Transposable Elements as Sources of Genomic Variation

Abstract

This chapter provides a general description of the types of genetic variation caused by transposable elements in animals and plants, and examines this variation within an evolutionary framework. It focuses on the variation induced by transposable elements in their host organisms. The host variation associated with transposable elements can result from several interconnected aspects of transposable element activity. Estimates of the frequencies of new transposable element-induced mutations have been made under laboratory conditions and varied over an enormous range. The partial or complete sterility associated with several systems of hybrid dysgenesis in Drosophila provides an interesting aspect of variation associated with transposable element activity. Heterochromatin proteins can recognize and silence transposable elements, some of which target heterochromatin for insertion. Thus, the evolution of heterochromatin could have led to a self-perpetuating expansion of domains rich in transposable elements. Two mechanisms are considered most likely to be responsible for transposable element-induced karyotypic changes. The best known mechanism is ectopic recombination, in which homologous recombination occurs between multiple copies of a transposable element present in a genome. A second mechanism for inducing genomic rearrangements is alternative transposition of class II elements in bacteria, plants, and animals. Some features of both transposable elements and hosts suggest coadaptations to mitigate the reduction of fitness expected from unfettered transposition, and to provide a wide range of new variations on which natural selection can act.