Plant genomes are unique in an intriguing feature: the range of their size variation is unprecedented among living organisms. Although polyploidization contributes to this variability, transposable elements (TEs) seem to play the pivotal role. TEs, often considered intragenomic parasites, not only affect the genome size of the host, but also interact with other genes, disrupting and creating new functions and regulatory networks. Coevolution of plant genomes and TEs has led to tight regulation of TE activity, and growing evidence suggests their relationship became mutualistic. Although the expansions of TEs represent certain costs for the host genomes, they may also bring profits for populations, helping to overcome challenging environmental (biotic/abiotic stress) or genomic (hybridization and allopolyploidization) conditions. In this paper, we discuss the possibility that the possession of inducible TEs may provide a selective advantage for various plant populations. 1. Transpositional Strategies, Distribution, and Regulation of TEs Transposable elements (TEs) comprise a palette of immensely diverse DNA structures that can be unified by the following definition: they all are (or have been) able to insert themselves (or new copies of themselves) into new locations within genome. According to their mechanism of transposition, TEs can be classified [1] into class I elements (retroelements) transposing through an RNA intermediate, and class II elements (DNA transposons) moving only via DNA. The major superfamilies of class I are Ty1-copia and Ty3-gypsy retrotransposons, while class II is represented by TIR (terminal inverted repeat) elements and Helitrons, which are sometimes classified separately [2]. Among both retrotransposons and transposons, nonautonomous forms (e.g., MITEs, SINEs, and LARDs) are quite prevalent [3], utilizing the transpositional machinery of autonomous TEs. A significant portion of plant genomes is constituted by class I elements (specifically LTR retrotransposons, with direct long terminal repeats at both ends), which replicate in a “copy-and-paste” manner. In brief (according to [3, 4]), the genomic DNA copy of a retroelement is transcribed into mRNA that enters the cytosol, similarly to standard DNA transcripts. The information of the mRNA is translated, typically creating a structural protein GAG and a polyprotein POL. These protein products associate with other retroelement mRNA copies and pack them into virus-like particles. Within these structures, dimerized mRNA copies are reversely transcribed into cDNA and the whole
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