Embryogeny and Phylogeny

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Typical Sea star life cycle represented by Crown-of thorns
Protostomes and Deuterostomes
Embryological Evidence for
Distinction of the
Chordate-Echinoderm Line
and the
Mollusc-Annelid line
(remembering they are all triploblastic)
General processes of embryogenesis,
from zygote to multicellular
triplobastic larva
1. Cell division
2. Cell differentiation
3. Formation of primary cell layers and tissues
4. Formation of the principal body cavity (coelom)
Similarities in early development
may be taken as indication of
common ancestry!!
(indeed, Ernst Haeckle went as far as
saying “Ontogeny Recapitulates Phylogeny”)
Why might early development be such a
reliable indicator of phylogeny?
5 Organogenesis
6. Growth
Phylogeny of the “big nine”:
Protostomes and Deuterostomes
Cellular
level
Multicellular
Patterns of embryonic cell division
Equal vs. unequal :
?
no bearing on phylogeny
Complete vs. incomplete: no bearing on phylogeny
(holoblastic vs. meroblastic)
?
Highly
Derived
Spiral vs. radial divisions: important…!!
The orientation of the cleavage plane is
inherent in the genetic program and this
appears to be a conservative character.
Protista
1
Planes of Division
Animal pole
Radial Cleavage
in echinoderms, acorn worms, chordates….
Longitudinal
plane
A-V Axis
Equatorial
plane
Vegetal pole
morula
Oblique
plane
Note how blastomeres end up stacked neatly one
on top of another or directly to the side
Radial Cleavage
in echinoderms, acorn worms, chordates….
morula
blastula
blastula
Note how blastomeres end up stacked neatly one
on top of another or directly to the side
Spiral Cleavage
in molluscs, annelids
Note oblique plane of mitotic spindle at 3rd division and the
spiral arrangement of the blastomeres from the third
division onward (better space use than radial)
Goulding MQ. 2009.
Cell Lineage of the Ilyanassa Embryo: Evolutionary
Acceleration of Regional Differentiation during Early
:10.1371/
journal.pone.0005506
Development. PLoS ONE 4(5): e5506. doi
The Third Cleavage Patterns of the Dextral and Sinistral Limnea stagnalis Embryos
and Their Adult Snails. (A) The 4-cell to the 8-cell stage of the sinistral and dextral L.
stagnalis embryos observed by light microscopy.
Shibazaki Y, Shimizu M, Kuroda R (2004) Body handedness is directed by genetically determined
cytoskeletal dynamic in the early embryo. Current Biology 14:1462-1467.P.Z. Myers://
scienceblogs.com/pharyngula/2006/04/spiral_cleavage.php
2
Sponges
cleavage and morphogenesis,
leading to the larvae. 1–4 Cleavage patterns in sponges: incurvational (1), polyaxial (2),
radial (3), and chaotic (4). Three
main form of sponges blastula: stomoblastula (5), coeloblastula
(6), and stereoblastula (7).
Different larval types of sponges: amphiblastula of Calcaronea
(Calcarea)
(8);; calciblastula of Calcinea (Calcarea) (9);; coeloblastula (10), parenchymella
(11), and disphaerula (12)
of
Halisarca (Halisarcida);; parenchymella of Vaceletia crypta
(Verticillitida) (13);; pseudoblastula of
Chondrosia
reniformis
(Chondrosida) (14);; trichimella of Oopsacas minuta (Hexactinellida)
(15);; juvenile of Tetilla under direct
development (16);; parenchymella of Tethya aurantium (Pallas, 1766;; Hadromerida) (17);; coeloblastula of Polymastia robusta Bowerbank,
1866
(Hadromerida)
(18);; parenchymella of Dictyoceratida
(19);; parenchymella of Incurvational polyaxial
freshwater Haplosclerida (20);; parenchymella of Poecilosclerida (21);; cinctoblastula of Homoscleromorpha (22).
What factors could potentially be
important in determining the fate of
individual cells in a developing embryo?
radial
chaotic
Cnidaria: radial, sometimes chaotic (different in sibs!)
Placozoans: radial
Ctenophores: biradial
Radial cleavage is the ancestral condition
Timing of cell fate-determination:
evidence of phylogenetic differences
Blastomere Separation Expts
Abnormal larvae
Small but
Normal larvae
Echinoderms ,chordates, etc. Regulative determination
Or indeterminate cleavage
Echinoderms ,chordates, etc.
Regulative determination or
indeterminate cleavage
Monozygotic twins
How do cells know where to go and what to
become?
Chemical signals in the cytoplasm
Other cells
Other physical factors
Timing of cell fate-determination
In echinoderms, acorn worms & vertebrates the
fate of embryonic cells is not established
until later in cleavage (16 cell or later)
In molluscs and other “protostomes” the fate
of embryonic cells is predetermined by
cytoplasmic factors very early in cleavage
Molluscs, Annelids etc.
Mosaic determination or
determinate cleavage
1 in 15 thousand
human pregnancies
result in monozygotic
Quadruplets
There is no
Two sets of healthy
Octuplets
No healthy nonuplets
Polyembryonic 9-banded Armadillos
(only six species of genus Dasypus)
can have
Purple sea urchins
But not in
Earthworms
Or Snails
consistent
phylogenetic
pattern in
the type of
blastula formed
Coeloblastula
Discoblastula
Stereoblastula
Periblastula
3
46
Fig. 7: The trend in sponge embryonic development publications
between 1870 and 2006.
Formation of larva in sponges
There is no
consistent
phylogenetic
pattern in how
gastrulation is
achieved
invagination
ingression
delamination
Fig. 8: Different types of
morphogenesis
in
sponges
resulting in larva formation: A. Cell delamination (Hexactinellida
– Oopsacas minuta);; B. Morula delamination (Demospongiae: Dendroceratida,
Dictyoceratida,
Halichondrida, Haplosclerida);; C. Invagination (Halisarca dujardini,
Demospongiae);; D. Multipolar ingression
(H.
dujardini,
Demospongiae);; E. Multipolar egression
(Homoscleromorpha,
Demospongiae);; F. Polarized delamination (Demospongiae: Poecilosclerida and Halichondrida);; G. Excurvation
(Calcaronea,
Calcarea);; H. Formation of blastula
(pseudoblastula) by means of
ingression of maternal cells into the
embryo (Chondrosia reniformis,
Demospongiae: Chondrosida);; I. Unipolar
proliferation
(Demospongiae: Verticillitida – Vaceletia crypta). (From: Ereskovsky and Dondua 2006).
epiboly
Ereskovsky 2007
Phylogenetic implications
of early development:
• Patterns of cell division
Important differences exist in:
the fate of the blastopore
Snail gastrula
sea urchin gastrula
• Timing of cell fate-determination
• Mechanism of blastula and gastrula formation
• Fate of the blastopore
• Origin of the mesoderm
• Method of coelom formation
Becomes the mouth
Becomes the anus
Important differences exist in:
the origin of the mesoderm
Snail gastrula
sea urchin gastrula
cleavage of 4D cell
outpocketing of archenteron
4
Important differences exist in:
the mechanism of coelom
formation
Enterocoelous coelom formation
Important differences exist in:
the mechanism of coelom
formation
Snail gastrula
schizocoelous coelom formation
Finally:
Protostomes
Deuterostomes
Molluscs, Annelids, etc. Echino., chordates etc.
Spiral cleavage
Radial cleavage
Mosaic cell fate
Blastopore is mouth
Regulative cell fate
Blastopore is anus
Mesoderm from 4D cell
Meso. from archenteron
Coelom by splitting of
solid mesodermal mass
Coelom by pinching off of
enterocoels
3 conditions with respect to the coelom:
Acoelomate
Pseudocoelomate
Coeloms, Pseudocoeloms
and Phylogeny
Review of Principal Body Plans
Eucoelomate
Acoelomate
Condition
No coelom
“False” coelom
“True” coelom
Traditional View is that these are homologous
primitive and derived states
Eucoelomate
condition
Pseudocoelomate
condition
5
Phylogeny of the “big nine”:
Protostomes and Deuterostomes
Cellular
level
Peritoneal cavity
Pleural Cavity
Pericardial Cavity
Multicellular
Are mesodermal lining
that surround organs
in vertebrates
Protista
6
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