origins of the metazoa
DESCRIPTION
ORIGINS OF THE METAZOA. Major evolutionary transitions. 1. Compartmentalization of replicating molecules - cells . 2. Coalescing of replicating molecules - chromosomes. 3. DNA and proteins as the genetic code. 4. Consolidate symbiotic cells to produce eukaryotic cells. - PowerPoint PPT PresentationTRANSCRIPT
ORIGINS OF THE METAZOA
Major evolutionary transitions
1. Compartmentalization of replicating molecules - cells
2. Coalescing of replicating molecules - chromosomes
3. DNA and proteins as the genetic code4. Consolidate symbiotic cells to produce eukaryotic cells
5. Sexual reproduction – production and fusion of haploid gametes
6. Evolution of multicellular organisms from unicellular ancestors
7. Establishment of social groups of discrete multicellular individuals
ORIGINS OF THE METAZOA
EARLY THEORIES
Syncytial theory
Metazoan ancestor - multinucleate, bilateral , ciliate protozoan
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Ciliated protozoan Hypothesized precursor
Cell walls form around nuclei
ORIGINS OF THE METAZOA
EARLY THEORIES
Colonial theory
Metazoan ancestor - multinucleate, bilateral , ciliate protozoan
Haeckel
Metschnikoffintrogression
invagination
What do more modern data tell us about how the Metazoa may have evolved?
What do more modern data tell us about how the Metazoa may have evolved?
-layer of algae/bacteria forms
-sediment is trapped
-a second algae/bacteria layer grows
- more sediment is trapped
Mat forming cyanophytes
Stromatolites
Distribution of multicellularity
For multicellularity to develop need:
1. Cell adhesion
2. Cell-to-cell communication
3. Programmed cell death
Types of Multicellular Development
1. Unicellular/clonal
- Clonal development from spore or zygote- - all multicellular aquatic forms and most terrestrial
- Genetic variation in cell lineage - low
- Cell specialization by epigenetic change
Types of Multicellular Development
2. Vegetative
- Produce multicellular vegetative propagules
3. Aggregative
- e.g. cellular slime molds
3. Aggregative
Cellular slime molds
3. Aggregative
Cellular slime molds
Distribution of multicellularity
Genetic consequences of different development patterns
One pattern for development of multicellularity
Advantages of multicellularity
Size related advantages
Scenedesmus acutus
Colonial form
Single cell form
Advantages of multicellularity
Division of labour
What are the candidates for the earliest multicellular animals
Proterospongia
Trichoplax
Salinella
Orthonectid
Placozoan- the earliest animal?
1 mm
Placozoan- the earliest animal?
1 mm
Placozoan- the earliest animal?
Their place in the evolutionary sequence
Three current theories
1. These are the sister group to all other Metazoans(Functional-morphology model)
Gallertoid
Flattened benthic planuloid
Gallertoid with internal duct system
Placozoa † † Sponges Cnidaria Ctenophora
Placozoan- the earliest animal?
Their place in the evolutionary sequence
Three current theories
1. These are the sister group to all other Metazoans(Functional-morphology model)
Placozoa
Sponges
Eumetazoa
Placozoan- the earliest animal?
Their place in the evolutionary sequence
Three current theories
2. Epitheliozoa Model
Placozoa
Sponges
Eumetazoa
- Considers shared characteristics
-desmosomes-ventral gland cells
Epitheliozoa
Some molecular support
Placozoan- the earliest animal?
Their place in the evolutionary sequence
Three current theories
2. Eumetazoa Model
Ctenophora
Sponges- Molecular analysis
EumetazoaCnidaria
Placozoa
Bilateria
Problems: statistically weak and ignores structure
What about the sponges?
Sponge Structure
- Specialized cells
- No true tissues
Schulze et al 1999. Proc.Roy.Soc.Lond.B. 266: 63
Schulze et al 1999. Proc.Roy.Soc.Lond.B. 266: 63
Schulze et al 1999. Proc.Roy.Soc.Lond.B. 266: 63
