Structure and polymorphism of the Adineta vaga Hebe retrotransposon. (a) Genomic environment of the subtelomeric Hebe copy (K in panel b), including two head-to-tail Athena retroelements and a retrovirus-like long terminal repeat (LTR) retrotransposon Juno. The region containing Athena elements has likely been acquired from another telomere, O3 (reference ), possibly by break-induced replication. T, telomeric repeats. Scale bar, 1 kb. (b) Structure of a full-length Hebe consensus copy, and alignment of genomic copies to the consensus. APE and RT denote the AP-like endonuclease and RT domains, respectively; vertical black bars denote the three CCHC zinc knuckle motifs; (A)n, poly(A) stretch;//, copies that were truncated by cloning and could not be sequenced to completion. Deletions are indicated by brackets; in-frame stop codons, by vertical lines; probes used for library screening, by double-headed arrows; 72-bp tandem repeat units in the 3' untranslated region are in green. Also shown are the base coordinates for the start and stop of open reading frame (ORF) in the consensus sequence and the position of the unique Spe I site. Scale bar, 0.1 kb. (c) Three zinc knuckle motifs in Hebe ORF1 (panel a) and comparison with selected non-LTR elements from Figure 4b. Highly conserved residues are designated by asterisks.