Cnidarian Double Whammy: Anemone Genome Completed and a Worm Thats a Jelly!

Its been a long time in the making and I know everyone is as excited as I am that the first cnidarian genome is finished! This is monumental for cnidarian biologists (such as I sometimes fancy myself to be). The lucky species is Nematostella vectensis (its pimp name is the starlet sea anemone),a small edwardsiid anemone quickly becoming a model organism in developmental biology and now comparative genomics.

Curiously, the genome more closely resembles that of the human and other vertebrate genomes rather than invertebrate genomes such as other model organisms Drosophila (Arthropoda) and C. elegans (Nematoda). Nicholas Putnam, lead author of the study elaborates that this surprising result may be due to greater retention of ancestral genes in the anemones and vertebrates, whereas these tend to be lost at higher rates in Drosophila and C. elegans.

"In many ways, the ancestral genome was not so different from ours; it was intron-rich and contained nearly complete toolkits for animal biochemistry and development,which can now be recognized as pan-eumetazoan, as well as the core gene set required to execute sophisticated neural and muscular function. The ancestor had blocks of linked genes that remain together in the modern human and anemone genomes—the oldest known conserved synteny outside of prokaryotic operons. Whereas fruit flies and soil nematodes have proven to be exquisite model systems for dissecting the genetic underpinnings of metazoan development and physiology, their genomes are relatively poor models for the ancestral eumetazoan genome, having lost introns, genes, and gene linkages."-Putnam et al. 2007. Science 317: 86-94

The sea anemone genome is estimated to contain about 18,000 genes compared to a reasonable estimate for the human genome at around 20,000 genes. These 18k genes are spread over 30 chromosomes. Furthermore, over 80% of the anemone's introns are in the same place as human's!

"Only 20 percent of the ancestral eumetazoan genes seem to be unique to animals. Fifteen percent of these seem to be completely novel - we can't identify any related gene in non-animals. The other five percent were formed through substantial modifications to very ancient genes."-Study co-author Daniel Rokhsar, quoted from UC-Berkeley press release on ScienceDaily.

“Nematostella’s genome may provide more insights into the functional evolution of human genes than many far more closely related animals.”-Co-author John Finnerty, quoted in Pennisi 2007. Science 317: 27, confirming the superiority of the sea anemone to all other phyla in being able to answer "What is the meaning life?" I know there is an anemone out there with 42 chromosomes...

This is fantastic start that will undoubtedly open many interesting doors in comparative genomics. I will be anxiously awaiting more results, especially in understanding the origin of novel genes to the animal kingdom relative to other eukaryotic kingdoms (Plantae and Fungi). The next genome? After reading the latest on Zooillogix today, it is definitely got to be Buddenbrockia plumatellae. It looks like a worm, but is completely symmetrical in cross-section. In the words of Peter Holland:

"It has no mouth, no gut, no brain and no nerve cord. It doesn’t have a left or right side or a top or bottom – we can’t even tell which end is the front!" (quoted from Physorg)

This study was also published in the latest issue of Science (way to bump up the invert presence!) and answers the paradox, what the $@#! is this thing?! Analysis of 50 genes (thats 31,092 amino acid alignments) confirms that 97% of the time Buddebrockia plumatellae clusters with medusozoan cnidarians. I would say that is pretty good evidence. The authors conclude that

"This active muscular worm increases the known diversity in cnidarian body plans and demonstrates that a muscular, wormlike form can evolve in the absence of overt bilateral symmetry."

Is that funding bells I hear ringing? This is an amazing evolutionary question on how body form is controlled at the genetic level. Let me clear things up a bit. Buddebrockia is a myxozoan. A strange group of typically amoeboid parasites, with Buddenbrockia being a parasite of bryozoans (see picture below of new cnidarian exiting a bryozoan zooid). Myxozoans have strange nematocyst-looking cells called polar capsules. Some consider them reduced cnidarians, though with the discovery of the worm-like Buddenbrockia plumatellae and some Hox genes support a bilateria origin. Confused or just weirded out? Hopefully this study lays to rest of some of this conundrum. Although, it opens up infinitely more conundrums. Such as, if these are really cnidarians, albeit highly derived parasitic forms, how can there be this amazing diversity of body form from medusoid, polypoid, amoeboid, worm-like, and planular larvae all within a single phylum. I remind you that the Cnidaria are a well-supported clade!

Photo credit: Sylvie Tops