Sea anemones, along with corals and jellyfish, are members of the phylum Cnidaria, and they possess a radial body plan that is distinct from the familiar bilateral body plan exhibited by vertebrates and many other animals. Despite their superficial dissimilarities, Cnidarians and other animals share a common ancestor--which existed over 500 million years ago--and, consequently, many genes. As illustrated by the new findings, scientists are now studying how these shared genes are used by radial and bilateral animals to learn how different body plans are determined and built at the molecular level, and how they arose during evolution.
The work is reported in the March 7th issue of Current Biology by David Matus and colleagues at the University of Hawaii and Stony Brook University.
To understand the evolution of body-plan patterning, the researchers isolated a particular set of genes from a tiny translucent cnidarian, the starlet sea anemone, Nematostella vectensis. The genes were the anemone's version of genes known to be responsible for organizing the dorso/ventral axis in bilateral animals.
Matus and colleagues found that two types of proteins--so-called TGFß ligands and an antagonist protein that functions in opposition to the ligands--encoded by these genes are expressed differently in anemones than they are in bilateral animals like flies and frogs. In those animals, the proteins are expressed on opposite sides of the dorsal/ventral axis very early in embryogenesis, during gastrulation. However, the researchers found that the anemone ligands and their antagonist protein are at first expressed together--then, as gastrulation proceeds, the proteins become segregated into different germ layers, with the TGFß ligands and downstream components deployed exclusively in the endoderm, or inner germ layer.
It therefore appears that the ancient, ancestral function for this signaling system may have been to determine the different germ layers of the developing embryo in animals that lacked a bilateral body plan. Over evolutionary time, the genes acquired a new and important role in establishing the dorso/ventral axis, eventually leading to the diversity of bilateral animals present today.