Isolation and expression analysis of a poriferan Antp-class Bar-/Bsh-like homeobox gene

A novel non- Hox Antp-class gene ( BarBsh- Hb) was isolated from the marine sponge Halichondria sp. This gene shares high sequence identity with eumetazoan genes from the Bsh and Bar gene families and can be distinguished from other non- Hox Antp-class genes by diagnostic residues. We also present an alignment of all known (full-length) poriferan non- Hox Antp-class genes. Maximum likelihood methods were employed to estimate phylogenetic relationships among non- Hox genes and BarBsh- Hb. We employed RT-PCR techniques to look at expression across different developmental stages (larval to rhagon). BarBsh- Hb product was present in newly released larvae, but expression was not detected 8-16 h post-release. Expression of BarBsh- Hb was detected in later-stage (>16 h post-release), free-swimming larvae until they settled and attached to the substratum, after which expression was down-regulated. In a separate set of experiments, low levels of expression were observed in normal adult tissue and disaggregated adult tissue, but BarBsh- Hb expression increased during tissue re-aggregation. These data increase the number of non- Hox homeobox genes identified in sponges and provide evidence of regulation of this non- Hox gene during sponge development. While the Bar and Bsh genes play important roles in the development of nervous tissue--especially visual systems--in metazoans, the specific role(s) BarBsh- Hb play(s) in sponge development is unclear and deserves greater attention.