Hox Gene Expression During Oreochromis niloticus Pharyngeal Arch Development : Discovering the Hox Code

Abstract

Hox genes encode transcription factors and have been extensively studied by evolutionary and developmental biologists. Hox genes are responsible for determining specific regional identities along the anterior-posterior and proximal-distal developmental axes. Multiple hox genes are often simultaneously expressed in a developing tissue. Understanding the spatial and temporal expression patterns of the hox genes involved in morphogenesis is therefore essential for the prediction of molecular mechanisms that direct morphogenesis and patterning. The combinatorial hox gene expression (spatial and temporal) within a defined compartment is dubbed the "hox code." This study investigated the spatial and temporal expression of paralogous group 3-6 hox genes expressed within the pharyngeal arches of Oreochromis niloticus (Nile tilapia) embryos. It was hypothesized that each pharyngeal arch of Nile tilapia developing embryos would have a distinct combinatorial code of hox gene expression. The data shows that while pharyngeal arch 3 expresses hoxa2a and hoxa3a genes, pharyngeal arch 4 expresses hoxd3a and hoxd4a in addition to hoxa2a and hoxa3a. Pharyngeal arch 5 continues to express the hox genes found within pharyngeal arches 3 and 4, as well as hox genes b3b, b4a, and d4b. Hox expression within pharyngeal arch 6 is more diverse than the previous arches. In pharyngeal arch 6, all the hox genes expressed in pharyngeal arch 5 are present, with the following notable exceptions: hox genes d3a and d4b are excluded, and hox genes a4a, a5a, and b5a are included. Lastly, pharyngeal arch 7 is host to numerous expressed hox genes. The expression in pharyngeal arch 7 is similar to that of pharyngeal arch 6, having lost only hoxb3b. Four hox genes, hox c4a, b5b, b6a, and b6b, have anterior pharyngeal expression limits that begin in pharyngeal arch 7. Our results clearly demonstrate that within each pharyngeal arch is expressed a unique combination of hox genes, or pharyngeal hox code.