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11.
The F5 and FM2 chromosome races of the Sceloporus grammicus complex form a hybrid zone in the Mexican state of Hidalgo. Previous studies of this zone have assessed genetic structure by averaging estimates of shape and width across three diagnostic chromosome markers. This approach is likely to mask subtle differences in cline shape among loci (e.g. selected vs. neutral), and obscure any displacement of cline centres (if present). Here we use maximum likelihood methods to construct the best fitting individual clines for three chromosomal markers, and also add two new markers; the mitochondrial DNA (mtDNA) locus, and the nuclear ribosomal DNA (rDNA) repeat. For each locus, hybrid zone models were fitted by cline shape and width, and the position and number of segments describing the centre of the zone. Pairwise comparisons between all clines revealed concordance between chromosomes 2 and 6, but significant discordance in cline structure among all other paired combinations. The concordance of chromosomes 2 and 6 suggests that these clines are maintained by genome-wide forces. The discordance of the chromosome 1 cline suggests an influence of asymmetric introgression, while the mtDNA cline is probably influenced by selection and drift. The rDNA locus reveals a pattern best explained by either extreme asymmetric introgression or gene conversion. The structure of zone indicates that genome-wide processes and locus specific selective forces as well as drift, are operating to different degrees on different loci. The locus-by-locus approach used here permits a finer discrimination among possible mechanisms responsible for the maintenance of the individual clines.  相似文献   
12.
We investigate the structure between and within colonies of Schedorhinotermes lamanianus (Isoptera: Rhinotermitidae) at a cluster of foraging galleries in Shimba Hills National Reserve, Kenya. Three independent methods (morphometrics of minor soldiers, multilocus fingerprinting from genomic DNA of workers, and aggression tests between workers) yielded concordant results concerning number and spatial extent of colonies as well as variation between and within colonies. At least three colonies exist in our study area. Genetic data reveal that the largest colony is genetically and spatially substructured in three subsidiary nests, which may form reproductive units. These subsidiary nests were not completely isolated as we were able to document exchange of workers. Subsidiary nests may facilitate foundation of colonies by budding which may generate isolation by distance (population viscosity).  相似文献   
13.
In the European Alps, Rhododendron ferrugineum can constitute dense populations with almost 100% of cover. The developmental pattern by layering and the resulting complexity of population structure make it difficult to identify distinct clones even by excavation. Therefore genotypic structure of a R. ferrugineum population, in the French Alps, was inferred from AFLP markers. In a first step, we analysed 400 samples using AFLP profiles generated by one selective primer pair. Seventeen bands out of 25 were polymorphic (68%). We identified a total of 32 multilocus genotypes. In a second step, the 32 genotypes were verified by applying two additional primer pairs to the two most distant samples from each genotype. The mean similarity (proportion of band sharing) between pairs of clones was 0.85 (range from 0.52 to 0.94). The spatial distribution of clones showed that vegetative spreading mainly occurred down a slope. Based on an annual shoot mean growth of 2.6 cm/year and the size of the widest clone, we estimated the age of the oldest individual to be at least 300 years. A single genotype can occupy a large surface and sometimes form a dense patch, suggesting that this species adopts a phalanx growth form with limited intermingling of some genets.  相似文献   
14.
Genetic mapping functions translate the observed recombination rate between two loci into the corresponding map distance in Morgan units. Different mapping functions give different weights to multiple crossing overs and therefore lead to different results. This points out that not every function is best suited to fit a data set. The data used in this study originated from 2214 sperm from 37 Norwegian bulls, which were genotyped for 11 markers. The optimal functions for the chromosomes 6, 23 and the sex chromosome of cattle were derived using the maximum likelihood method, the likelihood ratio test and empirical discriminant analysis. It became apparent that for each chromosome a different function fitted the data best. These were the function of Rao et al. (Human Heredity 1977, 27, 99–104) with p = 0·63 for chromosome 6, the function of Goldgar & Fain (American Journal of Human Genetics 1988, 43, 38–45) with c0 = 0·42, c1 = 0·47, c2 = 0·07 and c3 = 0·04 for chromosome 23 and the function of Felsenstein (Genetics 1979, 91, 769–75) with K = 0·23 for the sex chromosome. The well known functions of Haldane (Journal of Genetics 1919, 8, 299–309) and Kosambi (Annals of Eugenics 1944, 12, 172–5) were shown to be suboptimal in most cases. A function is said to be multilocus feasible if the evaluation of the probability of all possible recombination events does not lead to negative values. The optimal function for chromosome 23 turned out to be multilocus feasible, whereas the functions for chromosome 6 and the sex chromosome were not. The choice of the correct mapping function is shown to have a considerable impact in mapping studies, when double recombinations have to be taken into account. Since there is no unique best mapping function, it is argued that it might be useful to use a simple parametric mapping function (like the one of Felsenstein 1979) and to estimate the respective parameter specifically for a given data set.  相似文献   
15.
Individual multilocus heterozygosity estimates based on a limited number of loci are expected to correlate only weakly with the inbreeding level of an individual. Before using multilocus heterozygosity estimates in studies of inbreeding, their ability to capture information on inbreeding in the given setting should be tested. A convenient method for this is to compute the heterozygosity-heterozygosity correlation, i.e. the mean correlation between multilocus heterozygosity estimates calculated from random samples of loci, which should be positive if multilocus heterozygosity carries a signature of inbreeding. Rhh is an extension package for the statistical software r that estimates this correlation and calculates three measures of individual multilocus heterozygosity: homozygosity by loci, internal relatedness and standardized heterozygosity. The extension package is available through the CRAN (http://cran.r-project.org) and has a homepage at http://www.helsinki.fi/biosci/egru/research/software.  相似文献   
16.
17.
Microsatellites are powerful markers to infer population genetic parameters. We used 10 microsatellite loci to characterize the genetic diversity and structure of 79 samples of Sclerotinia sclerotiorum isolated from four Brazilian dry bean populations and observed that eight of them were polymorphic within populations. We identified 102 different haplotypes ranging from 6 to 18 per locus. Analyses based on genetic diversity and fixation indices indicated variability among and within populations of 28.79% (FST = 28793) and 71.21%, respectively. To examine genetic relatedness among S. sclerotiorum isolates, we used internal spacer (ITS1‐5.8S‐ITS2) restriction fragment length polymorphism (PCR‐RFLP) and sequencing analysis. PCR‐RFLP analysis of these regions failed to show any genetic differences among isolates. However, we detected variability within the sequence, which does not support the hypothesis of clonal populations within each population. High variability within and among populations may indicate the introduction of new genotypes in the areas analysed, in addition to the occurrence of clonal and sexual reproduction in the populations of S. sclerotiorum in the Brazilian Cerrado.  相似文献   
18.
Lineage, or true ‘species’, trees may differ from gene trees because of stochastic processes in molecular evolution leading to gene‐tree heterogeneity. Problems with inferring species trees because of excessive incomplete lineage sorting may be exacerbated in lineages with rapid diversification or recent divergences necessitating the use of multiple loci and individuals. Many recent multilocus studies that investigate divergence times identify lineage splitting to be more recent than single‐locus studies, forcing the revision of biogeographic scenarios driving divergence. Here, we use 21 nuclear loci from regional populations to re‐evaluate hypotheses identified in an mtDNA phylogeographic study of the Brown Creeper (Certhia americana), as well as identify processes driving divergence. Nuclear phylogeographic analyses identified hierarchical genetic structure, supporting a basal split at approximately 32°N latitude, splitting northern and southern populations, with mixed patterns of genealogical concordance and discordance between data sets within the major lineages. Coalescent‐based analyses identify isolation, with little to no gene flow, as the primary driver of divergence between lineages. Recent isolation appears to have caused genetic bottlenecks in populations in the Sierra Madre Oriental and coastal mountain ranges of California, which may be targets for conservation concerns.  相似文献   
19.
Bacterial populations can display high levels of genetic structuring but the forces that influence this are incompletely understood. Here, by combining modelling approaches with multilocus sequence data for the zoonotic pathogen Campylobacter, we investigated how ecological factors such as niche (host) separation relate to population structure. We analysed seven housekeeping genes from published C. jejuni and C. coli isolate collections from a range of food and wild animal sources as well as abiotic environments. By reconstructing genetic structure and the patterns of ancestry, we quantified C. jejuni host association, inferred ancestral populations, investigated genetic admixture in different hosts and determined the host origin of recombinant C. jejuni alleles found in hybrid C. coli lineages. Phylogenetically distinct C. jejuni lineages were associated with phylogenetically distinct wild birds. However, in the farm environment, phylogenetically distant host animals shared several C. jejuni lineages that could not be segregated according to host origin using these analyses. Furthermore, of the introgressed C. jejuni alleles found in C. coli lineages, 73% were attributed to genotypes associated with food animals. Our results are consistent with an evolutionary scenario where distinct Campylobacter lineages are associated with different host species but the ecological factors that maintain this are different in domestic animals such that phylogenetically distant animals can harbour closely related strains.  相似文献   
20.
A nested multiplex PCR (polymerase chain reaction) approach was used for multilocus genotyping of arbuscular mycorrhizal fungal populations. This method allowed us to amplify multiple loci from Glomus single spores in a single PCR amplification. Variable introns in the two protein coding genes GmFOX2 and GmTOR2 were applied as codominant genetic markers together with the LSU rDNA. Genetic structure of Glomus spp. populations from an organically and a conventionally cultured field were compared by hierarchical sampling of spores from four plots in each field. Multilocus genotypes were characterized by SSCP (single stranded conformation polymorphism) and sequencing. All spore genotypes were unique suggesting that no recombination was taking place in the populations. There were no overall differences in the distribution of genotypes in the two fields and identical genotypes could be sampled from both fields. Analysis of gene diversity indicated that Glomus populations are subdivided between plots within each field. There were however, no subdivision between the fields.  相似文献   
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