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 a ll plant genomes investigated so far. We have isolated the reverse tra nscriptase domains of the Ty1-copia group elements from several specie s, ranging in genome size from some 100 Mbp to 23 000 Mbp, and determi ned the distribution patterns of these retrotransposons on metaphase c hromosomes and within interphase nuclei by DNA:DNA in situ hybridizati on. With some exceptions, the reverse transcriptase domains were distr ibuted over the length of the chromosomes. Exclusion from rDNA sites a nd some centromeres (e.g., slash pine, 23 000 Mbp, or barley, 5500 Mbp ) is frequent, whereas many species exclude retrotransposons from othe r sites of heterochromatin (e.g., intercalary and centromeric sites in broad bean). In contrast, in the plant Arabidopsis thaliana, widely u sed for plant molecular genetic studies because of its small genome (c . 100 Mbp), the Ty1-copia group reverse transcriptase gene domains are concentrated in the centromeric regions, collocalizing with the 180 b p satellite sequence pAL1. Unlike the pAL1 sequence, however, the Ty1- copia 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 physica l 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.