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Evolution reveals an independent route for diversity in animal form

May 05, 2006

Researchers have found that Cnidaria, a group of marine animals noted for diverse morphology among its constituent species, actually lacks the ancient "Hox" gene system that is essential for the development of most other animals. The finding is surprising because the Hox system is largely responsible for so-called axial patterning, the developmental process that directs the formation of different morphological features along the anterior-posterior axis-the axis along which the head, trunk, and tail are arranged. The work indicates that despite the importance of the Hox system in the development of most animals, the evolution of a typical Hox system is not in fact a prerequisite for axial patterning during development or for the generation of diverse animal morphology.

The findings are reported in the May 9th issue of Current Biology by scientists Bernd Schierwater, Kai Kamm, and Wolfgang Jakob of Tierärztliche Hochschule Hannover, in Germany, Stephen Dellaporta of Yale University, and David Miller of James Cook University in Australia.

A remarkable feature of Hox-system genes is that they are typically clustered in groups along chromosomes, with their order along the chromosome reflecting their actual pattern of expression along the anterior-posterior axis of a developing animal. The Hox system has long been considered a defining characteristic of animals, and much of the variation seen in animal morphology has been attributed to evolutionary variations on how the Hox system is implemented during development in different species.

In the new work, the researchers investigated the Hox status of cnidarians-a group of primitive animals that diverged very early on in animal evolution from the lineage leading to most other animal species. Cnidarians include reef-building polyps and pelagic jellyfish, while the group from which it split-the Bilateria-has given rise to the vast majority of the animals we are familiar with, including worms, fish, molluscs, insects, and mammals. While all "higher" animals clearly either have a typical Hox system or had ancestors that did, the evolutionary origins of the Hox system are unclear, and the Hox-gene status of cnidarians has been controversial.

The authors of the new study demonstrate that although Hox-like genes are present in cnidarians, most of these have evolved by gene duplication independently since cnidarians diverged from the higher animal lineage. The researchers found no equivalent of a Hox cluster in two representative cnidarians, and the patterns of expression of the cnidarian Hox-like genes differ radically across the phylum and are inconsistent with the expression patterns exhibited by typical Hox-system genes. The authors conclude that the Hox system is an invention of higher animals (the Bilateria) and, therefore, is not a defining characteristic of metazoan animals.

Knowing this, the spectacular morphological diversity in Cnidaria comes as a surprise. The Cnidaria is one of the most diverse and species-rich of animal groups, ranging in shape and size from microscopic solitary polyps to massive and highly differentiated colonial siphonophores up to 40 meters long. One clear implication of the paper is that, contrary to expectations, the Hox system is not necessary for the elaboration of substantial morphological complexity.

Cell Press