Epigenetics and plant genome evolution.

Epigenetics was envisioned as a topic to inform evolutionary theory, but the interplay between epigenetics and evolution has received little attention. With the advent of high-throughput methods, it is now routine to measure the genome-wide distribution of epigenetic marks, and these genome-wide patterns are providing insights into evolutionary processes. For example, DNA methylation is associated with transposable element silencing but also with repression of the expression of nearby genes, perhaps caused by the spread of methylation into regulatory regions. This repressive effect, which is typically deleterious, is acted upon by purifying selection. These dynamics may also govern the outcome of hybridization and polyploid events by affecting homoeolog expression. Finally, genes are also often methylated, but the implications of genic methylation for plant gene and genome evolution are not yet characterized fully.

Diez CM ，Roessler K ，Gaut BS 《-》

The Role of Small RNA-Based Epigenetic Silencing for Purifying Selection on Transposable Elements in Capsella grandiflora.

To avoid negative effects of transposable element (TE) proliferation, plants epigenetically silence TEs using a number of mechanisms, including RNA-directed DNA methylation. These epigenetic modifications can extend outside the boundaries of TE insertions and lead to silencing of nearby genes, resulting in a trade-off between TE silencing and interference with nearby gene regulation. Therefore, purifying selection is expected to remove silenced TE insertions near genes more efficiently and prevent their accumulation within a population. To explore how effects of TE silencing on gene regulation shapes purifying selection on TEs, we analyzed whole genome sequencing data from 166 individuals of a large population of the outcrossing species Capsella grandiflora. We found that most TEs are rare, and in chromosome arms, silenced TEs are exposed to stronger purifying selection than those that are not silenced by 24-nucleotide small RNAs, especially with increasing proximity to genes. An age-of-allele test of neutrality on a subset of TEs supports our inference of purifying selection on silenced TEs, suggesting that our results are robust to varying transposition rates. Our results provide new insights into the processes affecting the accumulation of TEs in an outcrossing species and support the view that epigenetic silencing of TEs results in a trade-off between preventing TE proliferation and interference with nearby gene regulation. We also suggest that in the centromeric and pericentromeric regions, the negative aspects of epigenetic TE silencing are missing.

Horvath R ，Slotte T 《Genome Biology and Evolution》

Genic C-Methylation in Soybean Is Associated with Gene Paralogs Relocated to Transposable Element-Rich Pericentromeres.

Most plants are polyploid due to whole-genome duplications (WGD) and can thus have duplicated genes. Following a WGD, paralogs are often fractionated (lost) and few duplicate pairs remain. Little attention has been paid to the role of DNA methylation in the functional divergence of paralogous genes. Using high-resolution methylation maps of accessions of domesticated and wild soybean, we show that in soybean, a recent paleopolyploid with many paralogs, DNA methylation likely contributed to the elimination of genetic redundancy of polyploidy-derived gene paralogs. Transcriptionally silenced paralogs exhibit particular genomic features as they are often associated with proximal transposable elements (TEs) and are preferentially located in pericentromeres, likely due to gene movement during evolution. Additionally, we provide evidence that gene methylation associated with proximal TEs is implicated in the divergence of expression profiles between orthologous genes of wild and domesticated soybean, and within populations.

El Baidouri M ，Kim KD ，Abernathy B ，Li YH ，Qiu LJ ，Jackson SA ... -  《-》

Epigenetic control of transposon transcription and mobility in Arabidopsis.

The mobility of genetic elements called transposable elements (TEs) was discovered half a century ago by Barbara McClintock. Although she had recognized them as chromosomal controlling elements, for much of the consequent time TEs were primarily considered as parasites of the host genome. However the recent explosion of discoveries in the fields of genomics and epigenetics have unambiguously shown the importance of TEs in genome function and evolution. Bursts of endogenous TEs have been reported in plants with epigenetic misregulation, revealing the molecular mechanisms underlying their control. We review here the different steps in TE invasion of the host genome involving epigenetic control and environmental stress responses. As TEs propagate in plant genomes and attract epigenetic marks, their neo-insertions can lead to the formation of new, heritable epigenetic variants (epialleles) of genes in their vicinity and impact on host gene regulatory networks. The epigenetic interplay between TE and genes thus plays a crucial role in the TE-host co-evolution.

Bucher E ，Reinders J ，Mirouze M 《-》

Genetic and epigenetic aspects of polyploid evolution in plants.

Polyploidy, the condition of possessing more than 2 complete chromosome sets in the same nucleus, is frequent in nature and has implications for a species' prospects for evolution. Newly formed polyploids, so-called neopolyploids, undergo a wide spectrum of genomic changes upon genome merger and duplication. Here, we review recent literature describing genomic and transcriptomic changes along the pathway from neoallopolyploid formation to the stabilization of species and diversification at the allopolyploid level. We begin by reviewing pathways of polyploid formation and discuss the effects of genome doubling and hybridization on chromosome pairing. We then review our knowledge of epigenetic changes in allopolyploids, followed by a consideration of the effects of these structural genomic and epigenetic changes on the transcriptional activity of genes in allopolyploids. We discuss the effects of changes in gene expression in polyploids with respect to current evolutionary theory. Finally, we draw attention to the general question of the relationships between genomic and transcriptomic alteration and incipient diversification among sibling polyploid lines and populations.

Madlung A ，Wendel JF 《-》