Expanded expression of Sonic Hedgehog inAstyanaxcavefish:multiple consequences on forebrain development and evolution

• DOI: 10.1242/dev.02780
• Publication Date: 2007-01-25T05:46:37Z
• Authors: Arnaud Menuet; Alessandro Alunni; Jean-Stéphane Joly; William R. Jeffery; Sylvie Rétaux
• Keywords: OLFACTORY BULBS; Telencephalon; ASTYANAX MEXICANUS; LIM-HOMEODOMAIN; [SDV]Life Sciences [q-bio]; Teleost; [SDV.NEU.NB]Life Sciences [q-bio]/Neurons and Cognition [q-bio.NC]/Neurobiology; Proliferation; MESH: Neurons; MESH: Prosencephalon; GABA; Cell Movement; MESH: Animals; LIM-homeodomain; MESH: Cell Movement; MESH: Organ Specificity; Neurons; 0303 health sciences; HYPOTALAMUS; PROLIFERATION; Fishes; CAVEFISH; Biological Evolution; Olfactory Bulb; Subpallium; Organ Specificity; NKX; Nkx; SUBPALLIUM; [SDV.NEU]Life Sciences [q-bio]/Neurons and Cognition [q-bio.NC]; MESH: Body Patterning; MESH: Olfactory Bulb; Olfactory bulbs; DLX; Dlx; Evolution; Hypothalamus; 03 medical and health sciences; Prosencephalon; MESH: Cell Proliferation; MESH: Evolution; MESH: Homeodomain Proteins; [SDV.BA.ZV]Life Sciences [q-bio]/Animal biology/Vertebrate Zoology; BIOLOGIE DU DEVELOPPEMENT; Animals; [INFO]Computer Science [cs]; Hedgehog Proteins; MESH: Diencephalon; 14. Life underwater; Diencephalon; Body Patterning; Cell Proliferation; Homeodomain Proteins; TELENCEPHALON; MESH: Hedgehog Proteins; MESH: Hypothalamus; MESH: Fishes; [SDV.BDD.EO]Life Sciences [q-bio]/Development Biology/Embryology and Organogenesis; Astyanax mexicanus; TELEOST; Cavefish

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.