Components of both major axial patterning systems of the Bilateria are differentially expressed along the primary axis of a 'radiate' animal, the anthozoan cnidarian Acropora millepora

Abstract

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.