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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