29 mammalian genomes reveal novel exaptations of mobile elements for likely regulatory functions in the human genome

Arguments for standardizing transposable element annotation in plant genomes

Whole genome sequence assemblies have been generated for many plants. Annotation of transposable elements (TEs), which constitute the major proportion of genomes and play a significant role in epigenome alterations under stress, has not been given equal importance to that of genes. In this opinion article, we argue that the lack of focus dedicated to the fine-scale characterization of repeat fractions and the absence of consistent methods for their annotation impede our ability to critically understand the influence of TEs on the epigenome with implications in gene expression and non-Mendelian inheritance. Major structural changes occur over an evolutionary time scale. However, epigenetic regulation mediated by TEs can happen in a single generation, thus emphasizing the need for their standardized annotation.

Exaptation of transposable elements into novel cis-regulatory elements: is the evidence always strong?

Transposable elements (TEs) are mobile genetic sequences that can jump around the genome from one location to another, behaving as genomic parasites. TEs have been particularly effective in colonizing mammalian genomes, and such heavy TE load is expected to have conditioned genome evolution. Indeed, studies conducted both at the gene and genome levels have uncovered TE insertions that seem to have been co-opted--or exapted--by providing transcription factor binding sites (TFBSs) that serve as promoters and enhancers, leading to the hypothesis that TE exaptation is a major factor in the evolution of gene regulation. Here, we critically review the evidence for exaptation of TE-derived sequences as TFBSs, promoters, enhancers, and silencers/insulators both at the gene and genome levels. We classify the functional impact attributed to TE insertions into four categories of increasing complexity and argue that so far very few studies have conclusively demonstrated exaptation of TEs as transcriptional regulatory regions. We also contend that many genome-wide studies dealing with TE exaptation in recent lineages of mammals are still inconclusive and that the hypothesis of rapid transcriptional regulatory rewiring mediated by TE mobilization must be taken with caution. Finally, we suggest experimental approaches that may help attributing higher-order functions to candidate exapted TEs.

ZBED evolution: repeated utilization of DNA transposons as regulators of diverse host functions

ZBED genes originate from domesticated hAT DNA transposons and encode regulatory proteins of diverse function in vertebrates. Here we reveal the evolutionary relationship between ZBED genes and demonstrate that they are derived from at least two independent domestication events in jawed vertebrate ancestors. We show that ZBEDs form two monophyletic clades, one of which has expanded through several independent duplications in host lineages. Subsequent diversification of ZBED genes has facilitated regulation of multiple diverse fundamental functions. In contrast to known examples of transposable element exaptation, our results demonstrate a novel unprecedented capacity for the repeated utilization of a family of transposable element-derived protein domains sequestered as regulators during the evolution of diverse host gene functions in vertebrates. Specifically, ZBEDs have contributed to vertebrate regulatory innovation through the donation of modular DNA and protein interacting domains. We identify that C7ORF29, ZBED2, 3, 4, and ZBEDX form a monophyletic group together with ZBED6, that is distinct from ZBED1 genes. Furthermore, we show that ZBED5 is related to Buster DNA transposons and is phylogenetically separate from other ZBEDs. Our results offer new insights into the evolution of regulatory pathways, and suggest that DNA transposons have contributed to regulatory complexity during genome evolution in vertebrates.

Transposable elements as genetic regulatory substrates in early development

The abundance and ancient origins of transposable elements (TEs) in eukaryotic genomes has spawned research into the potential symbiotic relationship between these elements and their hosts. In this review, we introduce the diversity of TEs, discuss how distinct classes are uniquely regulated in development, and describe how they appear to have been coopted for the purposes of gene regulation and the orchestration of a number of processes during early embryonic development. Although young, active TEs play an important role in somatic tissues and evolution, we focus mostly on the contributions of the older, fixed elements in mammalian genomes. We also discuss major challenges inherent in the study of TEs and contemplate future experimental approaches to further investigate how they coordinate developmental processes.

How do mammalian transposons induce genetic variation? A conceptual framework: the age, structure, allele frequency, and genome context of transposable elements may define their wide-ranging biological impacts

In this essay, we discuss new insights into the wide-ranging impacts of mammalian transposable elements (TE) on gene expression and function. Nearly half of each mammalian genome is comprised of these mobile, repetitive elements. While most TEs are ancient relics, certain classes can move from one chromosomal location to another even now. Indeed, striking recent data show that extensive transposition occurs not only in the germline over evolutionary time, but also in developing somatic tissues and particular human cancers. While occasional germline TE insertions may contribute to genetic variation, many other, similar TEs appear to have little or no impact on neighboring genes. However, the effects of somatic insertions on gene expression and function remain almost completely unknown. We present a conceptual framework to understand how the ages, allele frequencies, molecular structures, and especially the genomic context of mammalian TEs each can influence their various possible functional consequences.