Origin of Origin of MulticellularityMulticellularity

A Unicellular Colony of A Unicellular Colony of SphaeroformaSphaeroforma arciticaarcitica

[Source: Ruiz[Source: Ruiz--TrilloTrillo et al. 2007, et al. 2007, Trends in Genetics Trends in Genetics 23: 11323: 113--118]118]

Innovations of Innovations of MulticellularMulticellular OrganismsOrganisms

•• Specialization of cells (ultimately tissues) Specialization of cells (ultimately tissues)

•• Adhesion of cells (and tissues)Adhesion of cells (and tissues)

•• Communication between cells (signaling Communication between cells (signaling molecules)molecules)

I. What traits define animals?

II. How did animals originate?

III. Did animals evolve twice?

Origin of Animals

SynapomorphiesSynapomorphies of Animalsof Animals

•• Cell JunctionsCell Junctions

Placazoa

Echinoderm

Sponge

Hydra

SeptateCell

Junctions

[Source: Nichols et al. 2006, [Source: Nichols et al. 2006, PNASPNAS 103: 12451103: 12451--12456]12456]

•• Cell JunctionsCell Junctions•• SpermatozoaSpermatozoa

SynapomorphiesSynapomorphies of Animalsof Animals

•• Cell JunctionsCell Junctions•• SpermatozoaSpermatozoa•• Muscle/nerve cellsMuscle/nerve cells

SynapomorphiesSynapomorphies of Animalsof Animals

0.25 μm

myosin filamentsmyosin filaments

actinactin filamentsfilaments

•• Cell JunctionsCell Junctions•• SpermatozoaSpermatozoa•• Muscle/nerve cellsMuscle/nerve cells•• CollagenCollagen

SynapomorphiesSynapomorphies of Animalsof Animals

Collagen fibers compose the extra-cellular matrix.

How did animals originate?How did animals originate?

A. Fossil RecordA. Fossil Record

B. Extant AnimalsB. Extant Animals

GlaessnerGlaessner (1984)(1984)

Precambrian FossilsPrecambrian Fossils(600 (600 myamya))

Radial Bilateral

The Most Basic Division of AnimalsThe Most Basic Division of Animals

ProtistProtist AncestorAncestor

multicellularitymulticellularity

tissuestissues

ParazoaParazoa EumetazoaEumetazoa

bilateral symmetrybilateral symmetry

Sponges

Sponges have a very simple body plan:

• specialized cells

• no tissue

Sponges feed with Sponges feed with choanocyteschoanocytes (collar cells).(collar cells).

choanoflagellate(Pleurasiga sp.)

demosponge(Callyspongia sp.)

calcarean sponge(Scypha sp.)

hexactinellidsponge

(Euplectella sp.)

Hypothesized Transition from Hypothesized Transition from ProtistProtist to Animalto Animal

A Colonial A Colonial ChoanoflagellateChoanoflagellate

MonosigaMonosiga brevicollisbrevicollis

ChoanoflagellatesChoanoflagellates possess some keypossess some keygenes used by animals.genes used by animals.

•• CadherinCadherin(cell adhesion)(cell adhesion)

•• Tyrosine Tyrosine kinasekinase(cell communication)(cell communication)

[Source: Ruiz[Source: Ruiz--TrilloTrillo et al. 2007, et al. 2007, Trends in Genetics Trends in Genetics 23: 11323: 113--118]118]

Did animals evolve more than once?

SynapomorphiesSynapomorphies of Animalsof Animals

•• Cell junctionsCell junctions•• SpermatozoaSpermatozoa•• Muscle/nerve cellsMuscle/nerve cells•• CollagenCollagen