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Diversity, dynamics and effects of long terminal repeat retrotransposons in the model grass Brachypodium distachyon.

ABSTRACT: Transposable elements (TEs) are the main reason for the high plasticity of plant genomes, where they occur as communities of diverse evolutionary lineages. Because research has typically focused on single abundant families or summarized TEs at a coarse taxonomic level, our knowledge about how these lineages differ in their effects on genome evolution is still rudimentary. Here we investigate the community composition and dynamics of 32 long terminal repeat retrotransposon (LTR-RT) families in the 272-Mb genome of the Mediterranean grass Brachypodium distachyon. We find that much of the recent transpositional activity in the B. distachyon genome is due to centromeric Gypsy families and Copia elements belonging to the Angela lineage. With a half-life as low as 66 kyr, the latter are the most dynamic part of the genome and an important source of within-species polymorphisms. Second, GC-rich Gypsy elements of the Retand lineage are the most abundant TEs in the genome. Their presence explains > 20% of the genome-wide variation in GC content and is associated with higher methylation levels. Our study shows how individual TE lineages change the genetic and epigenetic constitution of the host beyond simple changes in genome size.

SUBMITTER: Stritt C

PROVIDER: S-EPMC7497039 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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Diversity, dynamics and effects of long terminal repeat retrotransposons in the model grass Brachypodium distachyon.

Stritt Christoph C Wyler Michele M Gimmi Elena L EL Pippel Martin M Roulin Anne C AC

The New phytologist 20191209 6

Transposable elements (TEs) are the main reason for the high plasticity of plant genomes, where they occur as communities of diverse evolutionary lineages. Because research has typically focused on single abundant families or summarized TEs at a coarse taxonomic level, our knowledge about how these lineages differ in their effects on genome evolution is still rudimentary. Here we investigate the community composition and dynamics of 32 long terminal repeat retrotransposon (LTR-RT) families in th ...[more]

PMID: 31677277

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Project description:BackgroundBrachypodium distachyon (Poaceae), an annual Mediterranean Aluminum (Al)-sensitive grass, is currently being used as a model species to provide new information on cereals and biofuel crops. The plant has a short life cycle and one of the smallest genomes in the grasses being well suited to experimental manipulation. Its genome has been fully sequenced and several genomic resources are being developed to elucidate key traits and gene functions. A reliable germplasm collection that reflects the natural diversity of this species is therefore needed for all these genomic resources. However, despite being a model plant, we still know very little about its genetic diversity. As a first step to overcome this gap, we used nuclear Simple Sequence Repeats (nSSR) to study the patterns of genetic diversity and population structure of B. distachyon in 14 populations sampled across the Iberian Peninsula (Spain), one of its best known areas.ResultsWe found very low levels of genetic diversity, allelic number and heterozygosity in B. distachyon, congruent with a highly selfing system. Our results indicate the existence of at least three genetic clusters providing additional evidence for the existence of a significant genetic structure in the Iberian Peninsula and supporting this geographical area as an important genetic reservoir. Several hotspots of genetic diversity were detected and populations growing on basic soils were significantly more diverse than those growing in acidic soils. A partial Mantel test confirmed a statistically significant Isolation-By-Distance (IBD) among all studied populations, as well as a statistically significant Isolation-By-Environment (IBE) revealing the presence of environmental-driven isolation as one explanation for the genetic patterns found in the Iberian Peninsula.ConclusionsThe finding of higher genetic diversity in eastern Iberian populations occurring in basic soils suggests that these populations can be better adapted than those occurring in western areas of the Iberian Peninsula where the soils are more acidic and accumulate toxic Al ions. This suggests that the western Iberian acidic soils might prevent the establishment of Al-sensitive B. distachyon populations, potentially causing the existence of more genetically depauperated individuals.

Project description:Background and aimsBrachypodium distachyon is being widely investigated across the world as a model plant for temperate cereals. This annual plant has three cytotypes (2n = 10, 20, 30) that are still regarded as part of a single species. Here, a multidisciplinary study has been conducted on a representative sampling of the three cytotypes to investigate their evolutionary relationships and origins, and to elucidate if they represent separate species.MethodsStatistical analyses of 15 selected phenotypic traits were conducted in individuals from 36 lines or populations. Cytogenetic analyses were performed through flow cytometry, fluorescence in situ hybridization (FISH) with genomic (GISH) and multiple DNA sequences as probes, and comparative chromosome painting (CCP). Phylogenetic analyses were based on two plastid (ndhF, trnLF) and five nuclear (ITS, ETS, CAL, DGAT, GI) genes from different Brachypodium lineages, whose divergence times and evolutionary rates were estimated.Key resultsThe phenotypic analyses detected significant differences between the three cytotypes and demonstrated stability of characters in natural populations. Genome size estimations, GISH, FISH and CCP confirmed that the 2n = 10 and 2n = 20 cytotypes represent two different diploid taxa, whereas the 2n = 30 cytotype represents the allotetraploid derived from them. Phylogenetic analysis demonstrated that the 2n = 20 and 2n = 10 cytotypes emerged from two independent lineages that were, respectively, the maternal and paternal genome donors of the 2n = 30 cytotype. The 2n = 20 lineage was older and mutated significantly faster than the 2n = 10 lineage and all the core perennial Brachypodium species.ConclusionsThe substantial phenotypic, cytogenetic and molecular differences detected among the three B. distachyon sensu lato cytotypes are indicative of major speciation processes within this complex that allow their taxonomic separation into three distinct species. We have kept the name B. distachyon for the 2n = 10 cytotype and have described two novel species as B. stacei and B. hybridum for, respectively, the 2n = 20 and 2n = 30 cytotypes.

Project description:Comparative genomics have facilitated the mining of biological information from a genome sequence, through the detection of similarities and differences with genomes of closely or more distantly related species. By using such comparative approaches, knowledge can be transferred from the model to non-model organisms and insights can be gained in the structural and evolutionary patterns of specific genes. In the absence of sequenced genomes for allergenic grasses, this study was aimed at understanding the structure, organisation and expression profiles of grass pollen allergens using the genomic data from Brachypodium distachyon as it is phylogenetically related to the allergenic grasses. Combining genomic data with the anther RNA-Seq dataset revealed 24 pollen allergen genes belonging to eight allergen groups mapping on the five chromosomes in B. distachyon. High levels of anther-specific expression profiles were observed for the 24 identified putative allergen-encoding genes in Brachypodium. The genomic evidence suggests that gene encoding the group 5 allergen, the most potent trigger of hay fever and allergic asthma originated as a pollen specific orphan gene in a common grass ancestor of Brachypodium and Triticiae clades. Gene structure analysis showed that the putative allergen-encoding genes in Brachypodium either lack or contain reduced number of introns. Promoter analysis of the identified Brachypodium genes revealed the presence of specific cis-regulatory sequences likely responsible for high anther/pollen-specific expression. With the identification of putative allergen-encoding genes in Brachypodium, this study has also described some important plant gene families (e.g. expansin superfamily, EF-Hand family, profilins etc) for the first time in the model plant Brachypodium. Altogether, the present study provides new insights into structural characterization and evolution of pollen allergens and will further serve as a base for their functional characterization in related grass species.

Project description:RNA interference (RNAi) is a biological process in which small RNAs regulate gene silencing at the transcriptional or posttranscriptional level. The trigger for gene silencing is double-stranded RNA generated from an endogenous genomic locus or a foreign source, such as a transgene or virus. In addition to regulating endogenous gene expression, RNAi provides the mechanistic basis for small RNA-mediated communication between plant hosts and interacting pathogenic microbes, known as cross-kingdom RNAi. Two core protein components, Argonaute (AGO) and Dicer (DCL), are central to the RNAi machinery of eukaryotes. Plants encode for several copies of AGO and DCL genes; in Arabidopsis thaliana, the AGO protein family contains 10 members, and the DCL family contains four. Little is known about the conservation and specific roles of these proteins in monocotyledonous plants, which account for the most important food staples. Here, we utilized in silico tools to investigate the structure and related functions of AGO and DCL proteins from the model grass Brachypodium distachyon. Based on the presence of characteristic domains, 16 BdAGO- and 6 BdDCL-predicted proteins were identified. Phylogenetic analysis showed that both protein families were expanded in Brachypodium as compared with Arabidopsis. For BdDCL proteins, both plant species contain a single copy of DCL1 and DCL4; however, Brachypodium contains two copies each of DCL2 and DCL3. Members of the BdAGO family were placed in all three functional clades of AGO proteins previously described in Arabidopsis. The greatest expansion occurred in the AtAGO1/5/10 clade, which contains nine BdAGOs (BdAGO5/6/7/9/10/11/12/15/16). The catalytic tetrad of the AGO P-element-induced wimpy testis domain (PIWI), which is required for endonuclease activity, is conserved in most BdAGOs, with the exception of BdAGO1, which lacks the last D/H residue. Three-dimensional modeling of BdAGO proteins using tertiary structure prediction software supported the phylogenetic classification. We also predicted a provisional interactome network for BdAGOs, their localization within the cell, and organ/tissue-specific expression. Exploring the specifics of RNAi machinery proteins in a model grass species can serve as a proxy for agronomically important cereals such as barley and wheat, where the development of RNAi-based plant protection strategies is of great interest.

Dataset Information

Diversity, dynamics and effects of long terminal repeat retrotransposons in the model grass Brachypodium distachyon.

Publications

Diversity, dynamics and effects of long terminal repeat retrotransposons in the model grass Brachypodium distachyon.

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