Animal Ancestors - Sponges or Comb Jellies?

I read this on the newsletter of Friday Harbor lab and that got me interested in comb jellies.

For years textbooks have started discussion of animals with sponges. They were considered to lack true tissues and had a sessile life style, sitting on the ocean floor as adults and capturing small particles. However, after sequencing the entire genomes of several sponges, it became clear that they are more complex than previously thought. Studies published in 2008 suggested that ctenophores, or comb jellies, were at the base of the animal tree of life, and it became a priority to sequence one of these species. Actually two different species have been sequenced, which is important for comparative purposes. These two comb jelly genomes show remarkable congruence in their genomes and similarities to sponge genomes.

What are comb jellies? Wikipedia provides a nice description -

Ctenophora (/t??n?f?r?/; singular ctenophore, /?t?n?f?r/ or /?ti?n?f?r/; from the Greek ????? kteis ‘comb’ and ???? pher? ‘carry’; commonly known as comb jellies) is a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the “combs”, groups of cilia they use for swimming, and they are the largest animals that swim by means of cilia adults of various species range from a few millimeters to 1.5 m (4 ft 11 in) in size. Like cnidarians, their bodies consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, these layers are two cells deep, while those in cnidarians are only one cell deep. Ctenophores also resemble cnidarians in having a decentralized nerve net rather than a brain.

But I presume our readers will enjoy a video introduction even more -

A comb jelly genome paper came out late last year in Science.

The Base of the Animal Tree?

The identity of the most basal lineages of the animal kingdom evolutionary tree has long been contested. Ryan et al. (p. 10.1126/science.1242592; see the Perspective by Rokas) sequenced the genome of the ctenophore the warty comb jelly or sea walnut, Mnemiopsis leidyi, and conclude that ctenophores alone, not sponges or the clade consisting of both ctenophores and cnidarians, are the most basal extant animals. The results suggest a specific evolutionary process that likely occurredincluding repeated gains and loss of mesoderm, expansion of genes associated with the cell cycle, growth signaling, apoptosis, and epithelial and neural cell types. Furthermore, previous hypotheses regarding the evolution of animals may require re-evaluation.

One beautiful aspect of this increased focus on sponges, jellyfish and comb jellies is the development of understanding of where the Hox genes came from. David Ferrier’s group in UK has been doing wonderful work in this regard and the following slides are highly informative.

Another source of the state-of-the-art information is his recent article in elife -

The Hox-TALE has been wagging for a long time

Hox genes are famous for often being found in clusters, with the order of the genes within the cluster matching the order in which these genes are first activated along the head-to-tail axis of the embryo. These genes code for Hox proteins that can interact with DNA to switch other genes on or off. The number of different Hox proteins is relatively small, but they are able to target a wide spectrum of other genes, with their ability to bind to different target genes being modulated via interactions with other proteins known as co- factors. The ability of a relatively small number of Hox genes to specify the huge diversity of animal body forms observed in nature is a major puzzle in developmental biology.

Work on flies and mice has revealed that the major Hox co-factors belong to the so-called TALE class of homeobox genes (Holland et al., 2007). Now, in eLife, Bruno Hudry of Imperial College London, Samir Merabet of the Institut de

Gnomique Fonctionnelle de Lyon and co-workers have uncovered the origin of this cofactor interaction by focussing on an early branch of the animal family tree, the cnidarians, which includes jellyfish, corals and anemones (Hudry et al., 2014).

But above is only a small part of the hox puzzle and we have a lot more. Stay tuned for the next commentary.