THE TY1-COPIA GROUP OF RETROTRANSPOSONS IN PLANTS - GENOMIC ORGANIZATION, EVOLUTION, AND USE AS MOLECULAR MARKERS

The genomic organisation and diversity of the Ty1-copia group retrotra nsposons has been investigated in several crop plants and their relati ves from both dicotyledonous and monocotyledonous families, including potato (Solanum tuberosum), faba beans (Vicia faba), Vicia melanops, V icia sativa, barley (Hordeum vulgare), rye (Secale cereale), and onion (Allium cepa). Extreme heterogeneity in the sequence of the Ty1-copia retrotransposons from all these plants was revealed following sequenc e analysis of reverse transcriptase fragments. The estimated copy numb ers of the Ty1-copia group retrotransposons for the genomes of S, tube rosum, L. esculentum, A. cepa, S. cereale, and V. faba is highly varia ble, ranging from a few hundred to approximately a million copies per genome. In situ hybridisation data from metaphase and prophase chromos omes of V. faba, S. cereale, and H. vulgare suggest that retrotranspos on sequences are dispersed throughout the euchromatic regions of the g enome but are almost undetectable in most heterochromatic regions. In contrast, similar data from metaphase chromosomes of A. cepa suggests that although retrotransposon sequences are dispersed throughout the e uchromatic regions of the genome, they are predominantly concentrated in the terminal heterochromatin. These results are discussed in the co ntext of the role played by the Ty1-copia group retrotransposons in th e evolution of the plant genome. Lastly, the application of retrotrans poson sequences as genetic markers for mapping genomes and for studyin g genetic biodiversity in plants is presented.