Homeobox‐containing genes in the most primitive metazoa, the sponges

The porifera represent the most primitive phylum of the metazoa. We identified three homeobox‐containing genes in the freshwater sponge (Ephydatia fluviatilis). Genomic DNA of the sponge was subjected to amplification by PCR with two primers that corresponded to the helix‐1 and helix‐3 regions of the homeodomain. Using the amplified products as probes, we isolated two homeobox genes, designated proxl and prox2. The amino acid sequences of the homeodomains of prox1 and prox2 were 72% and 62% identical to those of the NK‐3 and Om(1 D) genes of Drosophila, respectively. Screening of a sponge genomic library with degenerate oligonucleotides that corresponded to helix 3 further revealed the presence of one more homeobox gene, prox3. The amino acid sequence of the homeodomain of the prox3 product was 77% identical to that of the msh gene product of human. These results indicate that, when the metazoa appeared during the course of evolution, the multiple and distinct classes of homeobox‐containing genes that have been identified in higher organisms already existed.