Japanese Medaka Hoxa1a Shows Divergent Gene Expression and Regulation from Evolutionarily Divergent Gnathostome Vertebrates

Hox genes are evolutionarily conserved developmental regulatory genes that function, in part, to pattern the anterior-posterior (AP) axis of organs and organ systems during animal embryonic development. Hoxa1, specifically, is shown to be expressed in the developing hindbrain, spinal cord, and pharyngeal arches, or transient compartments giving rise to the craniofacial skeletal elements, of mouse (Mus musculus), chicken (Gallus gallus), African clawed frog (Xenopus laevis), spotted cat shark (Scyliorhinus canicula) and zebrafish (Danio rerio).  The cis-regulatory elements (CREs), or short DNA sequences, that direct Hoxa1 expression in these embryonic domains have been mapped and functionally tested in the mouse as well.  However, it is unknown if this gene is expressed similarly and contains conserved CREs in other evolutionarily divergent vertebrate organisms.  Here, we present the hoxa1a expression pattern in the Japanese medaka fish (Oryzias latipes).  Our data shows that medaka hoxa1a is expressed strongly in the pharyngeal arches and weakly in the spinal cord.  However, we did not observe any expression within the hindbrain of medaka.  Furthermore, our comparative genomic DNA sequence analyses show that the functional CREs mapped in the mouse are conserved among the tetrapods and shark, but not within the Osteichthyans, or bony fishes.  Our analyses suggest that divergent hoxa1a expression patterns within the medaka may be a result from divergence within CREs.  Future studies involving other species of fish and tetrapods will be employed to further understand the divergence of the expression and genomic architecture of this gene.