Antennapedia ( Antp) class of Homeobox genes ( Hox, Para Hox and Nk genes) are considered to have played a very important role in diversity of animal body plan. Information obtained from genome sequences of diploblastic animals like Cntenophores and sponges reveal that they have only Nk genes from the Antp class and sponges lacked Hox genes from the beginning or had lost them during evolution.Information of these important developmental regulators in basal species provide valuable information for their role in higher organisms, as they are highly conserved through out evolution.

Complete genome sequence information from demosponge Amphimedon queenslandica led to the proposal that all all ANTP families found in nonsponges animals (eumetazoans) derived from an ancestral “proto-NK” six-gene cluster.But as a single sponge species cannot reveal ancestral metazoan traits authors Eve Gazave and others looked for Nk genes expression in Homoscleromorh sponges, a a sponge lineage recently phylogenetically classified outside demosponges and characterized by unique histological features.

By degenrate PCR they could identify new class of class Nk genes and named it HomoNK ,which is close to NK6 and NK 7 families of Cnidarians and Antp of Bilaterains.Five partial homeobox gene fragments were isolated from five homoscleromorph species (O. lobularis, O. imperialis, Pseudocorticium jarrei, Plakina jani, and Plakortis simplex). HomoNK genes are not closely related to any NK family previously identified from calcareous sponges or demosponges Amphimedon queenslandica.

Expression pattern analysis of HomoNK gene OlobNK in adult Oscarella lobularis showed that this gene is a strict marker of choanocytes, the most typical sponge cell type characterized by an apical flagellum surrounded by a collar of microvilli. This might be an important aspect as NK6 and NK7 family of genes in bilaterians have predominant neurosensory expression and recent proposal that choanocytes could be homologs of sensory cells in the sponge.

Reference :

Eve Gazave & Pascal Lapébie & Emmanuelle Renard ,Chantal Bézac & Nicole Boury-Esnault & Jean Vacelet ,Thierry Pérez & Michaël Manuel & Carole Borchiellini.
NK homeobox genes with choanocyte-specific expression in homoscleromorph sponges.
Dev Genes Evol.DOI 10.1007/s00427-008-0242-z

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