McClintock's challenge in the 21st century

The regulatory potential of transposable elements in maize

The genomes of flowering plants consist largely of transposable elements (TEs), some of which modulate gene regulation and function. However, the repetitive nature of TEs and difficulty of mapping individual TEs by short-read sequencing have hindered our understanding of their regulatory potential. Here we show that long-read chromatin fibre sequencing (Fiber-seq) comprehensively identifies accessible chromatin regions (ACRs) and CpG methylation across the maize genome. We uncover stereotypical ACR patterns at young TEs that degenerate with evolutionary age, resulting in TE enhancers preferentially marked by a novel plant-specific epigenetic feature: simultaneous hyper-CpG methylation and chromatin accessibility. We show that TE ACRs are co-opted as gene promoters and that ACR-containing TEs can facilitate gene amplification. Lastly, we uncover a pervasive epigenetic signature-hypo-5mCpG methylation and diffuse chromatin accessibility-directing TEs to specific loci, including the loci that sparked McClintock's discovery of TEs.

The regulatory potential of transposable elements in maize

The genomes of flowering plants consist largely of transposable elements (TEs), some of which modulate gene regulation and function. However, the repetitive nature of TEs and difficulty of mapping individual TEs by short-read-sequencing have hindered our understanding of their regulatory potential. We demonstrate that long-read chromatin fiber sequencing (Fiber-seq) comprehensively identifies accessible chromatin regions (ACRs) and CpG methylation across the maize genome. We uncover stereotypical ACR patterns at young TEs that degenerate with evolutionary age, resulting in TE-enhancers preferentially marked by a novel plant-specific epigenetic feature: simultaneous hyper-CpG methylation and chromatin accessibility. We show that TE ACRs are co-opted as gene promoters and that ACR-containing TEs can facilitate gene amplification. Lastly, we uncover a pervasive epigenetic signature - hypo-5mCpG methylation and diffuse chromatin accessibility - directing TEs to specific loci, including the loci that sparked McClintock's discovery of TEs.

Modeling transposable elements dynamics during polyploidization in plants

In this work we study the proliferation of transposable elements (TEs) and the epigenetic response of plants during the process of polyploidization. Through a deterministic model, expanding on our previous work on TE proliferation under epigenetic regulation, we study the long-term TE distribution and TE stability in the subgenomes of both autopolyploids and allopolyploids. We also explore different small-interfering RNA (siRNA) action modes on the subgenomes, including a model where siRNAs are not directed to specific genomes and one where siRNAs are directed - i.e. more active - in subgenomes with higher TE loads. In the autopolyploid case, we find long-term stable equilbria that tend to equilibrate the number of active TEs between subgenomes. In the allopolyploid case, directed siRNA action is fundamental to avoid a "winner takes all" outcome of the competition between the TEs in the different subgenomes. We also show that decaying oscillations in the number of TEs occur naturally in all cases, perhaps explaining some of the observed features of 'genomic shock' after hybridization events, and that the balance in the dynamics of the different types of siRNA is determinant for the synchronization of these oscillations.

Transcriptional repression in stochastic gene expression, patterning, and cell fate specification

Development is often driven by signaling and lineage-specific cues, yielding highly uniform and reproducible outcomes. Development also involves mechanisms that generate noise in gene expression and random patterns across tissues. Cells sometimes randomly choose between two or more cell fates in a mechanism called stochastic cell fate specification. This process diversifies cell types in otherwise homogenous tissues. Stochastic mechanisms have been extensively studied in prokaryotes where noisy gene activation plays a pivotal role in controlling cell fates. In eukaryotes, transcriptional repression stochastically limits gene expression to generate random patterns and specify cell fates. Here, we review our current understanding of repressive mechanisms that produce random patterns of gene expression and cell fates in flies, plants, mice, and humans.

CHD4 Conceals Aberrant CTCF-Binding Sites at TAD Interiors by Regulating Chromatin Accessibility in Mouse Embryonic Stem Cells

CCCTC-binding factor (CTCF) critically contributes to 3D chromatin organization by determining topologically associated domain (TAD) borders. Although CTCF primarily binds at TAD borders, there also exist putative CTCF-binding sites within TADs, which are spread throughout the genome by retrotransposition. However, the detailed mechanism responsible for masking the putative CTCF-binding sites remains largely elusive. Here, we show that the ATP-dependent chromatin remodeler, chromodomain helicase DNA-binding 4 (CHD4), regulates chromatin accessibility to conceal aberrant CTCF-binding sites embedded in H3K9me3-enriched heterochromatic B2 short interspersed nuclear elements (SINEs) in mouse embryonic stem cells (mESCs). Upon CHD4 depletion, these aberrant CTCF-binding sites become accessible and aberrant CTCF recruitment occurs within TADs, resulting in disorganization of local TADs. RNA-binding intrinsically disordered domains (IDRs) of CHD4 are required to prevent this aberrant CTCF binding, and CHD4 is critical for the repression of B2 SINE transcripts. These results collectively reveal that a CHD4-mediated mechanism ensures appropriate CTCF binding and associated TAD organization in mESCs.

Corticosterone dynamically regulates retrotransposable element expression in the rat hippocampus and C6 cells

The hippocampus is a highly plastic brain region sensitive to environmental stress. It shows dynamic changes in epigenetic marks associated with stress related learning. Previous work has shown that acute stress induces substantial transient changes in histone H3 lysine 9 trimethylation (H3K9me3). Moreover, increased H3K9me3 is enriched in hippocampal gene deserts accumulating within endogenous retroviruses and transposable elements. We have found that in response to acute glucocorticoid treatment, a similar change in global H3K9me3 is observed. However, when localized we found that H3K9me3 is markedly decreased at B2 short interspersed nuclear elements but not within intracisternal-A particle endogenous retroviruses. Further, decreased H3K9me3 valence within B2 elements was associated with increased transcript abundance. These data demonstrate the capacity for acute glucocorticoids to mobilize transposable elements via epigenetic unmasking. Reconciled with previous findings following acute stress, this suggests the capacity for mobile elements to potentially function as novel regulators given their dynamic regulation by stress and glucocorticoids.

Giant cells: Linking McClintock's heredity to early embryogenesis and tumor origin throughout millennia of evolution on Earth

The "life code" theory postulates that egg cells, which are giant, are the first cells in reproduction and that damaged or aged giant somatic cells are the first cells in tumorigenesis. However, the hereditary basis for giant cells remains undefined. Here I propose that stress-induced genomic reorganization proposed by Nobel Laureate Barbara McClintock may represent the underlying heredity for giant cells, referred to as McClintock's heredity. Increase in cell size may serve as a response to environmental stress via switching proliferative mitosis to intranuclear replication for reproduction. Intranuclear replication activates McClintock's heredity to reset the genome following fertilization for reproduction or restructures the somatic genome for neoplastic transformation via formation of polyploid giant cancer cells (PGCCs). The genome-based McClintock heredity functions together with gene-based Mendel's heredity to regulate the genomic stability at two different stages of life cycle or tumorigenesis. Thus, giant cells link McClintock's heredity to both early embryogenesis and tumor origin. Cycling change in cell size together with ploidy number switch may represent the most fundamental mechanism on how both germ and soma for coping with environmental stresses for the survival across the tree of life which evolved over millions of years on Earth.

A capture-based assay for detection and characterization of transposon polymorphisms in maize

Transposons can create allelic diversity that affects gene expression and phenotypic diversity. The detection of transposon polymorphisms at a genome-wide scale across a large population is difficult. Here we developed a targeted sequencing approach to monitor transposon polymorphisms of interest. This approach can interrogate the presence or absence of transposons reliably across various genotypes. Using this approach, we genotyped a set of 965 transposon-related presence/absence polymorphisms in a diverse panel of 16 maize (Zea mays L.) inbred lines that are representative of the major maize breeding groups. About 70% of the selected regions can be effectively assayed in each genotype. The consistency between the capture-based assay and PCR-based assay are 98.6% based on analysis of 24 randomly selected transposon polymorphisms. By integrating the transposon polymorphisms data with gene expression data, ∼18% of the assayed transposon polymorphisms were found to be associated with variable gene expression levels. A detailed analysis of 18 polymorphisms in a larger association panel confirmed the effects of 10 polymorphisms, with one of them having stronger association with expression than nearby SNP markers. The effects of seven polymorphisms were tested using a luciferase-based expression assay, and one was confirmed. Together, this study demonstrates that the targeted sequencing assay is an effective way to explore transposon function in a high-throughput manner.

Rapid evolution and conserved function of the piRNA pathway

Transposons are major genome constituents that can mobilize and trigger mutations, DNA breaks and chromosome rearrangements. Transposon silencing is particularly important in the germline, which is dedicated to transmission of the inherited genome. Piwi-interacting RNAs (piRNAs) guide a host defence system that transcriptionally and post-transcriptionally silences transposons during germline development. While germline control of transposons by the piRNA pathway is conserved, many piRNA pathway genes are evolving rapidly under positive selection, and the piRNA biogenesis machinery shows remarkable phylogenetic diversity. Conservation of core function combined with rapid gene evolution is characteristic of a host–pathogen arms race, suggesting that transposons and the piRNA pathway are engaged in an evolutionary tug of war that is driving divergence of the biogenesis machinery. Recent studies suggest that this process may produce biochemical incompatibilities that contribute to reproductive isolation and species divergence.

Construction and Optimization of a Large Gene Coexpression Network in Maize Using RNA-Seq Data

With the emergence of massively parallel sequencing, genomewide expression data production has reached an unprecedented level. This abundance of data has greatly facilitated maize research, but may not be amenable to traditional analysis techniques that were optimized for other data types. Using publicly available data, a gene coexpression network (GCN) can be constructed and used for gene function prediction, candidate gene selection, and improving understanding of regulatory pathways. Several GCN studies have been done in maize (Zea mays), mostly using microarray datasets. To build an optimal GCN from plant materials RNA-Seq data, parameters for expression data normalization and network inference were evaluated. A comprehensive evaluation of these two parameters and a ranked aggregation strategy on network performance, using libraries from 1266 maize samples, were conducted. Three normalization methods and 10 inference methods, including six correlation and four mutual information methods, were tested. The three normalization methods had very similar performance. For network inference, correlation methods performed better than mutual information methods at some genes. Increasing sample size also had a positive effect on GCN. Aggregating single networks together resulted in improved performance compared to single networks.

Current understanding of the pathways of flavonoid biosynthesis in model and crop plants

Flavonoids are a signature class of secondary metabolites formed from a relatively simple collection of scaffolds. They are extensively decorated by chemical reactions including glycosylation, methylation, and acylation. They are present in a wide variety of fruits and vegetables and as such in Western populations it is estimated that 20-50 mg of flavonoids are consumed daily per person. In planta they have demonstrated to contribute to both flower color and UV protection. Their consumption has been suggested to presenta wide range of health benefits. Recent technical advances allowing affordable whole genome sequencing, as well as a better inventory of species-by-species chemical diversity, have greatly advanced our understanding as to how flavonoid biosynthesis pathways vary across species. In parallel, reverse genetics combined with detailed molecular phenotyping is currently allowing us to elucidate the functional importance of individual genes and metabolites and by this means to provide further mechanistic insight into their biological roles. Here we provide an inventory of current knowledge of pathways of flavonoid biosynthesis in both the model plant Arabidopsis thaliana and a range of crop species, including tomato, maize, rice, and bean.

The dynamic genome: transposons and environmental adaptation in the nervous system

Classically thought as genomic clutter, the functional significance of transposable elements (TEs) has only recently become a focus of attention in neuroscience. Increasingly, studies have demonstrated that the brain seems to have more retrotransposition and TE transcription relative to other somatic tissues, suggesting a unique role for TEs in the central nervous system. TE expression and transposition also appear to vary by brain region and change in response to environmental stimuli such as stress. TEs appear to serve a number of adaptive roles in the nervous system. The regulation of TE expression by steroid, epigenetic and other mechanisms in interplay with the environment represents a significant and novel avenue to understanding both normal brain function and disease.

Control of Transposon-Mediated Directed Mutation by the Escherichia coli Phosphoenolpyruvate:Sugar Phosphotransferase System

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) has been shown to control transport, cell metabolism and gene expression. We here present results supporting the novel suggestion that in certain instances it also regulates the mutation rate. Directed mutations are defined as mutations that occur at higher frequencies when beneficial than when neutral or detrimental. To date, the occurrence of directed point mutations has not been documented and confirmed, but a few examples of transposon-mediated directed mutations have been reported. Here we focus on the first and best-studied example of directed mutation, which involves the regulation of insertion sequence-5 hopping into a specific site upstream of the glpFK glycerol utilization operon in Escherichia coli. This insertional event specifically activates expression of the glpFK operon, allowing the growth of wild-type cells with glycerol as a carbon source in the presence of nonmetabolizable glucose analogues which normally block glycerol utilization. The sugar-transporting PTS controls this process by regulating levels of cytoplasmic glycerol-3-phosphate and cyclic (c)AMP as established in previous publications. Direct involvement of the glycerol repressor, GlpR, and the cAMP receptor protein, Crp, in the regulation of transposon-mediated directed mutation has been demonstrated.

Dispersal of an ancient retroposon in the TP53 promoter of Bovidae: phylogeny, novel mechanisms, and potential implications for cow milk persistency

BACKGROUND

In recent years, the perception of transposable genetic elements has changed from "junk DNA" to a focus of interest when appearing near or inside genes. Bov-A2 is a short interspersed nuclear element (SINE) that was first found in Bovidae and later in other ruminants. This retroposon is mostly used as a marker for phylogenetic analysis.

RESULTS

We describe insertions of Bov-A2 in the promoter region of TP53, a key tumor suppressor gene that is indispensable for diverse developmental processes, in Antilopinae and Tragelaphini (belonging to the Bovinae subfamily). In Tragelaphini two Bov-A2 elements were inserted sequentially, whereas in 5 tribes of Antilopinae only one Bov-A2 element was inserted, in a different site and reverse orientation. The entrance site in both cases employed short palindromes that can form hairpin secondary structures. Interestingly, mutations that create or disrupt base pairing in the palindrome sequence dictated the presence or absence of Bov-A2, such as in the domestic cow and buffalo, which lack Bov-A2. Transcription factor binding site analysis revealed unique binding sites for STAT3 and NFκB within the Bov-A2 sequence, which together with TP53 itself are known to play a crucial role in mammary involution.

CONCLUSIONS

This report demonstrates how short palindromes serve as hot spots for Bov-A2 retroposon insertion into the mammalian genome. The strict correlation between point mutation in the palindromes and the presence/absence of Bov-A2 retroposon insertions, questions the use of singular insertion events as valid phylogenetic markers inside families. Bov-A2 insertion into the TP53 promoter in Antilopinae and Tragelaphini may not only provide a genetic network that regulates mammary involution, but can also answer the need for rapid mammary involution in Savanna antelopes after weaning, partially in response to predation stress. The absence of Bov-A2 in domestic bovids may constitute the molecular background for greater lactation persistency.