Role of transposable elements in the genome of Drosophila melanogaster

Abstract

The discovery of transposable elements (TEs) has raised the question of their role in host genomes. The experimental data accumulated over the last three decades has allowed scientists to draw conclusions on the role and behavior of TEs in genomes. It is currently known that the spontaneous rate of TE insertions in drosophila is 10 -4 -10 -5 events per site per genome per generation, which is one to two orders of magnitude higher than the rate of spontaneous recessive mutations. It is believed that most oligogenic (majorgenic) mutations result from TE insertion. Moreover, TEs can regulate the function and activity of major and minor genes, because they contain control system motifs and enhancers, which consist of several modules, and thus can bind to various transcription factors. Crossover between long terminal repeats can result in various types of chromosomal rearrangements: deletions, duplications, and inversions. Transposable elements can fill chromosome telomeres and take part in horizontal gene transfer. Various stress factors acting on genomes cre ate transposition explosions as a response. In this work we estimate the TE transposition induction rates in response to various external stress factors: cold and heat shock, ethanol vapor, and γ irradiation. We show that external stress factors increase the TE transposition rate to 10 -2 -10 -3 events per site per genome per gener ation, which is one to two orders of magnitude greater than the spontaneous transposition rate. Also, TE pat terns are significantly influenced by genetic factors: outbreeding, inbreeding, and selection. Thus, we con clude that the system of transposable elements in genomes is as substantial and universal as the SOS repara tion and hormonal control systems. It is reasonable to suggest that TEs act as receptors of stress signals, which initiate explosions of transpositional variation during critical phases of population evolution. This may cause a rapid adjustment of the specific homeostatic norm and, probably, speciation. We can thus state that the pres ence of TEs, in particular in eukaryotic genomes, allows a population to survive under dramatically altered environmental conditions. Therefore, we can suggest that TEs are directly involved in the expression of genes encoding traits in the course of selection and evolution.

Evolutionary interaction between W/Y chromosome and transposable elements

“…The above findings support our idea that transposable element transcript invasion in a cell (as a result of W/Y chromosome aging) may lead to increased expression of the PIWI protein complex, and thus entails some energetic costs for cell systems. Because the PIWI and Hsp90 heat-shock proteins also have other functions in addition to transposable element transcript silencing, in situations of environmental or internal stress the level of their expression may not be high enough to maintain genome defense at the required level (Vasil’eva et al 2011 ).…”

Profuse evolutionary diversification and speciation on volcanic islands: transposon instability and amplification bursts explain the genetic paradox

“…The stressors include both biotic factors, such as inbreeding [ 88 , 89 ] and interspecific hybridization [ 90 – 92 ], and abiotic factors. Among environmental stressors is the physical factor of heat shock which has been demonstrated to increase transposition rates of a variety of elements by one or two orders of magnitude in diverse organisms from yeast to flies [ 81 – 83 , 93 , 94 ]. Other physical factors include cold shock, UV radiation and γ-radiation [ 83 ].…”

“…Among environmental stressors is the physical factor of heat shock which has been demonstrated to increase transposition rates of a variety of elements by one or two orders of magnitude in diverse organisms from yeast to flies [ 81 – 83 , 93 , 94 ]. Other physical factors include cold shock, UV radiation and γ-radiation [ 83 ]. Likewise, a variety of chemicals and toxins can induce high levels of transposition, even if the exposure is very brief ( e.g ., 1.5 min exposure to ethanol vapors!)…”

“…Likewise, a variety of chemicals and toxins can induce high levels of transposition, even if the exposure is very brief ( e.g ., 1.5 min exposure to ethanol vapors!) [ 83 ]. Extrinsic environmental stressors as well as a variety of cellular stresses [ 95 ] trigger bursts of transposition resulting in dramatic amplification of TE copy numbers and generation of novel mutations, some of which may be adaptive [ 68 , 74 , 96 – 98 ] and promote survival in the local changed environment.…”

“…The induced TE mobilization would lead to a transient period of genomic instability, corresponding perhaps to the stochastic phase of genetic disorganization hypothesized by Carson [ 48 , 49 ] to be critical for speciation, before their activity is once more suppressed by silencing mechanisms, restoring genome stability (equivalent perhaps to the genetic reorganization phase in Carson’s hypothetical model). The situation may involve more than just relaxation of the epigenetic control mechanisms [ 59 ], with certain TEs acting as stress-responsive regulators of host gene expression [ 62 , 68 , 82 , 83 , 85 , 100 ], and the altered transcriptional patterns perhaps permitting the organism to survive the stressful conditions.…”