Transposable elements and the evolution of eukaryotic genomes

When transposable elements (TEs) were discovered in maize by Barbara McClintock >50 years ago they were regarded as a curiosity; now they are known to be the most abundant component of probably all eukaryotic genomes. They account for almost 50% of the human genome and >70% of the genomes of some grass species, including maize. As such, they make up the vast majority of the output of genome sequencing projects. The availability of so much new information has fueled a revolution in their analysis and studies of their interaction with the host.

In addition to discovering TEs, McClintock also uncovered disparate ways that TEs can alter genetic information. At one end of the spectrum she found that TEs could restructure genomes through element-mediated chromosomal rearrangements. At the other end she and others found they could generate new alleles by inserting into and around genes and altering their expression. Thus, the presence and extraordinary abundance of TEs in eukaryotic genomes promote a myriad of genome-altering events.

TEs are fragments of DNA that can insert into new chromosomal locations, and they often make duplicate copies of themselves in the process. Eukaryotic TEs are divided into two classes, according to whether their transposition intermediate is RNA (class 1) or DNA (class 2) (Fig. 1). For all class 1 elements, the element-encoded transcript (mRNA) forms the transposition intermediate. In contrast, with class 2 elements, the element itself moves from one site to another in the …

*E-mail: sue{at}plantbio.uga.edu