How did protosomes and deuterosomes differentiate?
Lorentz
Bilateral animals are divided into protosomes and deuterostomes.
The defining difference between them is in the eventual fate of the
large dimple that forms on the embryo during gastrulation. In
protosomes, the large dimple becomes a mouth (i.e., marks the anterior
part of the adult body). In deuterosomes, the large dimple becomes the
anus (i.e., marks the posterior part of the adult body). Molecular
studies confirm that this division is monophyletic. There must have
been a most recent common ancestor between protosomes and
deuterosomes.
Evolution almost always occurs in small steps. However, it is
hard to see how such a division could develop in small steps from an
animal that already had two openings in its body. I present my
speculations on the subject. However, I see problems with both.
Dawkins in his "The Ancestors Tale" pursues the issue of how the
nerve chord switched places. Deuterosomes mostly have dorsal nerve
chord, while proterosomes have a ventral nerve chord. He suggests a
behavior change where the animals starts to swim upside down. While
this explains the nerve chord pretty well, it happens NOT to explain
the primary embryological difference between the two groups. It
doesn't explain how the openings switched functions.
1) Both of them evolved from a cctenophore-like organism with one
opening in its body and bilaterial symmetry. The most recent common
ancestor in this theory has two openings both of which served to to
excrete waste and eat food.
-Problem: Gastrulation is very similar in both proterosomes
and deuterosomes. One would think that a break in symmetry would
require a difference in the process of gastrulation.
2) There is a smaller dimple (according to Wolpert) that develops on
the side opposite the big dimple. The function didn't really switch.
The small dimple grew larger and the large dimple shrunk. Each dimple
kept its function over time.
-Why would the two dimples switch sizes over time?
r norman
The short answer is that it is a lot more complex than simply
switching between mouth and anus. There is a good discussion on
exactly this topic at
http://nimravid.wordpress.com/2008/03/05/protostome-deuterostome/
See also "PROBLEMS WITH CHARACTERIZING THE
PROTOSTOME-DEUTEROSTOME ANCESTOR"
http://www.discovery.org/scripts/viewDB/filesDB-download.php?id=146
Here is some information on possible nervous system development
preceding the protostome/deuterostome split
http://content.karger.com/produktedb/produkte.asp?typ=fulltext&file=000151471
Lorentz
> On Sat, 26 Dec 2009 12:44:33 -0500 (EST), Lorentz
>
>
>
> > � � � �-Why would the two dimples switch sizes over time?
I should have stated to begin with. The "dimple" is called a
blastopore.
>
> The short answer is that it is a lot more complex than simply
> switching between mouth and anus. �There is a good discussion on
> exactly this topic at
> �http://nimravid.wordpress.com/2008/03/05/protostome-deuterostome/
>
> See also "PROBLEMS WITH CHARACTERIZING THE
> PROTOSTOME-DEUTEROSTOME ANCESTOR"
> �http://www.discovery.org/scripts/viewDB/filesDB-download.php?id=146
>
This link is interesting. The author proposes that the common
ancestor was a bit protosome-like. It had a blastopore was that, like
some annelids, puckered into a separate mouth and anus. Then, in the
lineage that led to deuterosomes, another mouth opened and the first
one shut.
I like this one because it is consistent with echinoderm
development. The "mouth" of a starfish or sea urchin is not the
"mouth" of its larva. So I can see a protosome living the life of a
"primitive" echinoderm. It could then differentiate into echinoderms
and protochordates.
However, it makes me wonder about the extant animals that use a
"slit blastopore." Is their gastrulation significantly different from
the gastrulation of other protosomes? My impression from reading
Wolpert is that gastrulation is the same in all bilaterans, other than
the eventual destination of the blastopore.
It does make me think about whether the chordate ancestory
includes a species that was fully sessile as an adult. The tunicate is
a protochordate with a fully sessile adult. Maybe our lineage went
from being a fully mobile adult protosome species, to a fully sessile
adult protosome species, to a split between protosomes and
deuterosomes.
r norman
Why in the world would you suspect being sessile to have anything at
all to do with it?
Lorentz
> On Thu, 31 Dec 2009 23:57:32 -0500 (EST), Lorentz
>
>
>
> <drosen0...@yahoo.com> wrote:
> >On Dec 27, 11:02�pm, r norman <r_s_nor...@comcast.net> wrote:
>
>
> >> > � � � �-Why would the two dimples switch sizes over time?
> > � � I should have stated to begin with. The "dimple" is called a
> >blastopore.
>
> >> The short answer is that it is a lot more complex than simply
> >> switching between mouth and anus. �There is a good discussion on
> >> exactly this topic at
> >> �http://nimravid.wordpress.com/2008/03/05/protostome-deuterostome/
>
> >> See also "PROBLEMS WITH CHARACTERIZING THE
> >> PROTOSTOME-DEUTEROSTOME ANCESTOR"
> >> �http://www.discovery.org/scripts/viewDB/filesDB-download.php?id=146
>
> >ancestor was a bit protosome-like. It had a blastopore was that, like
> >some annelids, puckered into a separate mouth and anus. Then, in the
> >lineage that led to deuterosomes, another mouth opened and the first
> >one shut.
> > � I like this one because it is consistent with echinoderm
> >development. The "mouth" of a starfish or sea urchin is not the
> >"mouth" of its larva. So I can see a protosome living the life of a
> >"primitive" echinoderm. It could then differentiate into echinoderms
> >and protochordates.
> > � However, it makes me wonder about the extant animals that use a
> >"slit blastopore." Is their gastrulation significantly different from
> >the gastrulation of other protosomes? My impression from reading
> >Wolpert is that gastrulation is the same in all bilaterans, other than
> >the eventual destination of the blastopore.
> > � �It does make me think about whether the chordate ancestory
> >includes a species that was fully sessile as an adult. The tunicate is
> >a protochordate with a fully sessile adult. Maybe our lineage went
> >from being a fully mobile adult protosome species, to a fully sessile
> >adult protosome species, to a split between protosomes and
> >deuterosomes.
>
> Why in the world would you suspect being sessile to have anything at
> all to do with it?
1) If an organism is moving, then there is a reason for having an
anterior and a posterior. The organism wants to leave it's excrement
behind. So a bilateral symmetry makes more sense than a radial
symmetry.
2) If the organism is sessile, then a bilateral symmetry doesn't make
as much sense. The sessile organism may as well be radial symmetric,
since it can look for food in all directions.
3) Echinoderms tend to be free traveling, bilateral larva and slow
moving, radial symmetric adults.
4) Barnacles look like free moving bilaterals when they are born, and
vaguely radial and sessile as adults.
5) Our early embryo stages (i.e., morula, blastula) are highly radial.
Maybe they really are recapitulating a sessile adult ancestory.
Yes, I know that ontology doesn't always recapitulate phylogeny.
However, sometimes it does |:-)
Lorentz
> � � How did protosomes and deuterosomes differentiate?
Sorry everybody. It's "protostomes" and "deuterostomes."