Diversity and clustered distribution of retrotransposable elements in the compact genome of the pufferfish Tetraodon nigroviridis.

We report the characterization and chromosomal distribution of retroelements in the compact genome of the pufferfish Tetraodon nigroviridis. We have reconstructed partial/complete retroelement sequences, established their phylogenetic relationship to other known eukaryotic retrotransposons, and performed double-color FISH analyses to gain new insights into their patterns of chromosomal distribution. We could identify 43 different reverse transcriptase retrotransposons belonging to the three major known subclasses (14 non-LTR retrotransposons from seven clades, 25 LTR retrotransposons representing the five major known groups, and four Penelope-like elements), and well as two SINEs (non-autonomous retroelements). Such a diversity of retrotransposable elements, which seems to be relatively common in fish but not in mammals, is astonishing in such a compact genome. The total number of retroelements was approximately 3000, roughly representing only 2.6% of the genome of T. nigroviridis. This is much less than in other vertebrate genomes, reflecting the compact nature of the genome of this pufferfish. Major differences in copy number were observed between different clades, indicating differential success in invading and persisting in the genome. Some retroelements displayed evidence of recent activity. Finally, FISH analysis showed that retrotransposable elements preferentially accumulate in specific heterochromatic regions of the genome of T. nigroviridis, revealing a degree of genomic compartmentalization not observed in the human genome.