Scientists at the University of California, Santa Barbara have discovered significant clues to the evolutionary origins of the nervous system by studying the genome of a sea sponge, a member of a group considered to be among the most ancient of all animals. The findings are published in the June 6 issue of the online, open-access journal PLoS ONE.
“It turns out that sponges, which lack nervous systems, have most of the genetic components of synapses,” said Todd Oakley, co-author and assistant professor in the Department of Ecology, Evolution and Marine Biology at UC Santa Barbara. Origination periods of post-synaptic genes. One possible configuration of the post-synaptic genes based on the known organization of the post-synaptic junction is illustrated. Each color represents the origination period (figure inset) of the gene family inferred from phylogenetic analyses (Figure S1). As further evidence for orthology, domain architectures of selected gene family members were compared (Figure S2). NCBI accession numbers for each gene family member are provided in Table S1. Some gene families may have been lost from the investigated genomes and originated with an earlier ancestor than shown. Question mark indicates insufficient traces to confirm this PDZ domain. doi:10.1371/journal.pone.0000506.g001
“Even more surprising is that the sponge proteins have ‘signatures’ indicating they probably interact with each other in a similar way to the proteins in synapses of humans and mice,” said Oakley. “This pushes back the origins of these genetic components of the nervous system to at or before the first animals –– much earlier than scientists had previously suspected.”
When analyzing something as complex as the nervous system, it is difficult to know where to begin, explained Ken Kosik, senior author and co-director of UCSB’s Neuroscience Research Institute, who holds the Harriman Chair in Neuroscience Research.
The first neurons and synapses appeared over 600 million years ago in “cnidarians,” creatures known today as hydra, sea anemones, and jellyfish. By contrast, sponges, the oldest known animal group with living representatives, have no neurons or synapses. They are very simple animals with no internal organs.
“We look at the evolutionary period between sponges and cnidarians as the period when the nervous system came into existence, about 600 million years ago,” said Kosik.
He explained that the research group made a list of all the genes expressed in a synapse in humans, since synapses epitomize the nervous system. Synapses are involved in cell communication, learning, and memory. Next, the researchers looked to see if any of the synapse genes were present in the sponge.
“That was when the surprise hit,” said Kosik. “We found a lot of genes to make a nervous system present in the sponge.” The research team also found evidence to show that these genes were working together in the sponge. The way two of the proteins interact, and their atomic structure, bear resemblance to the human nervous system. (A) Sponge PDZ3—CRIPT Homology Model.
The last five residues of the CRIPT protein (yellow) interact with PDZ3 residues (blue and orange) by making van der Waals contacts, hydrogen bonds, or electrostatic interactions of greater than 0.1 kcal/mol in magnitude in any of the PDZ3 homology models. (Figure S4). The subset of residues painted blue represent the core union set that interact directly with the ligand in the PDZ1 co-crystal (2I1N), the PDZ2 co-crystal (2G2L), or the PDZ3 co-crystal (1BE9) by either van der Waals contacts of 3.9 Å or shorter or by hydrogen-bond lengths of 3.5 Å or shorter. (B) Ligand-binding residues are very highly conserved within a specific type of PDZ domain. Conservation of the 13 binding residues compared to the remaining 61 more distant residues for 16 types of PDZ domains from Homo, Drosophila, Nematostella and Amphimedon. These frequencies are also calculated across all those domains at once (column 1). Comparison of the conservation of binding residues versus non-binding residues; *, p<0.05; **, p<0.01; ***, p<0.001 (Probability associated with a Student's two-sample unequal variance t-Test). doi:10.1371/journal.pone.0000506.g002
“We found this mysterious unknown structure in the sponge, and it is clear that evolution was able to take this entire structure, and, with small modifications, direct its use toward a new function,” said Kosik. “Evolution can take these ‘off the shelf’ components and put them together in new and interesting ways.”