Molecular evidence for the presence of a developmental gene in the lowest animals: identification of a homeobox-like gene in the marine sponge Geodia cydonium.

During the development of higher animals, morphogenetic programs are switched on which are frequently controlled by homeotic genes. Until now these genes have not been identified in the lowest animals, the marine sponges. Since sponges show (i) an antero-posterior and/or dorso-ventral axis during embryogenesis and (ii) a complex differentiation pattern during spicula formation, we hypothesized that in sponges homeotic genes--if present--are also involved in the control of these processes. Therefore, we searched for homeobox or homeobox-like sequences in the marine sponge Geodia cydonium. Here we describe a homeobox-like sequence from these animals; it was isolated from a cDNA library of an adult specimen. The deduced amino acid sequence of the complete homeodomain shares over 70% similarity with other homeodomain sequences, including those from hydra, insects and vertebrates. These data indicate that the sponge homeodomain-like sequence is similar with respect to structure to those of other animals and may suggest that the sponge homeodomain-like sequence(s) might function during developmental processes and/or during spiculogenesis in a similar manner to that known for higher animals.