A genomic view of 500 million years of cnidarian evolution

Abstract

Cnidarians (corals, anemones, jellyfish and hydras) are a diverse group of animals of interest to evolutionary biologists, ecologists and developmental biologists. With the publication of the genome sequences of Hydra and Nematostella, whose last common ancestor was the stem cnidarian, researchers are beginning to see the genomic underpinnings of cnidarian biology. Cnidarians are known for the remarkable plasticity of their morphology and life cycles. This plasticity is reflected in the Hydra and Nematostella genomes, which differ to an exceptional degree in size, base composition, transposable element content and gene conservation. It is now known what cnidarian genomes, given 500 million years, are capable of; as we discuss here, the next challenge is to understand how this genomic history has led to the striking diversity seen in this group.

Figure 1

Cnidarian Phylogeny. The figure shows the evolutionary relationships among sponges, bilaterians (protostomes and deuterostomes), and the five classes in the phylum Cnidaria. Hydrozoa, Scyphozoa, Cubozoa, and Staurozoa belong to the subphylum Medusozoa. Staurozoa is a recently defined class that includes the stauromedusae [3]. Its evolutionary relationship to the other medusozoan classes is not certain, but the current evidence supports a basal branching within Medusozoa. The Hydra picture was taken by Peter Bryant and the Nematostella picture by Jens Fritzenwanker.

Figure 2

Fate of Hox megacluster genes in Hydra and Nematostella. The figure shows the genes present in Hydra and Nematostella that derived from the Hox megacluster (top) present in the ancestor of cnidarians and bilaterians [45]. The megacluster contained the Hox and ParaHox genes, and genes of the NK cluster. Uncolored genes are present in both Hydra and Nematostella. Colored genes are missing from Hydra. Gene nomenclature and arrangements in the megacluster are from Garcia-Fernández [45]. Gene presence/absence data are from Chourrout et al. [46]. Note that some of the details of the current gene linkage patterns have been ignored for simplicity, e.g. the Evx gene in Nematostella is actually located between two Hox genes, and the Mnx gene is linked to the Hox/Evx cluster.