Isolation and Characterization of Retrotransposons in Wild and Cultivated Peanut Species

Retrotransposons are considered a possible source for mutations due to their potential of spreading in the genome using a 'copy and paste'-like mechanism. Here we report on the isolation and characterization of a new Ty3 -gypsy retrotransposon from allotetraploid peanut (Arachis hypogaea, 2n=4x=40) and its diploid ancestors A. duranensis (AA-genome, 2n=20) and A. ipaensis (BB-genome, 2n=20). We have identified two repetitive sequences, one showing high similarity at the amino acid level to the reverse transcriptase of Athila-type retrotransposons, the other being AT-rich with no similarities to gene bank sequences. Results from genome walking experiments gave first evidence that both sequences represented parts of the same Ty3-gypsy retrotransposon, the 5'-LTR (long terminal repeat)- and the pol (polyprotein)-region respectively. Fluorescent in situ hybridization (FISH) experiments showed that the element is dispersedly distributed on the chromosomes, absent from centromeres and telomeric regions, and more prominent in chromosomes of the A-genome. The element appeared to be moderately repetitive with copy numbers of about 830 (A. ipaensis ), 2,600 (A. duranensis), and 3,000 (Arachis hypogaea) per haploid genome. Phylogenetic analysis of the deduced amino acid sequences of 80 isolated reverse transcriptase clones from the three species shed light on its evolution within the peanut species. The isolated sequences contained multiple stop-codons and so far, no evidence has been found that the element is still active. An outlook is given regarding finding new tools for the advancement of Arachis breeding programs aimed at the transfer of resistance to biotic and abiotic stresses to peanut. (author)