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First published online 24 January 2007
doi: 10.1242/dev.02780

Development 134, 845-855 (2007)
Published by The Company of Biologists 2007
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Expanded expression of Sonic Hedgehog in Astyanax cavefish: multiple consequences on forebrain development and evolution

Arnaud Menuet1,*, Alessandro Alunni2,*, Jean-Stéphane Joly2, William R. Jeffery3 and Sylvie Rétaux1,{dagger}

1 CNRS-UPR2197 DEPSN, Institut Fessard, Avenue de la Terrasse, 91198 Gif-sur-Yvette cedex, France.
2 MSNC INRA Group, Institut Fessard, Avenue de la Terrasse, 91198 Gif-sur-Yvette cedex, France.
3 Department of Biology, University of Maryland, MD 20742, USA.

{dagger} Author for correspondence (e-mail: Sylvie.Retaux{at}iaf.cnrs-gif.fr)

Accepted 13 December 2006

Ventral midline Sonic Hedgehog (Shh) signalling is crucial for growth and patterning of the embryonic forebrain. Here, we report how enhanced Shh midline signalling affects the evolution of telencephalic and diencephalic neuronal patterning in the blind cavefish Astyanax mexicanus, a teleost fish closely related to zebrafish. A comparison between cave- and surface-dwelling forms of Astyanax shows that cavefish display larger Shh expression in all anterior midline domains throughout development. This does not affect global forebrain regional patterning, but has several important consequences on specific regions and neuronal populations. First, we show expanded Nkx2.1a expression and higher levels of cell proliferation in the cavefish basal diencephalon and hypothalamus. Second, we uncover an Nkx2.1b-Lhx6-GABA-positive migratory pathway from the subpallium to the olfactory bulb, which is increased in size in cavefish. Finally, we observe heterochrony and enlarged Lhx7 expression in the cavefish basal forebrain. These specific increases in olfactory and hypothalamic forebrain components are Shh-dependent and therefore place the telencephalic midline organisers in a crucial position to modulate forebrain evolution through developmental events, and to generate diversity in forebrain neuronal patterning.

Key words: Teleost, Telencephalon, Subpallium, Olfactory bulbs, LIM-homeodomain, Nkx, Dlx, GABA, Proliferation, Hypothalamus, Cavefish, Astyanax mexicanus






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