Are Obese Plant Genomes on a Diet?

Richard Dawkins' (1976) selfish DNA hypothesis (that the only purpose of DNA is to perpetuate itself) is clearly reflected in the case of repetitive DNA, especially retrotransposons. These ubiquitous, self-replicating DNA elements do not seem to do anything but invade the host's genome (Orgel and Crick 1980; Doolittle and Sapienza 1980). Mutations caused by the activity of retrotransposons may eventually be evolutionarily advantageous, but are more likely to be deleterious for the host organism and thus eliminated from the population (Charlesworth et al. 1994). However, insertions of those elements that do not alter any functional region of the genome may be perpetuated in the population.

In this scenario, differences in retrotransposon activity leading to the accumulation of multiple repeats of these elements can easily explain the large differences in genome size observed even among related organisms. The observation that a large genome is not correlated with the complexity of the organism is known as the C-value paradox (Thomas 1971). Amplification of mobile elements would be limited by loss of host fitness due to their deleterious effects, but at least in some species, other mechanisms for preventing retrotransposon activity must exist to explain differences in the amount of repetitive DNA. DNA methylation has been proposed to be one such mechanism (Martienssen 1998), although other mechanisms may exist in organisms that lack methylation.

Introducing a different perspective, Petrov et al. (1996) showed that there is not only limitation of transposon proliferation, but also deletion of repetitive DNA: In the unmethylated genome …