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First published online 12 April 2006
doi: 10.1242/dev.02352

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Differentiation of cerebellar cell identities in absence of Fgf signalling in zebrafish Otx morphants

¹ MRC Centre for Developmental Neurobiology, New Hunt's House, King's College London, London SE1 9RT, UK.
² The FIRC Institute of Molecular Oncology, Via Adamello, 16, 20139 Milano, Italy.
³ CEINGE Biotecnologie Avanzate, Via Comunale Margherita 482, 80145 Naples, Italy.
⁴ Institute of Genetics and Biophysics-ABT, Via G. Marconi 12, 80125 Naples, Italy.
⁵ Zebrafish Neurogenetics, GSF-Research Center for Environment and Health, Institute of Developmental Genetics, Ingolstaedter Landstrasse 1, D.85764 Neuherberg, Germany.
⁶ Institute of Virology, Technical University-Munich, Trogerstrasse 4b, D-81675 Munich, Germany.

^* Author for correspondence (e-mail: corinne.houart{at}kcl.ac.uk)

Accepted 9 March 2006

Although the secreted molecule Fgf8 is a key player of the isthmic organiser function, the mechanisms by which it acts remain unclear. Here, we present evidence indicating that Fgf8 is not instructive in establishing zebrafish cerebellar cell identities, although it is required for proliferation and morphogenesis of this territory. We first show that, as in mouse, lack of Otx function in zebrafish leads to transformation of the presumptive mesencephalon into an extended rhombomere 1 (r1). Expanded Fgf8 expression was proposed to be the cause of this fate transformation. However, this report demonstrates that zebrafish embryos lacking both Otx and fgf8 functions retain an extended r1 and display differentiation of at least two cerebellar cell fates. We show that this is not caused by presence of other Fgfs, which implies that in absence of Otx, Fgf function is not necessary for the differentiation of cerebellar cell types. Otx proteins are therefore potent repressors of cerebellar fates, kept out of r1 progeny by Fgf8. Because Otx transcripts are not present in presumptive r1 territory prior to fgf8 expression, Fgf8 is required to maintain, rather than induce, the posterior boundary of Otx expression. This maintenance is enough to allow cerebellar differentiation.

Key words: Cerebellum, Fgf8, Otx2, Isthmic organiser

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