Fig. 1.

Molecular mechanisms of repeated pelvic fin loss in sticklebacks. a Three-spined sticklebacks (Gasterosteus aculeatus) repeatedly colonized postglacial freshwater habitats. The adaptations in these independent populations are remarkably similar. b One common adaptation is the loss of the paired spiny pelvic fins. This loss is caused by the repeated deletion of a pelvic fin specific regulatory element that drives expression of pitx1, a crucial transcription factor for pelvic fin development. The exact deletions differ between freshwater populations and as Xie et al. show [3] are facilitated by sequence features in the genomic region that result in a non-canonical DNA conformation (Z-DNA) that causes double-strand breaks repaired by the more error-prone non-homologous end-joining repair