Transposons and Tandem Repeats Are Not Involved in the Control of Genomic Imprinting at the MEDEA Locus in Arabidopsis

Abstract

Transposons are selfish mobile DNA elements that caninsert into nonhomologous target sites, thereby amplifying their copy number in the genome. Yet, transpositionis considered tightly controlled, because unregulated amplification could have severe consequences for the fitnessof the host organism. Nonetheless, transposons constitute~45% of the human genome and ~80% of the maizegenome. Transposable elements can act as both "attractors" and "mediators" of epigenetic regulation across thegenome. The potential for transposons to show epigeneticactivity leading to effects on phenotypic variation wasfirst recognized by McClintock (see McClintock 1984) as"changes in phase." Some years ago it was proposed thattransposons may be involved in a variety of epigeneticphenomena such as gene silencing, paramutation, and genomic imprinting (Martienssen 1996; Matzke et al.1996). Indeed, there is growing evidence that transposonscan act as epigenetic mediators of phenotypic variation.Here, we briefly review the role of transposons and repeated sequences in epigenetic gene regulation and investigate their potential role in controlling genomic imprinting at the MEDEA locus of Arabidopsis...