Evolutionary Divergence of Pharyngeal Arch Specification in Teleosts

Meeting Abstract

31.3  Friday, Jan. 4  Evolutionary Divergence of Pharyngeal Arch Specification in Teleosts LE PABIC, Pierre*; SCEMAMA, Jean-Luc; STELLWAG, Edmund; East Carolina University stellwage@ecu.edu

We are interested in the effects of differential gene retention on Hox paralog group 2 (Hox PG2) function following a post-genome duplication event that occurred during the incipient stages of teleost evolution. At present, the evolutionary consequences of differential retention of Hox PG2 genes in teleosts remain obscure as functional studies have been limited to zebrafish which retained only two of the four hypothesized post-duplication Hox PG2 genes, and where hoxa2b and b2a function redundantly as selector genes of 2nd pharyngeal arch (PA2) identity. Here we present the first functional study of Hox PG2 genes in a teleost with three Hox PG2 genes, the Nile tilapia (tilapia). Microinjection of tilapia zygotes with antisense oligonucleotide morpholinos (MO) directed against hoxa2a, hoxa2b or hoxb2a mRNA�s resulted in distinct phenotypes, each dependent on the specific MO used for microinjection. Hoxa2a knock-down alone results in a mirror-image duplication of Meckel’s cartilage in place of the ceratohyal, providing evidence that unlike zebrafish, tilapia hoxa2a acts as the sole selector gene of PA2 identity. By comparison, knock-down of tilapia hoxa2b resulted in the unexpected production of a supernumerary posterior ceratobranchial cartilage. Hoxb2a-directed MO�s generated the broadest ranging perturbations, with variable fusions or loss of ceratobranchials 1 to 5, the absence of the ceratohyal, persistence of unfused hyomandibular and symplectic cartilages and a misshapen Meckel�s cartilage, a result that parallels the early hoxb2a expression in the ventral most region of the first pharyngeal arch (PA1) and constitutes the first documented instance of Hox gene function in PA1 patterning for any gnathostome. Our results provide evidence for extensive post-genome duplication divergence in Hox PG2 gene function among teleosts.

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