The behavior and evolution of Class II transposable elements in the malarial mosquito, Anopheles gambiae

Abstract

Transposable elements are DNA sequences with a unique ability to change their genomic location. Transposable elements are fascinating because of their ability to move, and their ubiquitous presence and contribution to the evolution of all prokaryotic and eukaryotic genomes. Their mobility properties have made them extremely useful as molecular tools in the laboratory. Transposable elements have also been proposed to be useful as genetic drive agents to introduce phenotype-altering genes in natural populations of mosquitoes, to control vector-borne diseases such as malaria. Presented in this thesis are studies on the behavior and evolution of two endogenous Class II transposable elements, Herves and Topi in natural populations of Anopheles gambiae, a species seriously being considered for population modification using genetic manipulation. In Chapters 2 and 4, results from the analysis of copy number, activity, and nucleotide sequence as well as structural diversity of Herves and Topi elements, respectively in 5-6 An.gambiae populations in Africa are described. In Chapter 3, studies to identify and assess the activity of the natural variants of Herves transposase in An.gambiae are described. The results from these studies show that both Herves and Topi elements have long histories in An.gambiae with Topi present in An.gambiae earlier than Herves. Herves, but not Topi, is still active in natural populations of An.gambiae with more than one active form of Herves transposase responsible for its activity. Both the elements, despite their long history in An.gambiae, have a very high percentage of individuals with complete forms of the element. This observation is an unusual feature of these elements, which would not be predicted for elements with such a long history. The presence of complete and active forms of Herves and Topi, elements with long histories in An.gambiae, argues against the possibility of rapid accumulation of deleted forms of transposable elements as a general feature of their evolution. Better understanding of the behavior and evolution of Class II transposable elements in An.gambiae shows that Class II transposable elements still hold promise as a genetic drive agent for this particular species.