MUTATIONS IN ZEBRAFISH GENES AFFECTING THE FORMATION OF THE BOUNDARY BETWEEN MIDBRAIN AND HINDBRAIN

Mutations in two genes affect the formation of the boundary between mi dbrain and hindbrain (MHB): no isthmus (noi) and acerebellar (ace), no i mutant embryos lack the MHB constriction, the cerebellum and optic t ectum, as well as the pronephric duct. Analysis of noi mutant embryos with neuron-specific antibodies shows that the MHB region and the dors al and ventral midbrain are absent or abnormal, but that the rostral h indbrain is unaffected with the exception of the cerebellum, Using mar kers that are expressed during its formation (eng, wnt1 and pax-b), we find that the MHB region is already misspecified in noi mutant embryo s during late gastrulation. The tectum is initially present and later degenerates, The defect in ace mutant embryos is more restricted: MHB and cerebellum are absent, but a tectum is formed, Molecular organisat ion of the tectum and tegmentum is disturbed, however, since eng, wntl and pax-b marker gene expression is not maintained, We propose that n oi and ace are required for development of the MHB region and of the a djacent mid- and hindbrain, which are thought to be patterned by the M HB region, Presence of pax-b RNA, and absence of pax-b protein, togeth er with the observation of genetic linkage and the occurrence of a poi nt mutation, show that noi mutations are located in the pax-b gene, pa x-b is a vertebrate orthologue of the Drosophila gene paired, which is involved in a pathway of cellular interactions at the posterior compa rtment boundary in Drosophila, Our results confirm and extend a previo us report, and show that at least one member of this conserved signall ing pathway is required for formation of the boundary between midbrain and hindbrain in the zebrafish.