Subterranean Fishes of the World

3082

2015

AIM 2015

Natural variations in sensory systems constitute adaptive responses to the environment. Here, we have compared the development of the sensory placodes in the blind, caveadapted morph and the eyed river-dwelling morph of the fish Astyanax mexicanus. Focusing on the lens and the olfactory placodes, we found a trade-off between these two sensory components in the two morphs: from neural plate stage onwards, cavefish have larger olfactory placodes and smaller lens placodes. Using lens placode laser ablation experiments, we ruled out the possibility of a defective community defect among cavefish lens cells that would explain their apoptotic death later in development. Searching for the developmental mechanisms underlying cavefish sensory evolution, we analyzed the roles of Shh hyper-signaling from the ventral midline, Fgf8 heterochronic signaling from the rostral forebrain, as well a newly discovered modification in Bmp4 expression dynamics in the prechordal plate of cavefish embryos. Using pharmacological treatments and injections to interfere with these three signaling systems at the end of gastrulation, we establish a regulatory network in which the subtle modifications of Shh, Fgf8 and Bmp4 signaling in space and time in cavefish embryos explain the observed changes and trade-off in lens versus olfactory placode fate, and account for the observed size of their sensory derivatives. We suggest that such evolutionary developmental mechanisms may be used throughout vertebrates to drive adaptive sensory specializations according to lifestyles and habitats.

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