The pattern of polymorphism in Arabidopsis thaliana.
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ABSTRACT:
SUBMITTER: Nordborg M
PROVIDER: S-EPMC1135296 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:We used hybridization to the ATH1 gene expression array to interrogate genomic DNA diversity in 23 wild strains (accessions) of Arabidopsis thaliana (arabidopsis), in comparison with the reference strain Columbia (Col). At <1% false discovery rate, we detected 77,420 single-feature polymorphisms (SFPs) with distinct patterns of variation across the genome. Total and pair-wise diversity was higher near the centromeres and the heterochromatic knob region, but overall diversity was positively correlated with recombination rate (R(2) = 3.1%). The difference between total and pair-wise SFP diversity is a relative measure contrasting diversifying or frequency-dependent selection, similar to Tajima's D, and can be calibrated by the empirical genome-wide distribution. Each unique locus, centered on a gene, has a diversity and selection score that suggest a relative role in past evolutionary processes. Homologs of disease resistance (R) genes include members with especially high levels of diversity often showing frequency-dependent selection and occasionally evidence of a past selective sweep. Receptor-like and S-locus proteins also contained members with elevated levels of diversity and signatures of selection, whereas other gene families, bHLH, F-box, and RING finger proteins, showed more typical levels of diversity. SFPs identified with the gene expression array also provide an empirical hybridization polymorphism background for studies of gene expression polymorphism and are available through the genome browser http://signal.salk.edu/cgi-bin/AtSFP.
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Project description:The level of within-species polymorphism differs greatly among genes in a genome. Many genomic studies have investigated the relationship between gene polymorphism and factors such as recombination rate or expression pattern. However, the polymorphism of a gene is affected not only by its physical properties or functional constraints but also by natural selection on organisms in their environments. Specifically, if functionally divergent alleles enable adaptation to different environments, locus-specific polymorphism may be maintained by spatially heterogeneous natural selection. To test this hypothesis and estimate the extent to which environmental selection shapes the pattern of genome-wide polymorphism, we define the "environmental relevance" of a gene as the proportion of genetic variation explained by environmental factors, after controlling for population structure. We found substantial effects of environmental relevance on patterns of polymorphism among genes. In addition, the correlation between environmental relevance and gene polymorphism is positive, consistent with the expectation that balancing selection among heterogeneous environments maintains genetic variation at ecologically important genes. Comparison of the gene ontology annotations shows that genes with high environmental relevance are enriched in unknown function categories. These results suggest an important role for environmental factors in shaping genome-wide patterns of polymorphism and indicate another direction of genomic study.
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Project description:The extensive research for predicting highly expressed genes in plant genome sequences has been going on for decades. The codon usage pattern of genes in Arabidopsis thaliana genome is a classical topic for plant biologists for its significance in the understanding of molecular plant biology. Here we have used a gene expression profiling methodology based on the score of modified relative codon bias (MRCBS) to elucidate expression pattern of genes in Arabidopsis thaliana. MRCBS relies exclusively on sequence features for identifying the highly expressed genes. In this study, a critical analysis of predicted highly expressed (PHE) genes in Arabidopsis thaliana has been performed using MRCBS as a numerical estimator of gene expression level. Consistent with previous other results, our study indicates that codon composition plays an important role in the regulation of gene expression. We found a systematic strong correlation between MRCBS and CAI (codon adaptation index) or other expression-measures. Additionally, MRCBS correlates well with experimental gene expression data. Our study highlights the relationship between gene expression and compositional signature in relation to codon usage bias and sets the ground for the further investigation of the evolution of the protein-coding genes in the plant genome.
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Project description:The composition of the individual eukaryote's genome and its variation within a species remain poorly defined. Even for a sequenced genome such as that of the model plant Arabidopsis thaliana accession Col-0, the large arrays of heterochromatic repeats are incompletely sequenced, with gaps of uncertain size persisting in them.Using geographically separate populations of A. thaliana, we assayed variation in the heterochromatic repeat arrays using two independent methods and identified significant polymorphism among them, with variation by as much as a factor of two in the centromeric 180 bp repeat, in the 45S rDNA arrays and in the Athila retroelements. In the accession with highest genome size as measured by flow cytometry, Loh-0, we found more than a two-fold increase in 5S RNA gene copies relative to Col-0; results from fluorescence in situ hybridization with 5S probes were consistent with the existence of size polymorphism between Loh-0 and Col-0 at the 5S loci. Comparative genomic hybridization results of Loh-0 and Col-0 did not support contiguous variation in copy number of protein-coding genes on the scale needed to explain their observed genome size difference. We developed a computational data model to test whether the variation we measured in the repeat fractions could account for the different genome sizes determined with flow cytometry, and found that this proposed relationship could account for about 50% of the variance in genome size among the accessions.Our analyses are consistent with substantial repeat number polymorphism for 5S and 45S ribosomal genes among accession of A. thaliana. Differences are also suggested for centromeric and pericentromeric repeats. Our analysis also points to the difficulties in measuring the repeated fraction of the genome and suggests that independent validation of genome size should be sought in addition to flow cytometric measurements.
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Project description:Pathogen resistance is an ecologically important phenotype increasingly well understood at the molecular genetic level. In this article, we examine levels of avrRpt2-dependent resistance and Rps2 locus DNA sequence variability in a worldwide sample of 27 accessions of Arabidopsis thaliana. The rooted parsimony tree of Rps2 sequences drawn from a diverse set of ecotypes includes a deep bifurcation separating major resistance and susceptibility clades of alleles. We find evidence for selection maintaining these alleles and identify the N-terminal part of the leucine-rich repeat region as a probable target of selection. Additional protein variants are found within the two major clades and correlate well with measurable differences among ecotypes in resistance to the avirulence gene avrRpt2 of the pathogen Pseudomonas syringae. Long-lived polymorphisms have been observed for other resistance genes of A. thaliana; the Rps2 data suggest that the long-term maintenance of phenotypic variation in resistance genes may be a general phenomenon and are consistent with diversifying selection acting in concert with selection to maintain variation.
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