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Neutral theory predicts the relative abundance and diversity of genetic elements in a broad array of eukaryotic genomes.

ABSTRACT: It is universally true in ecological communities, terrestrial or aquatic, temperate or tropical, that some species are very abundant, others are moderately common, and the majority are rare. Likewise, eukaryotic genomes also contain classes or "species" of genetic elements that vary greatly in abundance: DNA transposons, retrotransposons, satellite sequences, simple repeats and their less abundant functional sequences such as RNA or genes. Are the patterns of relative species abundance and diversity similar among ecological communities and genomes? Previous dynamical models of genomic diversity have focused on the selective forces shaping the abundance and diversity of transposable elements (TEs). However, ideally, models of genome dynamics should consider not only TEs, but also the diversity of all genetic classes or "species" populating eukaryotic genomes. Here, in an analysis of the diversity and abundance of genetic elements in >500 eukaryotic chromosomes, we show that the patterns are consistent with a neutral hypothesis of genome assembly in virtually all chromosomes tested. The distributions of relative abundance of genetic elements are quite precisely predicted by the dynamics of an ecological model for which the principle of functional equivalence is the main assumption. We hypothesize that at large temporal scales an overarching neutral or nearly neutral process governs the evolution of abundance and diversity of genetic elements in eukaryotic genomes.

SUBMITTER: Serra F

PROVIDER: S-EPMC3683013 | biostudies-literature | 2013

REPOSITORIES: biostudies-literature

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Neutral theory predicts the relative abundance and diversity of genetic elements in a broad array of eukaryotic genomes.

Serra François F Becher Verónica V Dopazo Hernán H

PloS one 20130614 6

It is universally true in ecological communities, terrestrial or aquatic, temperate or tropical, that some species are very abundant, others are moderately common, and the majority are rare. Likewise, eukaryotic genomes also contain classes or "species" of genetic elements that vary greatly in abundance: DNA transposons, retrotransposons, satellite sequences, simple repeats and their less abundant functional sequences such as RNA or genes. Are the patterns of relative species abundance and diver ...[more]

PMID: 23798991

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Project description:Recent genome-wide experiments in different eukaryotic genomes provide an unprecedented view of transcription factor (TF) binding locations and of nucleosome occupancy. These experiments revealed that a large fraction of TF binding events occur in regions where only a small number of specific TF binding sites (TFBSs) have been detected. Furthermore, in vitro protein-DNA binding measurements performed for hundreds of TFs indicate that TFs are bound with wide range of affinities to different DNA sequences that lack known consensus motifs. These observations have thus challenged the classical picture of specific protein-DNA binding and strongly suggest the existence of additional recognition mechanisms that affect protein-DNA binding preferences. We have previously demonstrated that repetitive DNA sequence elements characterized by certain symmetries statistically affect protein-DNA binding preferences. We call this binding mechanism nonconsensus protein-DNA binding in order to emphasize the point that specific consensus TFBSs do not contribute to this effect. In this paper, using the simple statistical mechanics model developed previously, we calculate the nonconsensus protein-DNA binding free energy for the entire C. elegans and D. melanogaster genomes. Using the available chromatin immunoprecipitation followed by sequencing (ChIP-seq) results on TF-DNA binding preferences for ~100 TFs, we show that DNA sequences characterized by low predicted free energy of nonconsensus binding have statistically higher experimental TF occupancy and lower nucleosome occupancy than sequences characterized by high free energy of nonconsensus binding. This is in agreement with our previous analysis performed for the yeast genome. We suggest therefore that nonconsensus protein-DNA binding assists the formation of nucleosome-free regions, as TFs outcompete nucleosomes at genomic locations with enhanced nonconsensus binding. In addition, here we perform a new, large-scale analysis using in vitro TF-DNA preferences obtained from the universal protein binding microarrays (PBM) for ~90 eukaryotic TFs belonging to 22 different DNA-binding domain types. As a result of this new analysis, we conclude that nonconsensus protein-DNA binding is a widespread phenomenon that significantly affects protein-DNA binding preferences and need not require the presence of consensus (specific) TFBSs in order to achieve genome-wide TF-DNA binding specificity.

Project description:BackgroundInterest in understanding the mechanisms that lead to a particular composition of the Gut Microbiota is highly increasing, due to the relationship between this ecosystem and the host health state. Particularly relevant is the study of the Relative Species Abundance (RSA) distribution, that is a component of biodiversity and measures the number of species having a given number of individuals. It is the universal behaviour of RSA that induced many ecologists to look for theoretical explanations. In particular, a simple stochastic neutral model was proposed by Volkov et al. relying on population dynamics and was proved to fit the coral-reefs and rain forests RSA. Our aim is to ascertain if this model also describes the Microbiota RSA and if it can help in explaining the Microbiota plasticity.ResultsWe analyzed 16S rRNA sequencing data sampled from the Microbiota of three different animal species by Jeraldo et al. Through a clustering procedure (UCLUST), we built the Operational Taxonomic Units. These correspond to bacterial species considered at a given phylogenetic level defined by the similarity threshold used in the clustering procedure. The RSAs, plotted in the form of Preston plot, were fitted with Volkov's model. The model fits well the Microbiota RSA, except in the tail region, that shows a deviation from the neutrality assumption. Looking at the model parameters we were able to discriminate between different animal species, giving also a biological explanation. Moreover, the biodiversity estimator obtained by Volkov's model also differentiates the animal species and is in good agreement with the first and second order Hill's numbers, that are common evenness indexes simply based on the fraction of individuals per species.ConclusionsWe conclude that the neutrality assumption is a good approximation for the Microbiota dynamics and the observation that Volkov's model works for this ecosystem is a further proof of the RSA universality. Moreover, the ability to separate different animals with the model parameters and biodiversity number are promising results if we think about future applications on human data, in which the Microbiota composition and biodiversity are in close relationships with a variety of diseases and life-styles.

Project description:Purpose: The goals of this study are to identify Mutator insertion sites and determine the affect of these insertions on nearby genes Methods: Wiseq-based Mu element profiling was performed as described previously using B73/Mo17 F1 hybrid progeny seedlings. The B73 parent was carried Mutator activity that had been introgressed into the B73 genetic background. The Mo17 parent lacked active Mu elements. Thus, all new insertions were into the B73 genome. Genomic DNA was extracted from 6-day-old seedlings of B73/Mo17 hybrid plants. Amplicon-based enrichment of Mu flanking DNA was then performed. The purified PCR products were subject to Miseq-based Wideseq pipeline at Purdue Genomics Core Facility (https://www.purdue.edu/hla/sites/genomics/wideseq-2/). Wideseq reads were mapped to the B73 reference genome as described previously. By identifying the Mu target site duplications (TSDs), a set of genes targeted by Mu insertions that segregated in hybrid progeny was obtained and those containing B73/Mo17 SNPs in their mRNA sequences were used for allele-specific expression analysis. Because new insertions were into the B73 genome, the effect of these insertions would be expected to be specific in all cases to the B73 allele. To quantify the allele frequency, we performed RT-PCR followed by Wideseq from the identical shoot tissues of the hybrid seedlings mentioned above. Total RNA was extracted using the RNA Extraction Kit (Zymo) and cDNAs were synthesized using Promega M-MLV Reverse Transcriptase. For for a subset of 16 genes that carried SNPs, RNA fragments containing B73/Mo17 SNPs were amplified by RT-PCR. Libraries were prepared using an mRNA preparation kit (Illumina) following a standard protocol (https://www.purdue.edu/hla/sites/genomics/wi-deseq-2/). The RT-PCR products were then sequenced by the “WideSeq” pipeline. Results: A knockdown index was deduced by normalizing the observed ratio to that observed in genes that lacked Mu insertions in both genetic backgrounds for each insertion. We found that none of the four promoter insertions changed the expression of nearby genes. A quarter of 5’ UTR insertions (5 out of 20) caused knockout or strong knockdown effects and one of seven intronic insertions resulted in a knockout effect. Collectively, of a total of 33 Mu insertions, all of which were within 200 bp of genes, only 11 significantly reduced gene expression, and only two knocked it out completely. Conclusions: These results indicate that that Mu element insertions near TSSs of host genes are often associated with quantitative and in many cases neglectable functional consequences on nearby gene expression.

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Neutral theory predicts the relative abundance and diversity of genetic elements in a broad array of eukaryotic genomes.

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Neutral theory predicts the relative abundance and diversity of genetic elements in a broad array of eukaryotic genomes.

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