2. Arbacia lixula
Mediterranean Sea Urchin

3. Protostomes and Deuterostomes
Embryological Evidence for Distinction of the
Chordate-Echinoderm Line and the
Mollusc-Annelid line
(remembering they are all triploblastic)

4. Typical Sea star life cycle represented by Crown-of thorns

5. 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

6. Phylogeny of the “big nine”: Protostomes and Deuterostomes
Cellular
level ?
Multi-
cellular ?
Highly
Derived
Protista

7. 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

8. 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.

9. Planes of Division Animal pole Vegetal pole Longitudinal plane
A-V Axis
Equatorial
plane
Vegetal pole
Oblique
plane

10. 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

11. 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)

12. Goulding MQ Cell Lineage of the Ilyanassa Embryo: Evolutionary Acceleration of Regional Differentiation during Early Development. PLoS ONE 4(5): e5506. doi: /journal.pone

13. Cnidaria: radial, sometimes chaotic (different in sibs!)
Incurvational polyaxial radial chaotic
Sponges
Cnidaria: radial, sometimes chaotic (different in sibs!)
Placozoans: radial
Ctenophores: biradial
Radial cleavage is the ancestral condition

14. 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

15. Blastomere Separation Expts
Timing of cell fate-determination: evidence of phylogenetic differences
Echinoderms ,chordates, etc. Regulative determination
Or indeterminate cleavage
Blastomere Separation Expts
Abnormal larvae
Small but
Normal larvae
Molluscs, Annelids etc.
Mosaic determination or
determinate cleavage
Echinoderms ,chordates, etc. Regulative determination or
indeterminate cleavage

16. 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

17. But not in Earthworms Or Snails Purple sea urchins 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
Uterine constraint

18. 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

19. There is no consistent phylogenetic pattern in
the type of blastula formed
Coeloblastula Stereoblastula
Discoblastula
Periblastula

20. There is no consistent phylogenetic pattern in how gastrulation is achieved
invagination
ingression
delamination
epiboly

21. Formation of larva in sponges
Ereskovsky 2007

22. Important differences exist in: the fate of the blastopore
Snail gastrula sea urchin gastrula
Becomes the mouth Becomes the anus

23. 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

24. Important differences exist in: the origin of the mesoderm
Snail gastrula sea urchin gastrula
cleavage of 4D cell outpocketing of archenteron

26. So Far: Molluscs, Annelids, etc. Echino., chordates etc.
Spiral cleavage Radial cleavage
Mosaic cell fate Regulative cell fate
Blastopore is mouth Blastopore is anus
(Protostomes) (Deuterostomes)
Mesoderm from 4D cell Meso. from archenteron

27. 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

28. Important differences exist in: the mechanism of coelom formation
Enterocoelous coelom formation

29. Important differences exist in: the mechanism of coelom formation
Snail gastrula
schizocoelous coelom formation

30. Finally: Protostomes Deuterostomes
Molluscs, Annelids, etc. Echino., chordates etc.
Spiral cleavage Radial cleavage
Mosaic cell fate Regulative cell fate
Blastopore is mouth Blastopore is anus
Mesoderm from 4D cell Meso. from archenteron
Coelom by splitting of Coelom by pinching off of
solid mesodermal mass enterocoels