Transposable elements as sources of variation in animals and plants

Abstract

A tremendous wealth of data is accumulating on the variety and distribution of transposable elements (TEs) in natural populations. There is little doubt that TEs provide new genetic variation on a scale, and with a degree of sophistication, previously unimagined. There are many examples of mutations and other types of genetic variation associated with the activity of mobile elements. Mutant phenotypes range from subtle changes in tissue specificity to dramatic alterations in the development and organization of tissues and organs. Such changes can occur because of insertions in coding regions, but the more sophisticated TE-mediated changes are more often the result of insertions into 5' flanking regions and introns. Here, TE-induced variation is viewed from three evolutionary perspectives that are not mutually exclusive. First, variation resulting from the intrinsic parasitic nature of TE activity is examined. Second, we describe possible coadaptations between elements and their hosts that appear to have evolved because of selection to reduce the deleterious effects of new insertions on host fitness. Finally, some possible cases are explored in which the capacity of TEs to generate variation has been exploited by their hosts. The number of well documented cases in which element sequences appear to confer useful traits on the host, although small, is growing rapidly.

Figure 1

Rearrangements associated with TE activity near a gene result in altered tissue specificity. (A) In the original isolate, a Tam3 element had inserted 64 bp upstream of the start of transcription of the niv gene of Antirrhinum. The result of this insertion was a reduced level of expression. (B) A derivative of the initial insertion allele carries an inversion flanked by two copies of the transposon. This allele confers an additional reduction in the level of expression. (C) Excision of the element closest to the niv gene left a short (26 bp) “footprint.” This rearrangement resulted in an increase in the level of niv gene expression as well as a novel pattern of expression, presumably due to the juxtaposition of a novel sequence with the niv gene TATA box and coding sequences [adapted from Lister et al. (20)].