Transposable elements in the genomes: parasites, junks or drivers of evolution?

In many organisms with large and complex genomes, transposable elements (TEs) constitute up to 50% of the genomic DNA. TEs have been widely studied and they showed high similarities across kingdoms. Several reasons have been adduced for the diverse TEs among plants and animal species. Activities of TEs could give rise to altered gene or genome at very high frequencies in both germinal and somatic tissues. TE-induced genetic variability can range widely; from changes in the arrangement of the whole genome to changes in single nucleotides. This may produce major effects on the phenotypic traits or small silent changes detectable only at the DNA sequence level. TE-induced mutation in the regulatory sequences may be of evolutionary significance and insertions in promoter sequences can alter tissue-specific patterns of gene expression. In addition, transposons can be involved in amplification and dispersal of genes by taking up portions of other sequences within the TEs themselves, moving them to new locations, thereby increasing their copy number. Therefore, mobilization of TEs might benefit the host through enhancement of genetic diversity. However, TE movement have been linked with undesirable traits in plants, hybrid dysgenesis in Drosophila and genome instability and diseases in mammals. This review summarises the diversity of TEs across plant and animal kingdoms and their impact and possible role in genomic evolution. It also describes some adverse effects of TEs as agents of genomic instability and diseases such as cancers, and the genetic regulations of their activities.