Common ancestor of protostomes and deuterostomes
Remus
embryogenesis recently and became interested in what is generally
considered to be the common ancestor of the protostomes and the
deuterostomes. I couldn't really find much on the subject. Is it
generally considered that both branched off from cnidarians
separately, or is there generally considered to be an ancestor of all
bilateria that then specialized to produce the protostomes and the
deuterostomes? Or are even the cnidarians a seperate branch, with the
bilaterians or even both the protostomes and the deuterostomes
branching off from something like the ctenophora?
Lorentz
This hypothesis came from a lunchtime discussion of the subject
by two physicists who should have been doing something else.
He pointed out that the dimple in the blastula becomes an anus in
deuterosomes, and becomes the mouth in protosomes. It seems unlikely
that in the common ancestor, the dimple could change to be a mouth in
one and an anus in the other. The two creatures must have evolved
gastrulation separately.
I pointed out that in gastrulation, there are actually two
dimples formed on both sides of the blastula. The mechanism that
causes the pinching in is small cords of protein. I read this in a
book on embryology, but it could be deduced from simple physics. By
Newtons Third Law, the force has to be applied to both sides of the
blastula. So actually, there are two dimples. The difference in size
is just a matter of which side is "stiffer." The difference is merely
one of consistency.
He immediately came to the conclusion that there was a common
ancestor. The variation that split the two lineages was a difference
in the consistency of the two sides. So the "moth" side and the "anus"
side didn't switch with regards to the body coordinates, just the
consistency of the sides changed. The "mouth" side became larger in
the protosomes and smaller in the deuterosomes.
I also want to point out that there is an asymmetry in the
blastula of the protosomes. So this may have influenced that
"consistency." The deuterosome blastula doesn't coil like the
protosome blastula. So maybe the real question is, "Why does the
blastula coil in one and not the other?"
In Deuterosomes, the big dimple becomes the anus and the small dimple
becomes the
Paul Ciszek
Since prostomes and deuterostomes are both triploblasts, I doubt they
branched separately off of either cnidaria or ctenophora.
There are triploblasts whose gut has only a single opening. So far as
I know, they are all classified as prostomes. Which is the more likely
scenario?
A) On two separate occasions, a triploblast with a blind gut developed
a second opening
B) A triploblast with a two-ended gut changed how it assigned the
functions of the two openings
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Lorentz
> I know, they are all classified as prostomes. Which is the more likely
> scenario?
>
> A) On two separate occasions, a triploblast with a blind gut developed
> a second opening
>
> B) A triploblast with a two-ended gut changed how it assigned the
> functions of the two openings
>
In B, why does the assignment of the functions of the two
openings have to change? Maybe the way we assign the two openings
isn't anatomically correct.
As I understand it, in a troblastic embryo, gastrulation starts
with two dimples. One is larger than the other, but the two dimples
start at the same time. The basic gastrulation process involves
protein cords that pull at the two sides, so both dimples start at the
same time. Maybe:
C) A triploblast with a two ended gut changed the relative size of the
two openings, but didn't change the assigned function of the two
openings. In other words, what changed is which indentation is
noticeable first.