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 ? Multi- cellular ? 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 Vegetal 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

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

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

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

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

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

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 Stereoblastula Discoblastula Periblastula

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

Formation of larva in sponges Ereskovsky 2007

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

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 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