| Transposable elements as selfish DNA

Abstract

Abstract Biologists seek functional explanations for the attributes of organisms. Such explanations show the advantage that the attribute gives to the organism. Any attribute, including components of the genome, can potentially be explained in this way. The belief that such functional explanations are appropriate rests on the tenet of the Neo-Darwinian evolutionary theory that states that the only systematic mechanism by which genes or other DNA sequences can spread through populations is through differences in the fitnesses of their carriers. Transposable elements, however, increase in number either directly or indirectly during transposition, and thus could potentially spread without increasing the fitness of their hosts. This non-Darwinian behaviour illegitimizes functional explanations and requires us to seek causal explanations for the presence of transposable elements in genomes, incorporating both their own molecular properties and their effects on their hosts. One class of explanations are those in which the effects of transposable elements on their hosts are negative. In other words, they are ‘selfish DNAs’ (1, 2). In this chapter, I will discuss some of the ideas and experiments indicating the selfishness or otherwise of transposable genetic elements. For details of the structures of various classes of transposable elements and the mechanisms of their transposition, the reader should consult other chapters in this volume. I will concentrate mainly on the elements in sexually reproducing diploid eukaryotes, for which the data sets of the distributions and frequencies of elements are often more complete, and for which the Neo-Darwinian population genetics synthesis provides a framework for considerations of transposable element evolution. I will strive, however, to highlight the ways in which the evolutionary process affecting prokaryotic transposable elements shows important similarities to and differences from the diploid paradigm.