Bilaterally symmetrical animals share a common ancestor with a CNS?
The common ancestor of Bilateria ( ~= bilaterally symmetric animals ~= “most animals including vertebrates, arthropods, molluscs, etc” - - tolweb) is thought to have had a nervous system. Question: did it have a centralized nervous system? Or did centralization in the nervous system evolve separately in chordates and in other bilaterally symmetric animals?
Evidence supporting the view that nervous system centralization evolved separately include:
- the central nerve cord is dorsal in chordates but ventral in non-chordates
- CNS patterning in each of the fruitfly and the nematode, both in Bilateria, are significantly different from chordates, although there are also similarities
- hemichordate worms, also in Bilateria and quite close to chordates, have a decentralized nervous system
However, there is recent genetic evidence that in fact all of Bilateria shares a common evolutionary origin for some of its CNS patterning machinery. Experiments show that there are significant similarities between the general patterning of the CNS in chordates and in annelids. Further experiments show that some patterning in the medial forebrain is conserved between chordates and annelids as well; specifically, some genetics related to a class of cells which are both photoreceptors and neurosecretory.
The authors suggest that annelids are closest to the common bilateria ancestor, that chordata is relatively close, and that insects and nematodes have evolved relatively “more” since then, which is why annelids are more similar to chordates even where insects and nematodes are dissimilar.
I’m not an expert in this stuff and I might have gotten this all wrong. Please lemme know if so.
Go invert! (-ebrate neurobiology)
the “general” patterning study:
A. Denes, G. Jékely, P. Steinmetz, F. Raible, H. Snyman, B. Prud’homme, D. Ferrier, G. Balavoine, D. Arendt. Molecular Architecture of Annelid Nerve Cord Supports Common Origin of Nervous System Centralization in Bilateria. Cell, Volume 129, Issue 2, Pages 277-288.
Commentary: Maximilian J. Telford, A Single Origin of the Central Nervous System?, Cell, Volume 129, Issue 2, 20 April 2007, Pages 237-239.
Press release
the photoreceptors/neurosecretory cell patterning study:
Kristin Tessmar-Raible, Florian Raible, Foteini Christodoulou, Keren Guy, Martina Rembold, Harald Hausen and Detlev Arendt, Conserved Sensory-Neurosecretory Cell Types in Annelid and Fish Forebrain: Insights into Hypothalamus Evolution, Cell, Volume 129, Issue 7, 29 June 2007, Pages 1389-1400.
Press release
Related weblog post at The Other 95%
July 20th, 2007 at 5:38 pm
Thanks for linking to my post! I learned from reading another recent paper on the cnidarian genome (that I posted on here: http://other95.blogspot.com/2007/07/cnidarian-double-whammy-anemone-genome.html) that fly and nematode evolve at a faster rate relative to the genomes of other model organisms and they a greater percentage of intron loss and ancestral gene loss. Perhaps the annelids are more conserved and evolve relatively slowly. I know that both insects and nematodes are very diverse taxa relative to chordates and annelids (and cnidarians for that matter).
July 20th, 2007 at 6:00 pm
Thanks, that’s neat! I added a link to http://other95.blogspot.com/2007/07/cnidarian-double-whammy-anemone-genome.html in the story above (around ‘have evolved relatively “more”’)
August 18th, 2007 at 2:42 am
This seems like a no brainer…so to speak. Bilateralizing came about from duplicating, which implies two parallel nervous systems, which equals in toto one non-centralized nervous system. QED
August 18th, 2007 at 4:13 pm
Yeah but the question is, did the common ancestor have a CENTRALIZED nervous system?