Freethought & Rationalism Archive

Oolon Colluphid · 01-13-2003, 02:59 AM

From the current issue of Nature: 421, 158-160 (2003)...

Larval stages of a living sea lily (stalked crinoid echinoderm)

Hiroaki Nakano, Taku Hibino, Tatsuo Oji, Yuko Hara & Shonan Amemiya

Quote:

The embryos and larvae of stalked crinoids, which are considered the most basal group of extant echinoderms, have not previously been described. In contrast, much is known about the development of the more accessible stalkless crinoids (feather stars), which are phylogenetically derived from stalked forms. Here we describe the development of a sea lily from fertilization to larval settlement. There are two successive larval stages: the first is a non-feeding auricularia stage with partly longitudinal ciliary bands (similar to the auricularia and bipinnaria larvae of holothurian and asteroid echinoderms, respectively); the second is a doliolaria larva with circumferential ciliary bands (similar to the earliest larval stage of stalkless crinoids). We suggest that a dipleurula-type larva is primitive for echinoderms and is the starting point for the evolution of additional larval forms within the phylum. From a wider evolutionary viewpoint, the demonstration that the most basal kind of echinoderm larva is a dipleurula is consistent with Garstang's auricularia theory for the phylogenetic origin of the chordate neural tube.

[My emphasis]

Make of it what you will... but one thing�s clear: yet again, nature reveals what evolutionary theory predicts it should.

Cheers, DT

Valentine Pontifex · 01-13-2003, 03:55 PM

The article abstract said in part:

Quote:

The embryos and larvae of stalked crinoids, which are considered the most basal group of extant echinoderms, have not previously been described.

I am really surprised that his has not been done before. Crinoids are an important group to the degree it is really surprising that one one had done that work before.

I have seen what seems to be a million of them in the rocks of Oklahoma, Arizona, and Utah -- especially Utah in the San Juan River.

For those who don't know what a crinoid is: think sea lily.

Nic Tamzek · 01-13-2003, 08:47 PM

Me, I've always been a rabid fan of Garstang's auricularia theory. I mean, who wouldn't?

nic

lpetrich · 01-13-2003, 10:43 PM

Here's a nice link on marine-invertebrate larvae. Look down to the bottom to see Garstang's hypothesis of auricularia -> vertebrate evolution. Also check out this page on the subject.

This kind of larva, like several other kinds of marine-invertebrate larvae, has a cilia band on it; but in Garstang's hypothesis, part of the cilia band moves away from the mouth and forms the neural tube, that band supplying nerves for that tube.

The recent genome sequencing of the sea squirt Ciona intestinalis should help here; sea squirts have a tadpole larva which may be much like the earliest chordates. However, the sea-squirt lineage has some evolutionary innovations like a sessile adult, so sea-squirt larvae may have some differences from the earliest chordates. Evolution works like a tree, not like a ladder.

But the genome sequencing will help in unraveling the development mechanisms of sea squirts, which will make possible some strong tests of Garstang's hypothesis as echinoderm and hemichordate development mechanisms are unraveled.

Nic Tamzek · 01-14-2003, 03:25 AM

That's a great page, I actually have a vague idea of Garstang's auricularia hypothesis now.

However, the terms in this field would be perfect for some of Douglas Adams' "the worst poets in the Universe".

And "Garstang's auricularia hypothesis" would be a great name for a band.

nic

PS: Seriously, IIRC human left-right asymmetry is determined by a current created by cilia beating in a certain direction in the very early embryo. If you have defective cilia you have a 50-50 chance of having the heart on the right or left.

Here's the thing: could those embryonic cilia be considered homologous to the cilia of these larva? That would be cool...

If I'm right (instead of crazy/clueless), you can expect my paper in Nature next week...

lpetrich · 01-14-2003, 05:44 AM

Some more papers:

http://www.mbl.edu/CASSLS/levine.html -- evidence that the chordate notochord originates from the anterior gut, in the fashion of the hemichordate stomochord

http://www.swarthmore.edu/NatSci/sgi.../notochord.htm -- more on the subject of the notochord and its fate

http://inanna.ecs.soton.ac.uk/~swh/C...data/devel.pdf -- comparison on insect and vertebrate central-nervous-system development. There are remarkable similarities between these, and a possible ancestral configuration was multiple nerve cords along the length of the body.

http://wwwlib.bionet.nsc.ru/Developm...12/dev9537.pdf -- evidence that the sea-squirt-larva neural tube has some overall patterning mechanisms similar to the vertebrate one.

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01-13-2003, 08:47 PM	#3
Nic Tamzek Senior Member Join Date: Jan 2002 Location: California Posts: 646	Me, I've always been a rabid fan of Garstang's auricularia theory. I mean, who wouldn't? nic

01-13-2003, 10:43 PM	#4
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	Here's a nice link on marine-invertebrate larvae. Look down to the bottom to see Garstang's hypothesis of auricularia -> vertebrate evolution. Also check out this page on the subject. This kind of larva, like several other kinds of marine-invertebrate larvae, has a cilia band on it; but in Garstang's hypothesis, part of the cilia band moves away from the mouth and forms the neural tube, that band supplying nerves for that tube. The recent genome sequencing of the sea squirt Ciona intestinalis should help here; sea squirts have a tadpole larva which may be much like the earliest chordates. However, the sea-squirt lineage has some evolutionary innovations like a sessile adult, so sea-squirt larvae may have some differences from the earliest chordates. Evolution works like a tree, not like a ladder. But the genome sequencing will help in unraveling the development mechanisms of sea squirts, which will make possible some strong tests of Garstang's hypothesis as echinoderm and hemichordate development mechanisms are unraveled.

01-14-2003, 03:25 AM	#5
Nic Tamzek Senior Member Join Date: Jan 2002 Location: California Posts: 646	That's a great page, I actually have a vague idea of Garstang's auricularia hypothesis now. However, the terms in this field would be perfect for some of Douglas Adams' "the worst poets in the Universe". And "Garstang's auricularia hypothesis" would be a great name for a band. nic PS: Seriously, IIRC human left-right asymmetry is determined by a current created by cilia beating in a certain direction in the very early embryo. If you have defective cilia you have a 50-50 chance of having the heart on the right or left. Here's the thing: could those embryonic cilia be considered homologous to the cilia of these larva? That would be cool... If I'm right (instead of crazy/clueless), you can expect my paper in Nature next week...

01-14-2003, 05:44 AM	#6
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	Some more papers: http://www.mbl.edu/CASSLS/levine.html -- evidence that the chordate notochord originates from the anterior gut, in the fashion of the hemichordate stomochord http://www.swarthmore.edu/NatSci/sgi.../notochord.htm -- more on the subject of the notochord and its fate http://inanna.ecs.soton.ac.uk/~swh/C...data/devel.pdf -- comparison on insect and vertebrate central-nervous-system development. There are remarkable similarities between these, and a possible ancestral configuration was multiple nerve cords along the length of the body. http://wwwlib.bionet.nsc.ru/Developm...12/dev9537.pdf -- evidence that the sea-squirt-larva neural tube has some overall patterning mechanisms similar to the vertebrate one.

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