Evolution and development of white stripe patterns in clownfishes (Pomacentridae)

How can we explain the fabulous diversity of color patterns in coral reef fishes? To address this question, we chose to work on the monophyletic group clownfishes. Clownfishes or Amphiprionini tribe comprises 30 species that possess a characteristic colour pattern composed of zero to three vertical white stripes over an orange to black body. Here, we wanted to understand the ultimate and proximate causes at the origin of this diversity of colour patterns by combining eco/evo/devo studies. First, we characterized the cellular nature of the white stripe and showed using drug treatment that those cells correspond to iridophores and that they are indispensable to initiate the formation of the colour pattern. Then, we showed that the common ancestor of clownfishes had three vertical white stripes and, during evolution, we observed a successive caudal to rostral losses of stripes. These results suggested a developmental constrain that we highlighted by studying the development of Amphiprion ocellaris (3 white stripes) and Amphiprion frenatus (1 white stripe). Interestingly, the white stripes of A. ocellaris always appear in a rostral to caudal stereotyped sequence during larval to juvenile transition. Surprisingly, whereas A. frenatus has only one stripe at the adult stage, they acquire three white stripes in a same sequential manner as A. ocellaris during larval to juvenile transition. Later on, these stripes disappear caudo-rostrally during the juvenile phase leading to the definitive colour pattern. Remarkably, the reduction of stripe number over ontogeny matches the sequences of stripe losses during evolution, showing that colour pattern diversification among clownfish lineages results from changes in developmental processes. Finally, we provided evidence that the diversity of striped patterns plays a role for species recognition.