TheTy1-copia group retrotransposons ofAllium cepa are distributed throughout the chromosomes but are enriched in the terminal heterochromatin

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Abstract
The genomic organization and diversity of theTy1-copia group retrotransposons has been investigated in a monocotyledonous plant,Allium cepa. We used the polymerase chain reaction (PCR) to generate sequences corresponding to a conserved domain of the reverse transcriptase gene ofTy1-copia retrotransposons in this plant. Sequence analysis of 27 of these PCR products shows that they are a highly heterogeneous population, a feature which is common in plants but not in yeast andDrosophila. Slot-blot analysis shows there are 100 000–200 000 copies ofTy1-copia group retrotransposons within theA. cepa genome (2C=31.7 pg), indicating that they are a significant component of the genome of this plant.In situ hybridization to metaphase chromosomes reveals thatTy1-copia retrotransposons are distributed throughout the euchromatin of all chromosomes ofA. cepa but are enriched in the terminal heterochromatic regions, which contain tandem arrays of satellite sequences. This is the first clear evidence for the presence ofTy1-copia retrotransposons in the terminal heterochromatin of plants and contrasts with the distribution of these elements in other plant species.