lim6, a novel LIM homeobox gene in the zebrafish: comparison of its expression pattern with lim1.

A novel LIM class homeobox gene, lim6, was isolated from a zebrafish embryonic cDNA library. The encoded protein shares a high degree of sequence similarity with the previously described Lim1 and Lim5 proteins. This study compares the spatial and temporal expression pattern of the closely related lim6 and lim1 genes during early embryogenesis. Generally, lim6 mRNA was found at rather low amounts compared to lim1 mRNA. At the shield stage, lim6 mRNA, similar to lim1 mRNA, was predominantly expressed in the shield. Lim6 was transiently expressed in a restricted region of the anterior neural plate at the bud stage, distinct from the expression of lim1 in the notochord and the pronephros and pronephric ducts. During the segmentation period, the lim6 gene started to be expressed in single cells in the spinal cord, followed by a gradually increasing wide-spread expression throughout the CNS. During this stage, lim1 mRNA disappeared in the notochord and pronephric ducts and was found in the pronephroi and single cells in the CNS. In 24 hr embryos, lim6 and lim1 were expressed in the fore-, mid-, and hindbrain and the spinal cord, except that lim1 mRNA was limited to two small domains in the telencephalon, whereas lim6 mRNA was widely expressed in this region. A comparison of expression of lim1 and lim6 and of the previously characterized lim5 show that, in spite of close sequence similarity, distinct expression patterns imply nonredundant functions for each member of this group of genes.