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 create 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 generation, which is one to two orders of magnitude greater than the spontaneous transposition rate. Also, TE patterns are significantly influenced by genetic factors: outbreeding, inbreeding, and selection. Thus, we conclude that the system of transposable elements in genomes is as substantial and universal as the SOS reparation 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 presence 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.