The first non-LTR retrotransposon characterised in the cephalochordate amphioxus, BfCR1, shows similarities to CR1-like elements.

BfCR1 is the first non-long terminal repeat retrotransposon to be characterised in the amphioxus genome. Sequence alignment of the predicted translation product reveals that BfCR1 belongs to the CR1-like retroposon class, a family widely distributed in vertebrate and invertebrate lineages. Structural analysis shows conservation of the specific motifs of the ORF2-CR1 elements: the N-terminal endonuclease, the reverse transcriptase and the C-terminal domains. The BfCR1 element possesses an atypical 3' terminus consisting of the tandem repeat (AAG)6. We gathered evidence supporting the mobility of this element and report an estimated 15 copies of BfCR1, mostly truncated, per haploid genome, a remarkably low number when compared to that of vertebrates. Phylogenetic analysis, including the amphioxus element, seems to indicate that (i) CR1-like retroposons cluster in a monophyletic group and (ii) the CR1-like family was already present in the chordate ancestor. Our data provide further support for the horizontal transmission of CR1-like elements during early vertebrate evolution.