Transposable elements in mosquitoes and other insect species

Insect transposable elements occur as intracellular parasitic DNA sequences which are amplified in genomes either by a pure DNA replication mechanism, or for the most frequent elements, via reverse transcription of an RNA intermediate. Due to their structural properties and their coding potential, many of them are potentially able to invade the chromosomes. However, selection pressures and evolution have apparently established true host-parasite relationships between these “selfish” sequences and the genomes which harbour them. Transposable elements seem to be expressed only occasionally, upon stimulation by genetic or environmental factors or in situations of “genomic shocks” induced by stresses. Such an activation of a given entity in the germline will result in the amplification and dispersion of daughter copies into the host genome. This appears to have occurred recently in mosquito chromosomes for the Juan retroposons: they have been probably amplified from one master element present in a unique population which has since spread worldwide. Horizontal transfers between species might also contribute to the spread of some elements, a fact which can restrict the use of recombinant organisms in the field. There is strong evidence that transposable elements are responsible for variability and biodiversity of insect populations. For example, in Culex pipiens mosquitoes, transposition events are responsible for polymorphism in the region of the esterase B gene locus and for insecticide resistance properties. Because they may affect the biological properties of individuals carrying them, transposable elements can be tools for the development of efficient genetical control methods of pest species.