THE EXPRESSION PATTERN OF 2 ZEBRAFISH ACHAETE-SCUTE HOMOLOG (ASH) GENES IS ALTERED IN THE EMBRYONIC BRAIN OF THE CYCLOPS MUTANT

Two achaete-scute homolog sequences, Zash-1a and Zash-1b, were isolate d from a zebrafish embryonic cDNA library. The Zash-1a cDNA encodes a protein very similar to rat Mash-1 and Xenopus Xash-1, with over 94% i dentity in the C-terminal three-fourths of all three polypeptides. The Zash-1b cDNA encodes a more distantly related protein, with 80% ident ity of amino acids to Mash-1 in this part of the sequence. At 24 hr, t he Zash-1a transcripts are found in the hindbrain in two bilaterally s ymmetrical lines of cells which mark the boundary between the alar and basal plates and in rhombomere 1 in ventral cells near the floorplate . The gene is also expressed in particular regions of the telencephalo n and diencephalon, in the epiphysis, the ventral tegmentum, the neura l retina, and in specific cells in the spinal cord. Zash-1b transcript s are found in the hindbrain in segmentally arranged fan-like groups o f cells which are located close to the anterior and posterior boundari es of each of rhombomeres 2-6 and in ventral cells close to the floor plate of most rhombomeres. The gene is also expressed at sites distinc t from cells expressing Zash-1a in the tegmentum, diencephalon, telenc ephalon, and spinal cord. In the mutant cyclops, Zash-1a transcripts a re absent from the ventral region of the tegmentum and in the ventral cells of rhombomere 1, while more dorsal expression regions are unaffe cted. The effects of the mutation on Zash-1b expression, however, are more complex. In the hindbrain, the ventral expression zone of this ge ne is absent, the more dorsal segmented expression is disorganized, an d ectopic expression in the alar plate is observed. A dramatic ectopic expression is also observed in the anterior tegmentum. The cyclops ge ne, therefore, has both positive and negative effects on the CNS of th e wild-type embryo: it is required for activation of both Zash-1a and -1b in particular ventral cells, but it also restricts the expression of Zash-1b in other ventral cells and in some dorsal regions. Zash-1a and -1b gene probes will be extremely useful in the analysis of additi onal mutations affecting development of the central nervous system in zebrafish embryos. (C) 1994 Academic Press, Inc.