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| Sodium Channels Evolved Before Nervous Systems They Enable |
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| SciMed - Biology | |||
| TS-Si News Service | |||
| Wednesday, 18 May 2011 15:00 | |||
 Austin, TX, USA. Sodium channels evolved prior to the  evolution of the nervous system, demonstrating how key innovations in complex traits can evolve gradually, often from parts that evolved for other purposes.New findings help explain our current configuration, while highlighting the potential importance of evolutionary developments. Genetic mutations or changes in our morphology can be dangerous, or they can prove useful when solving an organism's current problems, factors that influence further evolutionary development.Nervous systems and neuron cells allow communication across the vast distances between cells in the body and led to sensory perception, behavior and the evolution of complex animal brains. The first such systems appeared in jellyfish-like animals about six hundred million years ago. Lacking further evidence, scientists thought that sodium channels evolved around the same time. Monosiga brevicollis is a single-celled marine choanoflagellate. Their colonies live in fresh- and salt-water ecosystems, occupying an important place in the marine carbon cycle between large organisms and nanoplankton. The discovery of genes for sodium channels that occur in animal neurons in M. brevicollis showed that the sodium channels evolved before neurons. Photo courtesy of Mark J. Dayel, University of California, Berkeley.Investigators from the University of Texas have now discovered that sodium channels were around well before nervous systems evolved. Zakon, a professor of neurobiology, and his coauthors, Professor David Hillis and graduate student Benjamin Liebeskind, published their findings this week in the Proceedings of the National Academy of Sciences (PNAS). Sodium channels are an integral part of a neuron's complex machinery. The channels are like floodgates lodged throughout a neuron's levee-like cellular membrane. When the channels open, sodium floods through the membrane into the neuron, and this generates nerve impulses. The researchers discovered the genes for such sodium channels hiding within an organism that isn't even made of multiple cells, much less any neurons. The single-celled organism is a choanoflagellate, and it is distantly related to multi-cellular animals such as jellyfish and humans. Because choanoflagellates are among the closest single-celled relatives of animals and are so ancient, they provide an important laboratory for exploring the origin and diversity of animal phyla. The researchers constructed evolutionary trees, or phylogenies, showing the relationship of those genes in the single-celled choanoflagellate to multi-cellular animals, including jellyfish, sponges, flies and humans. Because the sodium channel genes were found in choanoflagellates, the scientists propose that the genes originated not only before the advent of the nervous system, but even before the evolution of multicellularity itself. "These genes were then co-opted by the nervous systems evolving in multi-cellular animals," says Hillis, the Alfred W. Roark Centennial Professor in Natural Sciences. "This study shows how complex traits, such as the nervous system, can evolve gradually, often from parts that evolved for other purposes." "Evolutionarily novel organs do not spring up from nowhere," adds Zakon, "but from pre-existing genes that were likely doing something else previously." Liebeskind, a graduate student in the university's ecology, evolution and behavior program, is directing his next research efforts toward understanding what the sodium channels do in choanoflagellates. CitationEvolution of sodium channels predates the origin of nervous systems in animals. Benjamin J. Liebeskind, David M. Hillis, Harold H. Zakon. Proceedings of the National Academy of Sciences 2011; ePub ahead of print. doi:10.1073/pnas.1106363108
Abstract Voltage-dependent sodium channels are believed to have evolved from calcium channels at the origin of the nervous system. A search of the genome of a single-celled choanoflagellate (the sister group of animals) identified a gene that is homologous to animal sodium channels and has a putative ion selectivity filter intermediate between calcium and sodium channels. Searches of a wide variety of animal genomes, including representatives of each basal lineage, revealed that similar homologs were retained in most lineages. One of these, the Placozoa, does not possess a nervous system. We cloned and sequenced the full choanoflagellate channel and parts of two placozoan channels from mRNA, showing that they are expressed. Phylogenetic analysis clusters the genes for these channels with other known sodium channels. From this phylogeny we infer ancestral states of the ion selectivity filter and show that this state has been retained in the choanoflagellate and placozoan channels. We also identify key gene duplications and losses and show convergent amino acid replacements at important points along the animal lineage. Data deposition: Accession numbers and relevant databases are provided in the paper. The sequences have been deposited in the GenBank database (accession nos. JF827087, JF905561, JF905562 and JF905563). Keywords: eumetazoan, inactivation gate, pore motif.
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| Last Updated on Wednesday, 18 May 2011 15:21 |
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