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EvoDevo  2012 

Broken colinearity of the amphioxus Hox cluster

DOI: 10.1186/2041-9139-3-28

Keywords: Hox gene regulation, Hox cluster evolution, Amphioxus, Hox colinearity, Retinoic acid

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Abstract:

Here we report an extended expression profile of the European amphioxus Branchiostoma lanceolatum Hox genes and describe that all Hox genes, except Hox13, are expressed during development. Interestingly, we report the breaking of both spatial and temporal colinearity for at least Hox6 and Hox14, which thus have escaped from the classical Hox code concept. We show a previously unidentified Hox6 expression pattern and a faint expression for posterior Hox genes in structures such as the posterior mesoderm, notochord, and hindgut. Unexpectedly, we found that amphioxus Hox14 had the most divergent expression pattern. This gene is expressed in the anterior cerebral vesicle and pharyngeal endoderm. Amphioxus Hox14 expression represents the first report of Hox gene expression in the most anterior part of the central nervous system. Nevertheless, despite these divergent expression patterns, amphioxus Hox6 and Hox14 seem to be still regulated by retinoic acid.Escape from colinearity by Hox genes is not unusual in either vertebrates or amphioxus and we suggest that those genes escaping from it are probably associated with the patterning of lineage-specific morphological traits, requiring the loss of those developmental constraints that kept them colinear.Hox genes code for a subfamily of homeodomain-containing transcription factors and have been found in all eumetazoans studied so far. Hox genes are responsible for giving the identity to morphological structures along the anterior-posterior (A-P) axis in most bilaterian animals [1-4]. Generally, these genes lie in the same genomic region and form gene clusters, usually one in invertebrates, and multiple clusters in vertebrates because of multiple rounds of genome duplication that took place at their origin (Figure 1) [5,6]. In most groups of animals, the position of Hox genes within any cluster corresponds with their mode of expression: genes placed more toward the 3′ end are expressed and pattern more anterior structures than

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