Georgia Journal of Science


Hoxb6 is an evolutionarily conserved developmental regulatory gene that functions, in part, to pattern several organs and organ systems within the embryonic trunk during vertebrate embryogenesis. The cis-regulatory circuitry mediating trunk expression in mouse (Mus musculus) may be conserved across gnathostome vertebrates, as several other species show similar trunk expression patterns, including chicken (Gallus gallus), dogfish shark (Scyliorhinus canicula), and several teleost fishes. A whole genome duplication event that occurred in the lineage leading to teleost fishes has generated at least two Hoxb6 genes, hoxb6a and b6b. Two teleost fishes of the superorder Acanthopterygii, Japanese medaka (Oryzias latipes) and Nile tilapia (Oreochromis niloticus), exhibit divergent Hoxb6 expression patterns from those of non-teleost vertebrates. This includes an anterior expansion of expression for both hoxb6a and b6b into pharyngeal arch 7, the posterior-most pharyngeal arch that, along with the other posterior pharyngeal arches, gives rise to the pharyngeal jaw apparatus in teleost fishes. While these patterns of expression are observed for both duplicate Hoxb6 genes in Acanthopterygians, it is uncertain whether this pharyngeal arch expression is shared with other teleost taxa. Here we present the expression patterns of hoxb6a and b6b in zebrafish (Danio rerio), a member of the Ostariophysi superorder. We show that, unlike the strict orthologs from medaka and tilapia, zebrafish hoxb6a is expressed in pharyngeal arches 5-7, whereas hoxb6b is not expressed in any of the pharyngeal arches. Further, we show through comparative genomic DNA sequence analyses that, although all teleost-specific sequences exhibit moderate conservation with the region functionally tested in mouse, zebrafish hoxb6a and b6b exhibit little to no conservation in sequence with their strict orthologs of medaka or tilapia outside of this region. Our data suggest that divergence in the cis-regulatory circuitry post-genome duplication has generated divergent hoxb6a and b6b expression patterns among teleost fishes.