Download Embryogeny and Phylogeny

Arbacia lixula
Mediterranean Sea Urchin
Protostomes and Deuterostomes
Embryological Evidence for
Distinction of the
Chordate-Echinoderm Line
and the
Mollusc-Annelid line
(remembering they are all triploblastic)
Typical Sea star life cycle represented by Crown-of thorns
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)
5 Organogenesis
6. Growth
Phylogeny of the “big nine”:
Protostomes and Deuterostomes
Cellular
level
Multicellular
?
?
Highly
Derived
Protista
Phylogenetic implications
of early development:
•Patterns of cell division
•Timing of cell fate-determination
•Mechanism of blastula and gastrula formation
•Fate of the blastopore
•Origin of the mesoderm
•Method of coelom formation
Patterns of embryonic cell division
Equal vs. unequal :
no bearing on phylogeny
Complete vs. incomplete: no bearing on phylogeny
(holoblastic vs. meroblastic)
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.
Planes of Division
Animal pole
Longitudinal
plane
A-V Axis
Equatorial
plane
Vegetal pole
Oblique
plane
Radial Cleavage
in echinoderms, acorn worms, chordates….
morula
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 Development. PLoS ONE 4(5): e5506.
doi
:10.1371/journal.pone.0005506
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 Sponges Homoscleromorpha (22).
radial
chaotic
Cnidaria: radial, sometimes chaotic (different in sibs!)
Placozoans: radial
Ctenophores: biradial
Radial cleavage is the ancestral condition
What factors could potentially be
important in determining the fate of
individual cells in a developing embryo?
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:
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
Molluscs, Annelids etc.
Mosaic determination or
determinate cleavage
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
Monozygotic twins
1 in 15 thousand
human pregnancies
result in monozygotic
Quadruplets
Two sets of healthy
Octuplets
No healthy nonuplets
Polyembryonic 9-banded Armadillos
(only six species of genus Dasypus)
Purple sea urchins
But not in
Earthworms
Or Snails
Phylogenetic implications
of early development:
•Patterns of cell division
•Timing of cell fate-determination
•Mechanism of blastula and gastrula formation
•Fate of the blastopore
•Origin of the mesoderm
•Method of coelom formation
There is no
consistent
phylogenetic
pattern in
the type of
blastula formed
Coeloblastula
Discoblastula
Stereoblastula
Periblastula
There is no
consistent
phylogenetic
pattern in how
gastrulation is
achieved
invagination
ingression
delamination
epiboly
Formation of larva in sponges
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 a nd 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).
Ereskovsky 2007
Important differences exist in:
the fate of the blastopore
Snail gastrula
Becomes the mouth
sea urchin gastrula
Becomes the anus
Phylogenetic implications
of early development:
•Patterns of cell division
•Timing of cell fate-determination
•Mechanism of blastula and gastrula formation
•Fate of the blastopore
•Origin of the mesoderm
•Method of coelom formation
Important differences exist in:
the origin of the mesoderm
Snail gastrula
sea urchin gastrula
cleavage of 4D cell
outpocketing of archenteron
So Far:
Molluscs, Annelids, etc.
Spiral cleavage
Mosaic cell fate
Blastopore is mouth
(Protostomes)
Mesoderm from 4D cell
Echino., chordates etc.
Radial cleavage
Regulative cell fate
Blastopore is anus
(Deuterostomes)
Meso. from archenteron
Phylogenetic implications
of early development:
•Patterns of cell division
•Timing of cell fate-determination
•Mechanism of blastula and gastrula formation
•Fate of the blastopore
•Origin of the mesoderm
•Method of coelom formation
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