Combinatorial epigenetics, “junk DNA”, and the evolution of complex organisms

Plant Tolerance to Drought Stress with Emphasis on Wheat

Environmental stresses, such as drought, have negative effects on crop yield. Drought is a stress whose impact tends to increase in some critical regions. However, the worldwide population is continuously increasing and climate change may affect its food supply in the upcoming years. Therefore, there is an ongoing effort to understand the molecular processes that may contribute to improving drought tolerance of strategic crops. These investigations should contribute to delivering drought-tolerant cultivars by selective breeding. For this reason, it is worthwhile to review regularly the literature concerning the molecular mechanisms and technologies that could facilitate gene pyramiding for drought tolerance. This review summarizes achievements obtained using QTL mapping, genomics, synteny, epigenetics, and transgenics for the selective breeding of drought-tolerant wheat cultivars. Synthetic apomixis combined with the msh1 mutation opens the way to induce and stabilize epigenomes in crops, which offers the potential of accelerating selective breeding for drought tolerance in arid and semi-arid regions.

Association of Polymorphisms within HOX Transcript Antisense RNA (HOTAIR) with Type 2 Diabetes Mellitus and Laboratory Characteristics: A Preliminary Case-Control Study

Type 2 diabetes mellitus (T2DM) is a complex heterogeneous disease resulting from the environment and genetic interactions. Lately, genetic association studies have shown that polymorphisms in long noncoding RNAs (lncRNAs) are associated with T2DM susceptibility. This preliminary study is aimed at investigating if HOX transcript antisense RNA (HOTAIR) polymorphisms contribute to T2DM development. Five hundred clinically diagnosed T2DM cases and 500 healthy controls were recruited from the southeast Iranian population. Genomic DNA was isolated from nucleated blood cells and genotyped for MspI (C/T) (rs920778) and AluI (A/G) (rs4759314) polymorphisms using the PCR-RFLP technique. For genotyping rs12826786 C/T and rs1899663 G/T variants, ARMS-PCR method was applied. Our findings indicated that HOTAIR rs920778 C/T, rs12826786 C/T, and rs4759314 A/G polymorphisms have a significant positive association with T2DM, while a negative association was observed between rs1899663 G/T T2DM susceptibility. Significant associations were also observed between rs920778 C/T and HDL-C as well as s4759314 A/G and both FBS and LDL-C in T2DM patients. Haplotype analysis indicated that the CGCG, CTTG, TGTA, and TTTG haplotypes of rs920778/rs1899663/rs12826786/rs4759314 significantly enhanced T2DM risk by 1.47, 1.96, 2.81, and 4.80 folds, respectively. No strong linkage disequilibrium was found between the four HOTAIR SNPs. We firstly reported that HOTAIR rs1899663 G/T, rs12826786 C/T, rs4759314 A/G, and rs920778 C/T polymorphisms might influence T2DM susceptibility by modulating different signaling pathways and could be regarded as potential prognostic markers in T2DM patients.

Environmental Epigenetics and a Unified Theory of the Molecular Aspects of Evolution: A Neo-Lamarckian Concept that Facilitates Neo-Darwinian Evolution

Environment has a critical role in the natural selection process for Darwinian evolution. The primary molecular component currently considered for neo-Darwinian evolution involves genetic alterations and random mutations that generate the phenotypic variation required for natural selection to act. The vast majority of environmental factors cannot directly alter DNA sequence. Epigenetic mechanisms directly regulate genetic processes and can be dramatically altered by environmental factors. Therefore, environmental epigenetics provides a molecular mechanism to directly alter phenotypic variation generationally. Lamarck proposed in 1802 the concept that environment can directly alter phenotype in a heritable manner. Environmental epigenetics and epigenetic transgenerational inheritance provide molecular mechanisms for this process. Therefore, environment can on a molecular level influence the phenotypic variation directly. The ability of environmental epigenetics to alter phenotypic and genotypic variation directly can significantly impact natural selection. Neo-Lamarckian concept can facilitate neo-Darwinian evolution. A unified theory of evolution is presented to describe the integration of environmental epigenetic and genetic aspects of evolution.

Epigenetics in adaptive evolution and development: the interplay between evolving species and epigenetic mechanisms: extract from Trygve Tollefsbol (ed.) (2011) Handbook of epigenetics--the new molecular and medical genetics. Chapter 26. Amsterdam, USA: Elsevier, pp. 423-446

By comparing epigenetics of current species with fossil records across evolutionary transitions, we can gauge the moment of emergence of some novel mechanisms in evolution, and recognize that epigenetic mechanisms have a bearing on mutation. Understanding the complexity and changeability of these mechanisms, as well as the changes they can effect, is both fascinating and of vital practical benefit. Our most serious pandemics of so-called 'non-communicable' diseases - mental and cardiovascular disorders, obesity and diabetes, rooted in the 'metabolic syndrome' - are evidently related to effects on our evolutionary mechanisms of agricultural and food industrialization, modern lifestyle and diet. Pollution affects us directly as well as indirectly by its destruction of ecologically essential biosystems. Evidently such powerful conditions of existence have epigenetic effects on both our health and our continuing evolution. Such effects are most profound during reproductive and developmental processes, when levels of hormones, as affected by stress particularly, may be due to modern cultures in childbearing such as excessive intervention, separation, maternal distress and disruption of bonding. Mechanisms of genomic imprinting seem likely to throw light on problems in assisted reproductive technology, among other transgenerational effects.

Transgenerational healing: Educating children in genesis of healthy children, with focus on nutrition, emotion, and epigenetic effects on brain development

Although our continuing evolution can never achieve our perfection, we long for our children's birth and health to be near-perfect. Many children are born healthy, though fewer than is possible. Birthing and health rapidly improved generally due to modern housing, sanitation and medicine, as well as birth interventions. Arguably interventions have exceeded the optimal level, without enough regard for natural physical and intuitive resources. Conception, often too easy, receives too little personal preparation unless a couple has problems. Nurturing the health of sperm and ovum seems hard to focus on, yet is needed by both parents - and even by the four grandparents. What are the key factors? Positive: The fields of hormones/emotions and of nutrition/metabolism. Negative: stress, poor nutrition, toxins, diseases; much being due to poverty. Positive and negative both have structural and also epigenetic effects. Interventions, essential or inessential, are seldom without negative side effects. Health can best, and most economically, be generated at the beginning of life, through healthy conception, gestation and birth. Understanding prime needs improves initial health. It also informs therapy of any early-life problems. Healing is therefore more efficient when transgenerational, and much more powerful than individual healing. My vision of healing is safeguarding our evolution in progress. Children's choices - eating, exercise, emotional attitudes and relationships - are already profoundly affecting any children they may have, their mental and physical health. The most practical starting point seems to be educating boys as well as girls. Childhood is therefore the time to educate them in choices. The correction of often unnoticed problems- nutrient deficits, toxins, uro-genital disease - has enabled nearly nine out of ten couples to bear fully healthy babies, even following severe problems - infertility, miscarriages, stillbirths and malformations. Correcting problems before conception prevents both structural faults and wrong setting of gene-switches. Children's habits set. Once courting most are preoccupied and many pregnant unintentionally. Childhood is the time to be adopting a healthy lifestyle, the way to healthy babies The mother's nutritional and emotional status throughout pregnancy continues to affect her child's future physical and mental health, behaviour and ability. Before conception a woman needs to build her appropriate body stores - vitamins and minerals, proteins, docosahexaenoic acid. Before bearing another child, a replenishment time of 3 years is desirable. A return to childbearing in the 20s and early 30s could reduce risks that have risen with the recent shift towards conception by school children and by women in their late 30s or more. Governments, schoolteachers, health professionals, need to adopt this policy of transgenerational health. Empowerment with knowledge is the one way to fend off the growing pandemic of mental ill health and related disorders and to make the most of a nation's genetic potential. Financially there could be no better investment, let alone in enhancing people's lives. Childhood is the most appropriate time for education in this way to generating a healthy, able and peaceful human race. Essential to our amazing genetic systems are the resources of land, sea and air. We are one with our biosphere. We need urgently to follow up the vital work of Developmental Origins of Health and Disease, and of Far East initiatives in sea-bed and sea husbandry.

Long noncoding RNA plays a key role in metastasis and prognosis of hepatocellular carcinoma

Long noncoding RNAs (lncRNAs) have been attracting immense research interests. However, only a handful of lncRNAs had been thoroughly characterized. They were involved in fundamental cellular processes including regulation of gene expression at epigenetics as well as tumorogenesis. In this paper, we give a systematic and comprehensive review of existing literature about lncRNA involvement in hepatocellular carcinoma. This review exhibited that lncRNAs played important roles in tumorigenesis and subsequent prognosis and metastasis of hepatocellular carcinoma and elucidated the role of some specific lncRNAs such as MALAT1 and HOTAIR in the pathophysiology of hepatocellular carcinoma and their potential of being therapeutic targets.

DNA methylation, epigenetics, and evolution in vertebrates: facts and challenges

DNA methylation is a key epigenetic modification in the vertebrate genomes known to be involved in biological processes such as regulation of gene expression, DNA structure and control of transposable elements. Despite increasing knowledge about DNA methylation, we still lack a complete understanding of its specific functions and correlation with environment and gene expression in diverse organisms. To understand how global DNA methylation levels changed under environmental influence during vertebrate evolution, we analyzed its distribution pattern along the whole genome in mammals, reptiles and fishes showing that it is correlated with temperature, independently on phylogenetic inheritance. Other studies in mammals and plants have evidenced that environmental stimuli can promote epigenetic changes that, in turn, might generate localized changes in DNA sequence resulting in phenotypic effects. All these observations suggest that environment can affect the epigenome of vertebrates by generating hugely different methylation patterns that could, possibly, reflect in phenotypic differences. We are at the first steps towards the understanding of mechanisms that underlie the role of environment in molding the entire genome over evolutionary times. The next challenge will be to map similarities and differences of DNA methylation in vertebrates and to associate them with environmental adaptation and evolution.

The DNA Habitat and its RNA Inhabitants: At the Dawn of RNA Sociology

Most molecular biological concepts derive from physical chemical assumptions about the genetic code that are basically more than 40 years old. Additionally, systems biology, another quantitative approach, investigates the sum of interrelations to obtain a more holistic picture of nucleotide sequence order. Recent empirical data on genetic code compositions and rearrangements by mobile genetic elements and noncoding RNAs, together with results of virus research and their role in evolution, does not really fit into these concepts and compel a reexamination. In this review, we try to find an alternate hypothesis. It seems plausible now that if we look at the abundance of regulatory RNAs and persistent viruses in host genomes, we will find more and more evidence that the key players that edit the genetic codes of host genomes are consortia of RNA agents and viruses that drive evolutionary novelty and regulation of cellular processes in all steps of development. This agent-based approach may lead to a qualitative RNA sociology that investigates and identifies relevant behavioral motifs of cooperative RNA consortia. In addition to molecular biological perspectives, this may lead to a better understanding of genetic code evolution and dynamics.

Viral ancestors of antiviral systems

All life must survive their corresponding viruses. Thus antiviral systems are essential in all living organisms. Remnants of virus derived information are also found in all life forms but have historically been considered mostly as junk DNA. However, such virus derived information can strongly affect host susceptibility to viruses. In this review, I evaluate the role viruses have had in the origin and evolution of host antiviral systems. From Archaea through bacteria and from simple to complex eukaryotes I trace the viral components that became essential elements of antiviral immunity. I conclude with a reexamination of the 'Big Bang' theory for the emergence of the adaptive immune system in vertebrates by horizontal transfer and note how viruses could have and did provide crucial and coordinated features.

Viral ancestors of antiviral systems

All life must survive their corresponding viruses. Thus antiviral systems are essential in all living organisms. Remnants of virus derived information are also found in all life forms but have historically been considered mostly as junk DNA. However, such virus derived information can strongly affect host susceptibility to viruses. In this review, I evaluate the role viruses have had in the origin and evolution of host antiviral systems. From Archaea through bacteria and from simple to complex eukaryotes I trace the viral components that became essential elements of antiviral immunity. I conclude with a reexamination of the 'Big Bang' theory for the emergence of the adaptive immune system in vertebrates by horizontal transfer and note how viruses could have and did provide crucial and coordinated features.

Quantification of nanoscale density fluctuations by electron microscopy: probing cellular alterations in early carcinogenesis

Most cancers are curable if they are diagnosed and treated at an early stage. Recent studies suggest that nanoarchitectural changes occur within cells during early carcinogenesis and that such changes precede microscopically evident tissue alterations. It follows that the ability to comprehensively interrogate cell nanoarchitecture (e.g., macromolecular complexes, DNA, RNA, proteins and lipid membranes) could be critical to the diagnosis of early carcinogenesis. We present a study of the nanoscale mass-density fluctuations of biological tissues by quantifying their degree of disorder at the nanoscale. Transmission electron microscopy images of human tissues are used to construct corresponding effective disordered optical lattices. The properties of nanoscale disorder are then studied by statistical analysis of the inverse participation ratio (IPR) of the spatially localized eigenfunctions of these optical lattices at the nanoscale. Our results show an increase in the disorder of human colonic epithelial cells in subjects harboring early stages of colon neoplasia. Furthermore, our findings strongly suggest that increased nanoscale disorder correlates with the degree of tumorigenicity. Therefore, the IPR technique provides a practicable tool for the detection of nanoarchitectural alterations in the earliest stages of carcinogenesis. Potential applications of the technique for early cancer screening and detection are also discussed.

MicroRNA in the regulation and expression of serotonergic transmission in the brain and other tissues

In addition to transcriptional regulation, the translation of protein-coding genes is modulated by MicroRNA-binding miRNAs (miRNAs), which have emerged to fulfil important roles in the control and expression of serotonergic transmission. Thus, miR-96 and miR-510 inhibit the translation of serotonin (5-HT)(1B) receptors and 5-HT(3E) receptor subunits respectively, and their susceptibility to repression is modified by polymorphisms in the 3'-UTR (miRNA-binding) regions of their mRNAs. Contrasting susceptibility of human subjects to miRNA-induced alterations in the translation of cerebral 5-HT(1B) receptors and intestinal 5-HT(3E) receptor subunits is related to differential aggressive behaviour and incidence of irritable bowel syndrome, respectively. Fluoxetine promotes the biogenesis of miR-16, leading to translational repression of 5-HT transporters in mouse serotonergic neurones. While the precise mechanism of action of fluoxetine is uncertain, studies of Aplysia have shown that 5-HT inhibits the generation of miR-124, thereby promoting de-repression of CREB and facilitation of synaptic plasticity. Interestingly, 5-HT(2C) receptors harbour a miRNA (miR-448) in their 4th intron that - oppositely to 5-HT(2C) sites - reduces adipocyte differentiation. Finally, interactions amongst 5-HT and miRNAs control processes of bone formation, as well as growth, motility and survival of tumours. The present article discusses the functionally and clinically important interplay amongst miRNAs and serotoninergic mechanisms in the brain, peripheral organs and cancerous tissue.

Introns form compositional clusters in parallel with the compositional clusters of the coding sequences to which they pertain

This report deals with the study of compositional properties of human gene sequences evaluating similarities and differences among functionally distinct sectors of the gene independently of the reading frame. To retrieve the compositional information of DNA, we present a neighbor base dependent coding system in which the alphabet of 64 letters (DNA triplets) is compressed to an alphabet of 14 letters here termed triplet composons. The triplets containing the same set of distinct bases in whatever order and number form a triplet composon. The reading of the DNA sequence is performed starting at any letter of the initial triplet and then moving, triplet-to-triplet, until the end of the sequence. The readings were made in an overlapping way along the length of the sequences. The analysis of the compositional content in terms of the composon usage frequencies of the gene sequences shows that: (i) the compositional content of the sequences is far from that of random sequences, even in the case of non-protein coding sequences; (ii) coding sequences can be classified as components of compositional clusters; and (iii) intron sequences in a cluster have the same composon usage frequencies, even as their base composition differs notably from that of their home coding sequences. A comparison of the composon usage frequencies between human and mouse homologous genes indicated that two clusters found in humans do not have their counterpart in mouse whereas the others clusters are stable in both species with respect to their composon usage frequencies in both coding and noncoding sequences.

Chromosome territories

Chromosome territories (CTs) constitute a major feature of nuclear architecture. In a brief statement, the possible contribution of nuclear architecture studies to the field of epigenomics is considered, followed by a historical account of the CT concept and the final compelling experimental evidence of a territorial organization of chromosomes in all eukaryotes studied to date. Present knowledge of nonrandom CT arrangements, of the internal CT architecture, and of structural interactions with other CTs is provided as well as the dynamics of CT arrangements during cell cycle and postmitotic terminal differentiation. The article concludes with a discussion of open questions and new experimental strategies to answer them.

Functional nuclear architecture studied by microscopy: present and future

In this review we describe major contributions of light and electron microscopic approaches to the present understanding of functional nuclear architecture. The large gap of knowledge, which must still be bridged from the molecular level to the level of higher order structure, is emphasized by differences of currently discussed models of nuclear architecture. Molecular biological tools represent new means for the multicolor visualization of various nuclear components in living cells. New achievements offer the possibility to surpass the resolution limit of conventional light microscopy down to the nanometer scale and require improved bioinformatics tools able to handle the analysis of large amounts of data. In combination with the much higher resolution of electron microscopic methods, including ultrastructural cytochemistry, correlative microscopy of the same cells in their living and fixed state is the approach of choice to combine the advantages of different techniques. This will make possible future analyses of cell type- and species-specific differences of nuclear architecture in more detail and to put different models to critical tests.

Repeat performance: how do genome packaging and regulation depend on simple sequence repeats?

Non-coding DNA has consistently increased during evolution of higher eukaryotes. Since the number of genes has remained relatively static during the evolution of complex organisms, it is believed that increased degree of sophisticated regulation of genes has contributed to the increased complexity. A higher proportion of non-coding DNA, including repeats, is likely to provide more complex regulatory potential. Here, we propose that repeats play a regulatory role by contributing to the packaging of the genome during cellular differentiation. Repeats, and in particular the simple sequence repeats, are proposed to serve as landmarks that can target regulatory mechanisms to a large number of genomic sites with the help of very few factors and regulate the linked loci in a coordinated manner. Repeats may, therefore, function as common target sites for regulatory mechanisms involved in the packaging and dynamic compartmentalization of the chromatin into active and inactive regions during cellular differentiation.

Epigenetic responses to environmental change and their evolutionary implications

Chromatin is a complex of DNA, RNA, histones and non-histone proteins and provides the platform on which the transcriptional machinery operates in eukaryotes. The structure and configuration of chromatin are manipulated by families of enzymes, some catalysing the dynamic addition and removal of chemical ligands to selected protein amino acids and some directly altering or displacing the basic structural units. The activities of many of these enzymes are sensitive to environmental and metabolic agents and can thereby serve as sensors through which environmental agents can alter gene expression. Such changes can, in turn, precipitate either local or cell-wide changes as the initial effect spreads through multiple interactive networks. This review discusses the increasingly well-understood mechanisms through which these enzymes alter chromatin function. In some cases at least, it seems that the effects on gene expression may persist even after the removal of the inducing agent, and can be passed on, through mitosis, to subsequent cell generations, constituting a heritable, epigenetic change. If such changes occur in germ cells or their precursors, then they may be passed on to subsequent generations. Mechanisms are now known to exist through which an epigenetic change might give rise to a localized change in DNA sequence exerting the same functional effect, thereby converting an epigenetic to a genetic change. If the induced genetic change has phenotypic effects on which selection can act, then this hypothetical chain of events constitutes a potential route through which the environment might directly influence evolution.

A role for the MS analysis of nucleic acids in the post-genomics age

The advances of mass spectrometry in the analysis of nucleic acids have tracked very closely the exciting developments of instrumentation and ancillary technologies, which have taken place over the years. However, their diffusion in the broader life sciences community has been and will be linked to the ever evolving focus of biomedical research and its changing demands. Before the completion of the Human Genome Project, great emphasis was placed on sequencing technologies that could help accomplish this project of exceptional scale. After the publication of the human genome, the emphasis switched toward techniques dedicated to the exploration of sequences not coding for actual protein products, which amount to the vast majority of transcribed elements. The broad range of capabilities offered by mass spectrometry is rapidly advancing this platform to the forefront of the technologies employed for the structure-function investigation of these noncoding elements. Increasing focus on the characterization of functional assemblies and their specific interactions has prompted a re-evaluation of what has been traditionally construed as nucleic acid analysis by mass spectrometry. Inspired by the accelerating expansion of the broader field of nucleic acid research, new applications to fundamental biological studies and drug discovery will help redefine the evolving role of MS-analysis of nucleic acids in the post-genomics age.

Deconstructing the dogma: a new view of the evolution and genetic programming of complex organisms

Since the birth of molecular biology it has been generally assumed that most genetic information is transacted by proteins, and that RNA plays an intermediary role. This led to the subsidiary assumption that the vast tracts of noncoding sequences in the genomes of higher organisms are largely nonfunctional, despite the fact that they are transcribed. These assumptions have since become articles of faith, but they are not necessarily correct. I propose an alternative evolutionary history whereby developmental and cognitive complexity has arisen by constructing sophisticated RNA-based regulatory networks that interact with generic effector complexes to control gene expression patterns and the epigenetic trajectories of differentiation and development. Environmental information can also be conveyed into this regulatory system via RNA editing, especially in the brain. Moreover, the observations that RNA-directed epigenetic changes can be inherited raises the intriguing question: has evolution learnt how to learn?

Nuclear organization in the 3D space of the nucleus - cause or consequence?

Recent evidence suggests that dynamic three-dimensional genomic interactions in the nucleus exert critical roles in regulated gene expression. Here, we review a series of recent paradigm-shifting experiments that highlight the existence of specific gene networks within the self-organizing space of the nucleus. These gene networks, evidenced by long-range intrachromosomal and interchromosomal interactions, can be considered as the cause or consequence of regulatory biological programs. Changes in nuclear architecture are a hallmark of laminopathies and likely potentiate genome rearrangements critical for tumor progression, in addition to potential vital contribution of noncoding RNAs and DNA repeats. It is virtually certain that we will witness an ever-increasing rate of discoveries that uncover new roles of nuclear architecture in transcription, DNA damage/repair, aging, and disease.

Transgenerational epigenetic inheritance: prevalence, mechanisms, and implications for the study of heredity and evolution

This review describes new developments in the study of transgenerational epigenetic inheritance, a component of epigenetics. We start by examining the basic concepts of the field and the mechanisms that underlie epigenetic inheritance. We present a comprehensive review of transgenerational cellular epigenetic inheritance among different taxa in the form of a table, and discuss the data contained therein. The analysis of these data shows that epigenetic inheritance is ubiquitous and suggests lines of research that go beyond present approaches to the subject. We conclude by exploring some of the consequences of epigenetic inheritance for the study of evolution, while also pointing to the importance of recognizing and understanding epigenetic inheritance for practical and theoretical issues in biology.

Modulation of gene expression in U251 glioblastoma cells by binding of mutant p53 R273H to intronic and intergenic sequences

Missense point mutations in the TP53 gene are frequent genetic alterations in human tumor tissue and cell lines derived thereof. Mutant p53 (mutp53) proteins have lost sequence-specific DNA binding, but have retained the ability to interact in a structure-selective manner with non-B DNA and to act as regulators of transcription. To identify functional binding sites of mutp53, we established a small library of genomic sequences bound by p53^R273H in U251 human glioblastoma cells using chromatin immunoprecipitation (ChIP). Mutp53 binding to isolated DNA fragments confirmed the specificity of the ChIP. The mutp53 bound DNA sequences are rich in repetitive DNA elements, which are dispersed over non-coding DNA regions. Stable down-regulation of mutp53 expression strongly suggested that mutp53 binding to genomic DNA is functional. We identified the PPARGC1A and FRMD5 genes as p53^R273H targets regulated by binding to intronic and intra-genic sequences. We propose a model that attributes the oncogenic functions of mutp53 to its ability to interact with intronic and intergenic non-B DNA sequences and modulate gene transcription via re-organization of chromatin.

Bridging the gap: The developmental aspects of evolution

The commentaries onEvolution in Four Dimensionsreflect views ranging from total adherence to gene-centered neo-Darwinism, to the acceptance of non-genetic and Lamarckian processes in evolution. We maintain that genetic, epigenetic, behavioral, and cultural variations have all been significant, and that the developmental aspects of heredity and evolution are an important bridge that can unite seemingly conflicting research programs and different disciplines.

Structural families of genomic microsatellites

We present an analysis of tandem repeats of short sequence motifs (microsatellites) in twelve eukaryotes for which a large part of the genome has been sequenced and assembled. The pattern of motif abundance varies significantly in different species, but it is very similar in different chromosomes of the same species. The most abundant repeats can be classified in two main families. The first family has a rigid conformation, with purines in one strand and pyrimidines in the complementary strand, mainly A(n)/T(n) and (AG)(n)/(CT)(n). The second family has alternating, flexible sequences, such as (AT)(n), (AC)(n) and related sequences. In the pluricellular organisms the relative frequency of both families is rather constant. These observations indicate that microsatellites have structural information and may be involved in the organization of chromatin fibers and in chromosome architecture in general. An additional intriguing finding is the absence of microsatellites with sequences which appear to be forbidden, such as (AATT)(n).

Raising the estimate of functional human sequences

While less than 1.5% of the mammalian genome encodes proteins, it is now evident that the vast majority is transcribed, mainly into non-protein-coding RNAs. This raises the question of what fraction of the genome is functional, i.e., composed of sequences that yield functional products, are required for the expression (regulation or processing) of these products, or are required for chromosome replication and maintenance. Many of the observed noncoding transcripts are differentially expressed, and, while most have not yet been studied, increasing numbers are being shown to be functional and/or trafficked to specific subcellular locations, as well as exhibit subtle evidence of selection. On the other hand, analyses of conservation patterns indicate that only approximately 5% (3%-8%) of the human genome is under purifying selection for functions common to mammals. However, these estimates rely on the assumption that reference sequences (usually ancient transposon-derived sequences) have evolved neutrally, which may not be the case, and if so would lead to an underestimate of the fraction of the genome under evolutionary constraint. These analyses also do not detect functional sequences that are evolving rapidly and/or have acquired lineage-specific functions. Indeed, many regulatory sequences and known functional noncoding RNAs, including many microRNAs, are not conserved over significant evolutionary distances, and recent evidence from the ENCODE project suggests that many functional elements show no detectable level of sequence constraint. Thus, it is likely that much more than 5% of the genome encodes functional information, and although the upper bound is unknown, it may be considerably higher than currently thought.

The neoselectionist theory of genome evolution

The vertebrate genome is a mosaic of GC-poor and GC-rich isochores, megabase-sized DNA regions of fairly homogeneous base composition that differ in relative amount, gene density, gene expression, replication timing, and recombination frequency. At the emergence of warm-blooded vertebrates, the gene-rich, moderately GC-rich isochores of the cold-blooded ancestors underwent a GC increase. This increase was similar in mammals and birds and was maintained during the evolution of mammalian and avian orders. Neither the GC increase nor its conservation can be accounted for by the random fixation of neutral or nearly neutral single-nucleotide changes (i.e., the vast majority of nucleotide substitutions) or by a biased gene conversion process occurring at random genome locations. Both phenomena can be explained, however, by the neoselectionist theory of genome evolution that is presented here. This theory fully accepts Ohta's nearly neutral view of point mutations but proposes in addition (i) that the AT-biased mutational input present in vertebrates pushes some DNA regions below a certain GC threshold; (ii) that these lower GC levels cause regional changes in chromatin structure that lead to deleterious effects on replication and transcription; and (iii) that the carriers of these changes undergo negative (purifying) selection, the final result being a compositional conservation of the original isochore pattern in the surviving population. Negative selection may also largely explain the GC increase accompanying the emergence of warm-blooded vertebrates. In conclusion, the neoselectionist theory not only provides a solution to the neutralist/selectionist debate but also introduces an epigenomic component in genome evolution.

A new paradigm for developmental biology

It is usually thought that the development of complex organisms is controlled by protein regulatory factors and morphogenetic signals exchanged between cells and differentiating tissues during ontogeny. However, it is now evident that the majority of all animal genomes is transcribed, apparently in a developmentally regulated manner, suggesting that these genomes largely encode RNA machines and that there may be a vast hidden layer of RNA regulatory transactions in the background. I propose that the epigenetic trajectories of differentiation and development are primarily programmed by feed-forward RNA regulatory networks and that most of the information required for multicellular development is embedded in these networks, with cell–cell signalling required to provide important positional information and to correct stochastic errors in the endogenous RNA-directed program.