Sea anemone genome reveals the gene repertoire and
genomic organization of the eumetazoan ancestor
Nicholas H. Putnam[1], Mansi Srivastava[2], Uffe Hellsten[1], Bill Dirks[2], Jarrod Chapman[1],
Asaf Salamov[1], Astrid Terry[1], Harris Shapiro[1], Erika Lindquist[1], Vladimir V.
Kapitonov[3], Jerzy Jurka[3], Grigory Genikhovich[4], Igor Grigoriev[1], JGI Sequencing
Team[1], Robert E. Steele[5], John Finnerty[6], Ulrich Technau[4], Mark Q. Martindale[7],
Daniel S. Rokhsar[1,2]
[1] Department of Energy Joint Genome Institute, Walnut Creek, CA 94598
[2] Center for Integrative Genomics and Department of Molecular and Cell Biology, University
of California, Berkeley CA 94720
[3] Genetic Information Research Institute, 1925 Landings Drive, Mountain View, CA 94043
[4] Sars International Centre for Marine Molecular Biology, University of Bergen,
Thormoeøhlensgt 55; 5008, Bergen, Norway
[5] Department of Biological Chemistry and the Developmental Biology Center, University of
California, Irvine, CA 92697
[6] Department of Biology, Boston University, Boston, MA 02215
[7] Kewalo Marine Laboratory, University of Hawaii, Honolulu, HI 96813
Abstract
Sea anemones are seemingly primitive animals that, along with corals, jellyfish, and hydras,
constitute the Cnidaria, the oldest eumetazoan phylum. Here we report a comparative analysis of
the draft genome of an emerging cnidarian model, the starlet anemone Nematostella vectensis.
The anemone genome is surprisingly complex, with a gene repertoire, exon-intron structure, and
large-scale gene linkage more similar to vertebrates than to flies or nematodes. These results
imply that the genome of the eumetazoan ancestor was similarly complex, and that fly and
nematode genomes have been modified via sequence divergence, gene and intron loss, and
genomic rearrangement. Nearly one-fifth of the genes of the ancestor are eumetazoan novelties
in the sense that they have no recognizable homologs outside of animals, or contain new protein
domains and/or domain combinations that are not found in other eukaryotes. These eumetazoan-
specific genes are enriched for animal functions like cell signaling, adhesion, and synaptic
transmission, and analysis of diverse pathways suggests that these gene "inventions" along the
lineage leading to animals were already likely well integrated with pre-existing eukaryotic genes
in the eumetazoan progenitor. Subsequent diversification in the cnidarian and bilaterian lineages
was therefore associated with new regulatory linkages and higher-level integration of these pre-
existing pathways and networks.
Introduction
All living "tissue-grade" animals, or "eumetazoans," are descended from the last common
ancestor of bilaterians (flies, worms, snails, humans), cnidarians (anemones, jellyfish, hydra), and
ctenophores (comb jellies)(1, 2). This eumetazoan ancestor lived perhaps seven hundred million
years ago, but is not preserved in the fossil record(3). Yet we can infer many of its
characteristics -- flagellated sperm, development through a process of gastrulation, multiple germ
layers, true epithelia lying upon a basement membrane, a lined gut (enteron), a neuromuscular
system, multiple sensory systems, and fixed body axes -- since they are conserved features
retained by its modern descendants.
Similarly, we can characterize the genome of this long-dead eumetazoan progenitor by
comparing modern DNA and protein sequences and identifying conserved features in different
modern lineages. Our ability to recognize ancient genomic features depends on the availability of
sequences from diverse living animals, and can only illuminate genomic characteristics that have
an intrinsically slow rate of change and/or are preserved by selective pressures. Comparisons (4-
6) between fruit fly, nematode, and vertebrate genomes reveal greater genomic complexity in the
vertebrates (and other deuterostomes (7, 8)) as measured by gene content and structure, but at the
same time show that many genes and networks are shared across bilaterians. To probe the
ancestral eumetazoan genome requires sequences from even deeper branches of the animal tree,
comparing bilaterian and non-bilaterian phyla.
In comparison with bilaterians, cnidarians appear morphologically simple. The phylum is defined
(see., e.g., (2)) by a sac-like body plan with a single "oral" opening, two-epithelial tissue layers,
the presence of numerous tentacles, a nerve net, and the characteristic stinging cells (cnidocytes,
literally, "nettle cells") that give the phylum its name (Figure 1g). The class Anthozoa ("flower
animals") includes diverse anemones, corals, and sea pens, all of which lack a medusa stage. The
other Cnidarian classes are united by their pelagic medusae and uniquely linear mitochondrial
genomes (9) into the Medusozoa, including Hydra and related hydroids, jellyfish, and box
jellies. Some of the oldest animal body fossils (e.g., the Ediacaran Charnodiscus (10) but see
also (11, 12)) and fossil embryos (13) are plausibly relics of stem cnidarians, suggesting a
Precambrian origin for the phylum.
Among Anthozoan cnidarians, the starlet sea anemone Nematostella vectensis is an emerging
model system (14, 15). This estuarine burrowing anemone is found on the Atlantic and Pacific
coasts of North America, as well as the coast of southeast England (16) (Figure 1).
Nematostella cultures are easily maintained in the laboratory. With separate sexes, inducible
spawning, and external fertilization (14, 17), embryos are available throughout the year.
Fertilization is followed by cell divisions resulting in a hollow blastula, which gastrulates by
invagination and ingression to produce a ciliated, tear-drop-shaped planula larva that swims with
an apical tuft of sensory cilia at the front and the blastopore at the rear (Figure 1a-e, h, i). On the
seventh day after fertilization, the planula develops into a juvenile polyp, with the blastopore
becoming the mouth (14, 18, 19) (Figure 1f). Like many cnidarians, adult Nematostella are
apparently immortal, with prodigious powers of regeneration: animals cut in half heal into two
complete individuals, mimicking the natural process of asexual reproduction that occurs by
transverse pinching. Recent studies with Nematostella have addressed the evolutionary origins
of mesoderm, germ cell specification, and axial patterning in metazoans (Figure 1j, k) (15, 20-
25)
While cnidarians are often characterized as "simple" or "primitive," closer study of Nematostella
and its relatives is revealing considerable molecular and morphological complexity (15).
Signaling pathways and transcription factors involved in the early patterning and development of
bilaterians are present in cnidarians and active in development, indicating that these pathways
and regulatory mechanisms predate the eumetazoan radiation. Perhaps most strikingly, genes
that establish the main body axes in bilaterian embryos are also expressed asymmetrically in
Nematostella development, even though cnidarians are conventionally viewed as "radial" animals
[for a critical discussion, see (26)]. The expression domains occur with apparent bilaterial
symmetry, i.e., reflecting distinct directed axes both along and perpendicular to the main body
axis, and with a left-right plane of symmetry (27-29). Although anemones show only subtle
external morphological manifestations of bilateral symmetry (Figure 1k) (i.e., asymmetry in the
structure of the adult pharynx and associated mesenteries (30)), these results suggest the antiquity
of "bilaterian" patterning mechanisms.
Here we report the draft genome of the starlet sea anemone, and use its gene repertoire and
genome organization to reconstruct features of the ancestral eumetazoan genome. Analysis of the
Nematostella genome in the context of sequences from other eukaryotes reveals the genomic
complexity of this last common cnidarian-bilaterian ancestor, and begins to illuminate the rich
history of genes and gene networks already present at the base of the animal tree of life. The
emerging picture is one of surprising conservation in gene content, structure, and organization
between Nematostella and vertebrates, even to the point of retaining chromosome-scale gene
blocks whose linkage in modern genomes has been preserved from the genome of their common
ancestor. These are the most ancient conserved linkages known outside of prokaryotic operons.
In contrast, the fruit fly and nematode model systems have experienced significant gene loss,
intron loss, and genome rearrangement. Thus from a genomic perspective, the eumetazoan
ancestor more closely resembled modern vertebrates and anemones.
Genome Sequencing and Assembly
The draft sequence of the Nematostella was produced using a random shotgun strategy (31) from
approximately 6.5X paired-end sequence coverage from several shotgun libraries of a range of
insert sizes (32). The total assembly spans ~357 Mb, with half of this sequence in 181 scaffolds
longer than ~470 Kb. Metaphase spreads indicate a diploid chromosome number of 2N=30 (Fig
S2.4). Currently there are no physical or genetic maps of Nematostella, so we could not
reconstruct the genome as chromosomes. Nevertheless, since half of the predicted genes are in
scaffolds containing 48 or more genes, the present draft assembly is sufficiently long-range to
permit useful analysis of synteny with other species, as shown below. The typical locus in the
draft genome is in a contiguous gap-free stretch of nearly 20 Kb. Comparison of the assembled
sequence with open reading frames derived from expressed sequence tags (ESTs, see below)
shows that the assembly captures ~95% of the known protein coding content (32). While
approximately one-third of the shotgun sequences were not assembled, they could typically be