Tuesday, April 28, 2015

Ctenophore Genome Sparks the Theory that the Neuron has Evolved Twice


Recent research in neuroscience and genetics has led to the idea that the neuron, previously thought to have only evolved once, has gone through two separate spurts of evolution. By examining the genome and neuromuscular structure of ctenophores, researchers are finding that the nervous system and immune system of this clade of animals has come around almost entirely independent from other animal groups - from the closely related proiferans to our own bilaterian grouping. The researchers examined the genomes of two model ctenophores, Pacific sea gooseberries and comb jellies (pictured above), to examine how these species expressed genes involved in their muscular system, nervous system, and immune system. The researchers used the genomes to place ctenophores at the base of the phylogenetic tree (pictured below), adding to the idea that ctenophores likely evolved separately from the rest of animal phylogeny. The examination of the immune system genetics found that the ctenophore immune system differs greatly from bilaterians, sponges, and cnidarians, lacking rather important pattern recognition markers. Genes involved in body patterns and axis formations were also found lacking in the ctenophores, despite being present in all metazoans. The examination of the nervous system of ctenophores found that ctenophores use practically none of the neurotransmitters known to be used in cnidarians and bilaterians (i.e. serotonin, adrenaline, dopamine, glycine, acetylcholine), suggesting these neurotransmitters are adaptations of the later cnidarian and bilaterian lines. While there are some neuron-related genes in the ctenophore that are shared by bilaterians, the neurons of the ctenophore do not express these shared genes. The similarity between ctenophores and later metazoans along with the stark differences in the nervous, muscular, and immune system as well as the genetic identification of ctenophores as a basal group supports the idea that ctenophores evolved independent from and parallel to the later metazoans. 
In easier terms, the findings of this research give way to the idea that the ctenophore phylum evolved separately from other phylums. This contrast previous ideas of ctenophores as an ancestor to later species and groups. This finding is especially interesting in the scope of neuroscience, as the previous way of thinking had the ctenophore nervous system as an ancestral system for later species, including bilateria. This research leads us to believe that, instead of a single, continual development of the nervous system, the nervous system instead evolved in two separate events - that of the ctenophore and that of the following phylums. This researching finding means that there are two entirely separate organizations of the nervous system that have evolved entirely independent of each other. Looking at nervous system evolution through this light means that the origin and evolution of our own nervous system is not as clear as we had once thought.
(second phylogeny image source: https://whyevolutionistrue.wordpress.com/2013/12/20/the-outgroup-for-animals-ctenophores/) 

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