Homeobox gene diversification in the calcareous sponge, Sycon raphanus

Knowledge of the developmental mechanisms in living basal metazoan phyla is crucial for understanding the genetic bases of morphological evolution in early animal history, We looked for homeobox genes in the calcareous sponge , Sycon raphanus, using the polymerase chain reaction. Partial sequences of eight homeoboxes were recovered, five of which are assignable to the NK-2 class of homeoboxes. The three remaining sequences are related members of a new class of homeoboxes, the Sycox class, showing limited similarity to bi laterian Lbx, Hlx, HEX, En, and Cad classes. Among the five NK-2 class home oboxes are four closely related sequences occupying a divergent position wi thin the class, the remaining one on the contrary showing high sequence sim ilarity with members of the NK-2 family, a particular subgroup within the N K-2 class, previously known only from the Bilateria, This suggests that div ersification of the NK-2 class occurred early in metazoan history. Altogeth er, the results reveal an unexpected diversification of homeobox genes in S . raphanus. (C) 2000 Academic Press.