Editing Transposable element (section)

== Adaptive TEs ==

Transposable elements have been recognized as good candidates for stimulating gene adaptation, through their ability to regulate the expression levels of nearby genes.<ref name=":3">{{cite journal | vauthors = Mariño-Ramírez L, Lewis KC, Landsman D, Jordan IK | title = Transposable elements donate lineage-specific regulatory sequences to host genomes | journal = Cytogenetic and Genome Research | volume = 110 | issue = 1–4 | pages = 333–41 | year = 2005 | pmid = 16093685 | pmc = 1803082 | doi = 10.1159/000084965 }}</ref> Combined with their "mobility", transposable elements can be relocated adjacent to their targeted genes, and control the expression levels of the gene, dependent upon the circumstances.

The study conducted in 2008, "High Rate of Recent Transposable Element–Induced Adaptation in Drosophila melanogaster", used ''D. melanogaster'' that had recently migrated from Africa to other parts of the world, as a basis for studying adaptations caused by transposable elements. Although most of the TEs were located on introns, the experiment showed a significant difference in gene expressions between the population in Africa and other parts of the world. The four TEs that caused the selective sweep were more prevalent in ''D. melanogaster'' from temperate climates, leading the researchers to conclude that the selective pressures of the climate prompted genetic adaptation.<ref name=":4">{{cite journal | vauthors = González J, Lenkov K, Lipatov M, Macpherson JM, Petrov DA | title = High rate of recent transposable element-induced adaptation in Drosophila melanogaster | journal = PLOS Biology | volume = 6 | issue = 10 | pages = e251 | date = October 2008 | pmid = 18942889 | pmc = 2570423 | doi = 10.1371/journal.pbio.0060251 | doi-access = free }}</ref> From this experiment, it has been confirmed that adaptive TEs are prevalent in nature, by enabling organisms to adapt gene expression as a result of new selective pressures.

However, not all effects of adaptive TEs are beneficial to the population. In the research conducted in 2009, "A Recent Adaptive Transposable Element Insertion Near Highly Conserved Developmental Loci in Drosophila melanogaster", a TE, inserted between Jheh 2 and Jheh 3, revealed a downgrade in the expression level of both of the genes. Downregulation of such genes has caused ''Drosophila'' to exhibit extended developmental time and reduced egg to adult viability. Although this adaptation was observed in high frequency in all non-African populations, it was not fixed in any of them.<ref name=":5">{{cite journal | vauthors = González J, Macpherson JM, Petrov DA | title = A recent adaptive transposable element insertion near highly conserved developmental loci in Drosophila melanogaster | journal = Molecular Biology and Evolution | volume = 26 | issue = 9 | pages = 1949–61 | date = September 2009 | pmid = 19458110 | pmc = 2734154 | doi = 10.1093/molbev/msp107 }}</ref> This is not hard to believe, since it is logical for a population to favor higher egg to adult viability, therefore trying to purge the trait caused by this specific TE adaptation.

At the same time, there have been several reports showing the advantageous adaptation caused by TEs. In the research done with silkworms, "An Adaptive Transposable Element insertion in the Regulatory Region of the EO Gene in the Domesticated Silkworm", a TE insertion was observed in the cis-regulatory region of the EO gene, which regulates molting hormone 20E, and enhanced expression was recorded. While populations without the TE insert are often unable to effectively regulate hormone 20E under starvation conditions, those with the insert had a more stable development, which resulted in higher developmental uniformity.<ref>{{cite journal | vauthors = Sun W, Shen YH, Han MJ, Cao YF, Zhang Z | title = An adaptive transposable element insertion in the regulatory region of the EO gene in the domesticated silkworm, Bombyx mori | journal = Molecular Biology and Evolution | volume = 31 | issue = 12 | pages = 3302–13 | date = December 2014 | pmid = 25213334 | doi = 10.1093/molbev/msu261 | doi-access = free }}</ref>

These three experiments all demonstrated different ways in which TE insertions can be advantageous or disadvantageous, through means of regulating the expression level of adjacent genes. The field of adaptive TE research is still under development and more findings can be expected in the future.