The chromosomal distributions of Ty1-copia group retrotransposable elements in higher plants and their implications for genome evolution
Retrotransposons make up a major fraction - sometimes more than 40% - of all plant genomes investigated so far. We have isolated the reverse transcriptase domains of theTyl-copia group elements from several species, ranging in genome size from some 100 Mbp to 23,000 Mbp, and determined the distribution patterns of these retrotransposons on metaphase chromosomes and within interphase nuclei by DNA: DNA in situ hybridization. With some exceptions, the reverse transcriptase domains were distributed over the length of the chromosomes. Exclusion fromr DNA sites and some centromeres (e.g., slash pine, 23,000 Mbp, or barley, 5500 Mbp) is frequent, whereas many species exclude retrotransposons from other sites of heterochromatin (e.g., intercalary and centromeric sites in broad bean).In contrast, in the plantArabidopsis thaliana,widely used for plant molecular genetic studies because of its small genome (c. 100 Mbp), theTy1-copiagroup reverse transcriptase gene domains are concentrated in the centromeric regions, collocalizing with the 180 bp satellite sequencepAL1. Unlike thepAL1sequence, however, the Ty1-lcopia signal is also detectable as weaker, diffuse hybridization along the lengths of the chromosomes. Possible mechanisms for evolution of the contrasting distributions are discussed. Understanding the physical distribution of retrotransposons and comparisons of the distribution between species is critical to understanding their evolution and the significance for generation of the new patterns of variability and in speciation.