LINEs, SINEs and repetitive DNA: non-LTR retrotransposons in plant genomes.

Retroelements and remnants thereof constitute a large fraction of the repetitive DNA of plant genomes. They include LTR (long terminal repeat) retrotransposons such as Ty1-copia and Ty3-gypsy retrotransposons, which are widespread in plant genomes and show structural similarity to retroviruses. Recently, non-LTR retrotransposons, lacking the long terminal repeats and subdivided into LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements), have been discovered as ubiquitous components of nuclear genomes in many species across the plant kingdom. LINEs are probably the most ancient class of retrotransposons in plant genomes, but the evolutionary borders between non-LTR retrotransposons, LTR retrotransposons and retroviruses are indistinct as shown by the detection of intermediate forms in other eukaryotic taxa. Transposition of non-LTR retrotransposons is only rarely observed in plants indicating that the majority of these retroelements are inactive and/or under regulation of the host genome. Transposition is poorly understood, but experimental evidence from other genetic systems, in particular from insect and mammalian species, shows that LINEs are able to transpose autonomously, while non-autonomous SINEs depend on the reverse transcription machinery of other retrotransposons. Fluorescence in situ hybridization demonstrated that different classes of retrotransposons differ largely in their chromosomal organization and are often excluded from blocks of rapidly homogenizing tandem repeats. In particular, LINEs contribute considerably to the repetitive DNA of nuclear plant genomes.