Multiple TGF‐β receptor related genes in sponge and ancient gene duplications before the parazoan–eumetazoan split1

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Abstract
Members of the transforming growth factor β (TGF‐β) family mediate key events in cell growth and development. Various receptors for diverse members of the TGF‐β family have recently been isolated and sequenced. These receptors form a family (TβR family) with a Ser/Thr kinase domain in common. To understand the divergence pattern of the TβR family during animal evolution, we have conducted cloning of cDNAs encoding the TβR family members from Ephydatia fluviatilis, a freshwater sponge. We obtained seven cDNAs (sALK‐1–sALK‐7) which are closely related in structure to known family members. Including these sponge sequences, a phylogenetic tree of the family members was inferred by a maximum likelihood method. The phylogenetic tree suggests that the sponge receptors sALK‐1–sALK‐3, which are closely related to each other, are sponge homologs of vertebrate activin type I receptor (ActR‐I). sALK‐5 is likely to be a homolog of TGF‐β type II receptor. sALK‐4 and sALK‐6 might be ancestral precursors of type I and type II receptors, respectively, and sALK‐7 is possibly an ancestral precursor of both types. The tree revealed that most, if not all, of the gene duplications that gave rise to known subtypes with distinct ligand specificities antedate the divergence of parazoans and eumetazoans, the earliest divergence of extant animal phyla.