DId Comb Jellies Evolve Nervous Systems Separately?

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DId Comb Jellies Evolve Nervous Systems Separately?

#1  Postby lpetrich » Feb 26, 2019 3:24 pm

]Comb jellies (ctenophores) are very odd animals. They look something like jellyfish, if one does not look very closely. They are rounded, and they have strips of cilia extending across their bodies. These animals' name is from those strips of cilia, because they seem like combs. They often have tentacles, sometimes much longer than their main bodies. Their nervous systems are simple nerve nets, without any well-defined central nervous system.

Ctenophora (Wikipedia), Introduction to the Ctenophora, Comb Jellies | Chesapeake Bay Program, Comb jelly, Open Waters, Invertebrates, Beroe spp at the Monterey Bay Aquarium, Jellyfish and Comb Jellies | Smithsonian Ocean, Comb Jelly (Ctenophore) | the Shape of Life | The Story of the Animal Kingdom

How are these animals related to the rest of the animal kingdom? A traditional hypothesis is Coelenterata, that its closest relatives are cnidarians: jellyfish and sea anemones and coral animals and the like. Their nervous systems are also nerve nets. But molecular phylogeny tells a different story.

I must now introduce the rest of the animal kingdom (Metazoa): sea sponges (Porifera), placozoans (small, blob-like), and bilaterians (all the rest). Sponges and placozoans have no nervous systems, but bilaterians do -- and sometimes very well-developed ones.

Independent origins of neurons and synapses: insights from ctenophores | Philosophical Transactions of the Royal Society B: Biological Sciences

Its starting point is this phylogeny: (ctenophores, (sponges, (placozoans, (cnidarians, bilaterians)))) This is supported by work like Extracting phylogenetic signal and accounting for bias in whole-genome data sets supports the Ctenophora as sister to remaining Metazoa | BMC Genomics | Full Text and The ctenophore lineage is older than sponges? That cannot be right! Or can it? | Journal of Experimental Biology though not Genomic data do not support comb jellies as the sister group to all other animals -- that one finds (ctenophores, (placozoans, (cnidarians, bilaterians))) either side-by side with sponges or emerging from within the sponges. I note that the divergence of these major groups was relatively fast compared to their age, thus making their phylogeny difficult to resolve.

But there is a problem with nervous-system evolution. I'll label organisms with a nervous system N and those without _:
(ctenophores N, (sponges _, (placozoans _, (cnidarians N, bilaterians N))))

So if neurons originated once, then we'd have N (ctenophores, (X sponges, (X placozoans, (cnidarians, bilaterians)))) where X is a loss of a nervous system. But if neurons originated twice, we'd have

(N ctenophores, (sponges, (placozoans, N (cnidarians, bilaterians))))

There is a further problem. Ctenophores' neurotransmitters are different. They don't have most of the small-molecule neurotransmitters that cnidarians and bilaterians do: they have glutamate and gamma-aminobutyric acid (GABA), but not acetylcholine, serotonin, histamine, dopamine, noradrenaline, or octopamine. They also have different "neuropeptides", short protein molecules used as neurotransmitters. So with a single origin, we'd have either this:

N2 (ctenophores, (X sponges, (X placozoans, XN1 (cnidarians, bilaterians))))
or
N1 (XN2 ctenophores, (X sponges, (X placozoans, (cnidarians, bilaterians))))

The XN's are loss and regain of features.

But two origins is simpler:
(N2 ctenophores, (sponges, (placozoans, N1 (cnidarians, bilaterians))))

So we have a remarkable window into the early evolution of animals.
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Re: DId Comb Jellies Evolve Nervous Systems Separately?

#2  Postby lpetrich » Feb 26, 2019 3:25 pm

Ancient coexistence of norepinephrine, tyramine, and octopamine signaling in bilaterians | BMC Biology | Full Text
Two types of receptors for each kind of monoamine neurotransmitter are found for some species and presumed ancestral in Bilateria.

Ancestry of neuronal monoamine transporters in the Metazoa | Journal of Experimental Biology
The ancestral bilaterian had three kinds of transporters: SERT for serotonin, DAT for dopamine, and MAT for an assortment of monoamine neurotransmitters.

Anatomy and development of the nervous system of Nematostella vectensis, an anthozoan cnidarian - Marlow - 2009 - Developmental Neurobiology - Wiley Online Library N. v. is a common model system.

Cnidarians utilize both classical fast (acetylcholine, glutamate, GABA, and glycine) and slow (catecholamines and serotonin) transmitters and neu-ropeptides (especially of the RFamide and RWamidefamilies) for synaptic neurotransmission (Satterlie, 2002; Kass-Simon and Pierobon, 2007).

Catecholamines include epinephrine (adrenaline), norepinephrine (noradrenaline), and dopamine.

The paper mentioned that N.v. has a gene for a protein much like bilaterian dopamine beta hydroxylase, which converts dopamine into norepinephrine.

So cnidarians and bilaterians have a lot of neurotransmitters in common.
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