As the closest outgroup to the Bilateria, the Phylum Cnidaria is likely to be critical to understanding the origins and evolution of body axes. Proteins of the decapentaplegic (DPP)͞bone morphogenetic protein (BMP) 2͞4 subfamily are central to the specification of the dorsoventral (D͞V) axis in bilateral animals, albeit with an axis inversion between arthropods and chordates. We show that a dpp͞BMP2͞4 ortholog (bmp2͞4-Am) is present in the reef-building scleractinian coral, Acropora millepora (Class Anthozoa) and that it is capable of causing phenotypic effects in Drosophila that mimic those of the endogenous dpp gene. We also show that, during coral embryonic development, bmp2͞4-Am expression is localized in an ectodermal region adjacent to the blastopore. Thus, a representative of the DPP͞BMP2͞4 subfamily of ligands was present in the common ancestor of diploblastic and triploblastic animals where it was probably expressed in a localized fashion during development. A localized source of DPP͞BMP2͞4 may have already been used in axis formation in this ancestor, or it may have provided a means by which an axis could evolve in triploblastic animals.
Cnidarians are animals with a single (oral/aboral) overt body axis and with origins that nominally predate bilaterality. To better understand the evolution of axial patterning mechanisms, we characterized genes from the coral, Acropora millepora (Class Anthozoa) that are considered to be unambiguous markers of the bilaterian anterior/posterior and dorsal/ventral axes. Homologs of Otx/otd and Emx/ems, definitive anterior markers across the Bilateria, are expressed at opposite ends of the Acropora larva; otxA-Am initially around the blastopore and later preferentially toward the oral end in the ectoderm, and emx-Am predominantly in putative neurons in the aboral half of the planula larva, in a domain overlapping that of cnox-2Am, a Gsh/ind gene. The Acropora homologs of Pax-3/7, NKX2.1/vnd and Msx/msh are expressed in axially restricted and largely non-overlapping patterns in larval ectoderm. In Acropora, components of both the D/V and A/P patterning systems of bilateral animals are therefore expressed in regionally restricted patterns along the single overt body axis of the planula larva, and two 'anterior' markers are expressed at opposite ends of the axis. Thus, although some specific gene functions appear to be conserved between cnidarians and higher animals, no simple relationship exists between axial patterning systems in the two groups.
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