First insights on the retroelement Rex1 in the cytogenetics of frogs

While some transposable elements (TEs) have been found in the sequenced genomes of frog species, detailed studies of these elements have been lacking. In this work, we investigated the occurrence of the Rex1 element, which is widespread in fish, in anurans of the genus Physalaemus. We isolated and characterized the reverse transcriptase (RT)-coding sequences of Rex1 elements of five species of this genus.

Rex1 elements were found in the genomes of five species of Physalaemus but clustered in only the P. ephippifer karyotype, in contrast to observations in some species of fish, where large chromosomal sites with Rex1 elements are typically present.

The amino acid sequences deduced from the nucleotide sequences of the isolated fragments allowed us to unambiguously identify regions corresponding to domains 3-7 of RT. Some of the nucleotide sequences isolated from Physlaemus ephippifer and P. albonotatus had internal deletions, suggesting that these fragments are likely not active TEs, despite being derived from a Rex1 element. When hybridized with metaphase chromosomes, Rex1 probes were revealed at the pericentromeric heterochromatic region of the short arm of chromosome 3 of the P. ephippifer karyotype. Neither other heterochromatin sites of the P. ephippifer karyotype nor any chromosomal regions of the karyotypes of P. albonotatus, P. spiniger and P. albifrons were detected with these probes. Conclusions: Rex1 elements were found in the genomes of five species of Physalaemus but clustered in only the P. ephippifer karyotype, in contrast to observations in some species of fish, where large chromosomal sites with Rex1 elements are typically present.