Coordinated spatial and temporal expression of <it>Hox </it>genes during embryogenesis in the acoel <it>Convolutriloba longifissura</it>

<p>Abstract</p> <p>Background</p> <p><it>Hox </it>genes are critical for patterning the bilaterian anterior-posterior axis. The evolution of their clustered genomic arrangement and ancestral function has been debated since their discovery. As acoels appear to represent the sister group to the remaining Bilateria (Nephrozoa), investigating <it>Hox </it>gene expression will provide an insight into the ancestral features of the <it>Hox </it>genes in metazoan evolution.</p> <p>Results</p> <p>We describe the expression of anterior, central and posterior class <it>Hox </it>genes and the <it>ParaHox </it>ortholog <it>Cdx </it>in the acoel <it>Convolutriloba longifissura</it>. Expression of all three <it>Hox </it>genes begins contemporaneously after gastrulation and then resolves into staggered domains along the anterior-posterior axis, suggesting that the spatial coordination of <it>Hox </it>gene expression was present in the bilaterian ancestor. After early surface ectodermal expression, the anterior and central class genes are expressed in small domains of putative neural precursor cells co-expressing <it>ClSoxB1</it>, suggesting an evolutionary early function of <it>Hox </it>genes in patterning parts of the nervous system. In contrast, the expression of the posterior <it>Hox </it>gene is found in all three germ layers in a much broader posterior region of the embryo.</p> <p>Conclusion</p> <p>Our results suggest that the ancestral set of <it>Hox </it>genes was involved in the anterior-posterior patterning of the nervous system of the last common bilaterian ancestor and were later co-opted for patterning in diverse tissues in the bilaterian radiation. The lack of temporal colinearity of <it>Hox </it>expression in acoels may be due to a loss of genomic clustering in this clade or, alternatively, temporal colinearity may have arisen in conjunction with the expansion of the <it>Hox </it>cluster in the Nephrozoa.</p>