MOLECULAR HETEROGENEITY AMONG PRIMARY MOTONEURONS AND WITHIN MYOTOMESREVEALED BY THE DIFFERENTIAL MESSENGER-RNA EXPRESSION OF NOVEL ISLET-1 HOMOLOGS IN EMBRYONIC ZEBRAFISH

Zebrafish embryos have three or four identifiable primary motoneurons per hemisegment. We previously reported that, while several ventral ce lls initially express the zebrafish Islet-1 (Isl-2) gene, a member of the LIM/homeobox gene family, the expression of this gene becomes rest ricted to a single or a pair of cells slightly anterior to each segmen t border by 16 hr after fertilization. Double staining by in situ hybr idization and immunohistochemistry strongly suggested that these cells were mainly rostral primary motoneurons. Here, we have isolated two n ovel zebrafish cDNA clones for more Isl-1 family genes, termed zfIsl-2 and zfIsl-3. zfIsl-2 mRNA starts to be expressed in the ventral midse gmental cells per hemisegment around 15 hr. Double labeling experiment s have shown that these midsegmental cells are the caudal primary moto neuron (CaP) and its variant equivalence pair. Our results revealed th e heterogeneity in the expressed genes among primary motoneurons befor e the fates of the primary motoneurons are irreversibly determined, an d further suggest the involvement of the Isl-1 and zfIsl-2 genes in th e determination of cellular identities by primary motoneurons in embry onic zebrafish. zfIsl-3 mRNA is not expressed in motoneurons but is ex pressed at 17 hr, mainly in the ventral myotomes. This suggests that z fIsl-3 may be involved in the regional specification of the myotome an d also in target recognition by CaP. zfIsl-2 is also expressed through out the developing eye and tectal region of the midbrain, the target f or the retinal axons. In the ventral spinal cord of the spadetail muta nt embryo, which has defects in the somites, the cells expressing zfIs l-2 mRNA significantly decreased in number in contrast to the increase in cells expressing Isl-1 mRNA, suggesting the influence of the somit es on the expression of both genes. (C) 1995 Academic Press, Inc.