Transposable Elements and Stress in Vertebrates: An Overview

Abstract

Since their identification as genomic regulatory elements, Transposable Elements (TEs) were considered, at first, molecular parasites and later as an important source of genetic diversity and regulatory innovations. In vertebrates in particular, TEs have been recognized as playing an important role in major evolutionary transitions and biodiversity. Moreover, in the last decade, a significant number of papers has been published highlighting a correlation between TE activity and exposition to environmental stresses and dietary factors. In this review we present an overview of the impact of TEs in vertebrate genomes, report the silencing mechanisms adopted by host genomes to regulate TE activity, and finally we explore the effects of environmental and dietary factor exposures on TE activity in mammals, which is the most studied group among vertebrates. The studies here reported evidence that several factors can induce changes in the epigenetic status of TEs and silencing mechanisms leading to their activation with consequent effects on the host genome. The study of TE can represent a future challenge for research for developing effective markers able to detect precocious epigenetic changes and prevent human diseases.

Investigation of the activity of transposable elements and genes involved in their silencing in the newt Cynops orientalis, a species with a giant genome

“…This idea is consistent with the expression levels of TEs and genes encoding proteins involved in heterochromatin formation and silencing in this species. Overall, the activity of TEs can increase due to environmental stressors such as temperature 22 . In particular, the beginning of Carboniferous and the transition from Permian to Triassic were characterized by severe environmental changes 56 that might have provoked a reduction of the efficiency of silencing mechanisms in gonads, leading to a mobilization of TEs and thus to an increase of genome size in lungfish and newts 17 .…”

“…piRNAs are extremely heterogeneous since they derive from transposons inserted in piRNA clusters, genomic regions transcribed as long single-stranded precursor RNA molecules, later processed into mature piRNAs. The transcription of these genomic regions seems to be activated by the deposition of histone 3 lysine 9 trimethylation (H3K9me3) by the protein SETDB1 22 . Subsequently the piRNA precursor transcripts are shuttled by the HMG protein Maelstrom from the nucleus to the cytoplasm, where they are cleaved by the PLD6 nuclease to form the primary piRNAs and by the HMG protein Maelstrom to form primary and secondary piRNAs 23 , 24 .…”

Transposable Elements in the Genome of Human Parasite Schistosoma mansoni: A Review

“…The advancement of sequencing techniques allowed the study of diverse genomes, evidencing the occurrence of TEs in virtually all eukaryotic species [ 11 , 12 , 31 , 32 ]. TEs are commonly kept under control by cellular silencing mechanisms—such as small interfering RNAs, DNA methylation, or histone modifications—to prevent disadvantageous events resulting from transpositions [ 21 , 33 , 34 , 35 ]. Novel or stressful environmental conditions may alter epigenetic modifications leading to the activation of silent TEs, providing opportunities for genomic and phenotypic innovations that favor adaptation through natural selection [ 12 , 20 , 21 , 22 , 23 ].…”

“…TEs are DNA sequences apt to transpose into a host genome within the cell [ 10 , 11 ], enabling them to influence the evolutionary trajectory of the species [ 10 , 12 , 13 , 14 , 15 ]. Although deleterious or neutral effects are likely [ 16 , 17 , 18 , 19 ], TEs mobilization may cause genomic innovations that lead to fitness increment, especially under stress conditions [ 12 , 20 , 21 , 22 , 23 ]. As a result of transposition events, genes may have their regulatory or coding sequences altered, or new genes may be established [ 16 , 17 , 24 , 25 , 26 , 27 , 28 ], with a positive effect.…”

“…As a result of transposition events, genes may have their regulatory or coding sequences altered, or new genes may be established [ 16 , 17 , 24 , 25 , 26 , 27 , 28 ], with a positive effect. Due to these features, TEs have been considered a powerful and fast source of genetic variability by providing a broad spectrum of raw material on which natural selection may operate, especially in conditions of severe evolutionary pressure [ 12 , 14 , 21 ].…”