1.04k likes | 6.08k Views
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.
E N D
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 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 • • • • • • • • • • • • • • • • • • • • • • • • Ciliated protozoan Hypothesized precursor • • • • • • • • • • • • Cell walls form around nuclei
ORIGINS OF THE METAZOA EARLY THEORIES Colonial theory Metazoan ancestor - multinucleate, bilateral , ciliate protozoan Haeckel invagination introgression Metschnikoff
What does more modern data tell us about how the Metazoa may have evolved?
What does more modern data tell us about how the Metazoa may have evolved?
Mat forming cyanophytes -layer of algae forms -sediment is trapped -a second algal layer grows - more sediment is trapped
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
What are the candidates for the earliest multicellular animals Trichoplax Proterospongia Salinella Orthonectid
Placozoan- the earliest animal? Their place in the evolutionary sequence Three current theories 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 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 - Considers shared characteristics -desmosomes -ventral gland cells Sponges Placozoa Epitheliozoa Eumetazoa Some molecular support
Placozoan- the earliest animal? Their place in the evolutionary sequence Three current theories 2. Eumetazoa Model - Molecular analysis Sponges Ctenophora Cnidaria Eumetazoa Placozoa Bilateria Problems: statistically weak and ignores structure
Sponge Structure - Specialized cells - No true tissues