutant recN. Unlike previously reported mutants of E. coli or yeast with an enhanced sensitivity to ionising radiations, rorB appears to be near wild type in ability to rejoin DNA double-strand breaks. The rorB gene maps close to ilvGEDAC at 84.5 min of the E. coli chromosome. 相似文献
995.
Richard Holmquist 《Journal of molecular evolution》1972,1(3):211-222
Summary The stochastic model of molecular evolution was used to makea priori predictions for the total number of one-step nucleotide changes required to account for a given number of amino acid substitutions between two homologous proteins. These predictions are now found to be concordant with empirical data summarized by Dayhoff, Eck and Park (1969). Correction factors are derived for adjusting the leg lengths of phylogenetic trees. It is shown that the operations of constructing the phylogenetic tree and applying the correction algorithm are not commutative with respect to obtaining the leg lengths. The effect of this on certain published phylogenies is discussed. It is suggested that, as a first approximation, at any given point in evolutionary time, enthalpic (selective) forces determine the number and position of those codon sites which are free to vary, whereas within these variable sites, entropic (random) processes determine the course of evolution at the molecular level. 相似文献
996.
Summary The problem of choosing an alignment of two or more nucleotide sequences is particularly difficult for nucleic acids, such as 5S ribosomal RNA, which do not code for protein and for which secondary structure is unknown. Given a set of costs for the various types of replacement mutations and for base insertion or deletion, we present a dynamic programming algorithm which finds the optimal (least costly) alignment for a set of N sequences simultaneously, where each sequence is associated with one of the N tips of a given evolutionary tree. Concurrently, protosequences are constructed corresponding to the ancestral nodes of the tree. A version of this algorithm, modified to be computationally feasible, is implemented to align the sequences of 5S RNA from nine organisms. Complete sets of alignments and proto-sequence reconstructions are done for a large number of different con-figurations of mutation costs. Examination of the family of curves of total replacements inferred versus the ratio of transitions/trans-versions inferred, each curve corresponding to a given number of in-sertions-deletions inferred, provides a method for estimating relative costs and relative frequencies for these different types of mutation. 相似文献
997.
Walter M. Fitch 《Journal of molecular evolution》1973,2(2-3):181-186
Summary The distribution of human hemoglobin variants has previously been studied by Vogel (1972) who concluded that the distribution was random although no statistical analysis was presented. This work points out that there are four biases in the data, one in the manner in which the number of variants is counted, another in the method by which they are detected and which favors charge changes, a third in the fact that for a few codons the same amino acid replacement may be brought about by two or three single nucleotide replacements, and a fourth in the non-random sampling procedure which favors variants producing clinical symptoms. Nevertheless, the distribution of beta hemoglobin variants is confirmed to be random as Vogel suggests. The alpha hemoglobin variants are distinctly non-randomly distributed, the best fit requiring that 69 of the alpha positions be considered invariable. The above biases could account for this result but other considerations combine to suggest the following: 1, about half of all alterations of alpha hemoglobin will not survive to sampling whereas nearly all beta variants can; 2, deleterious mutants that survive to sampling but are destined to be eliminated by selection are more likely to be observed in beta than in alpha hemoglobin; and 3, mutations destined to go to fixation are more likely to occur in beta than in alpha hemoglobin. 相似文献
998.
Summary One of the major goals of population genetics is to discover the nature and amount of genetic variation in natural populations. Various measures, including the population heterozygosity at any locus and the number of alleles extant at the locus, have been used for this purpose. An important task of theoretical population genetics is thus to provide expressions for the mean values of these two quantities (when calculated from a sample of genes) for various models of selection, mutation and random drift. This aim has been achieved for the selectively neutral case, where all alleles at the locus are assumed to be selectively equivalent. It is, however, generally agreed that classes of (evolutionarily unimportant) selectively deleterious alleles exist, so that the neutral theory calculations should be extended to cover this case. This has previously been done only for extremely weak selection. In this paper we obtain, via the confluent hypergeometric function and three allied functions, concise and simple exact and approximate formulae for the means of the above measures of population variation for arbitrary selective values. These all derive from the allelic frequency spectrum, which is of independent interest in assessing likely models of population variation. 相似文献
999.
Barry G. Hall 《Journal of molecular evolution》1995,40(1):86-93
Selection-induced mutations, sometimes called directed, adaptive, or Cairnsian mutations, are spontaneous mutations that occur as specific responses to environmental challenges, usually during periods of prolonged stress, and that occur more often when they are selectively advantageous than when they are selectively neutral. In this study I show that lesions in uvrA, uvrB, uvrC, or uvrD increase the mutation rate from trpA46 to trpA
+ by 102– to 104–fold during tryptophan starvation, but those same lesions do not affect random mutation rates in growing cells when tryptophan is present. The increased selection-induced mutation rates remain specific to the gene that is under selection in that no increase in the mutation rate from trpA46 to trpA
+ is detected during proline starvation.Evidence is presented showing that proline starvation produces a state of cellular stress which results in a burst of mutations from trpA46 to trpA
+ when proline-starved cells are plated onto medium lacking tryptophan but containing proline.These results are consistent with the hypermutable state model for selection-induced mutagenesis. 相似文献
1000.
Mirjana Tosic Michel Dolivo Krystyna Domaska-Janik Jean-Marie Matthieu 《Journal of neurochemistry》1994,63(6):2210-2216
Abstract: Paralytic tremor ( pt ) is a sex-linked mutation in rabbit that affects myelination of the CNS. Myelin in the pt brains represents ∼30% of the normal levels. Previously we showed that the pt mutation affects primarily proteolipid protein ( Plp ) gene expression. In the present study we investigated the relative effect of the pt mutation on two distinctive Plp gene products, PLP- and DM-20-specific messenger RNAs. Our results showed that both PLP and DM-20 are affected and that the ratio DM-20/PLP was higher in pt rabbits than in age-matched controls. We sequenced normal rabbit PLP cDNA and characterized pt mutation at the DNA level. Rabbit PLP sequence, deduced from cDNA, differs from the human protein only at Thr198 . Sequence analysis of the mutant cDNA revealed a transversion T → A in exon 2 of the Plp gene. This point mutation, which is placed at the end of the first potential transmembrane domain, results in a substitution of His36 by a glutamine. This transversion abolishes a restriction site that enabled us to screen a large number of animals and observe a perfect correlation between the pt allele and the abnormal phenotype. 相似文献