Genomic sequence and spatiotemporal expression comparison of zebrafish mbx1 and its paralog, mbx2.

The expression of midbrain homeobox-1 (mbx1) defines a discrete region in the vertebrate neural plate that will give rise to the mesencephalon, as well as subregions of the diencephalon and retinal field. Here, we report on the identification and cloning of a second Mbx gene in zebrafish, termed mbx2. Genomic sequence comparison suggests that mbx1 and mbx2 are derived from the duplication of a single putative ancestral gene that is conserved in other vertebrates as a single copy gene. Furthermore, phylogenetic analyses indicate that the mbx genes belong to a novel subgroup of paired-like homeobox genes. Finally, quantitative reverse transcriptase-PCR and whole mount in situ hybridization experiments revealed a pattern of partial spatiotemporal expression divergence between the mbx paralogs that correlates with sequence divergence in noncoding regulatory domains. Our data support a subfunctionalization model that may explain the retention of duplicate mbx genes in teleosts.