week 3 - multicellularity Flashcards
evolution of life
overview
First billion years only heterotrophic bacteria that were floating in the ocean
Only about 2.5 billion years ago the first photosynthetic the first autotrophic bacteria appear
(in the fossil record)
First organisms that evolved were the simple consumers
* Easier to consume than make own food
Heterotrophs and autotrophs living in the ocean
* With the heterotrophs making the food and the autotrophs consuming it
Unicellular eukaryotic cells evolved ~1 billion years later
Multicellular organisms only about 1 billion years ago
All of the organisms evolved from the ocean
* Because the ocean provided an environment that was both nourishing and protective
Land was uninhabitable (UV too harsh)
* Huge impact of plants
o Through photosynthesis autotrophic bacteria (photosynthetic multicellular organisms) were making and releasing oxygen
o This led to the formation of the ozone
o 500 million years ago (plant and animals can survive on land)
evolution of multicellularity
- Multicellularity is present in (nearly) all eukaryotic groups
- Some organisms are multicellular only some of the time
o This tells us there is some advantage to being multicellular - Some prokaryotes are also multicellular
multicellular bacteria:
- Cyanobacterium
- Enterobacterium
- Myxobacterium
which Multicellular eukaryotes form fruiting bodies.
- Dictyostelium
multicellularity and altruism
- Some cells die (stalk), others become the next generation (spores)
multicellularity summary
Multicellularity is prevalent…
… in all kingdoms of life
multicellularity summary
Multicellularity has arisen…
independently multiple times
multicellularity summary
In some multicellular organisms not all cells…
go on to reproduce (altruism)
multicellularity summary
Some organisms are multicellular only…
under certain conditions
what is multicellularity?
- Adhesion: cells stick together
- Communication: cells communication with one another (in addition to responding to external signals)
- Differentiation: cells within the organisms had specialized functions
- Dependency: cells are dependent on one another survival (perhaps not so much in colonial multicellular organisms/biofilms)
molecular tool kits: how to become multicellular
cell adhesion: cell stick together
- linked to cytoskeleton
Cell signalling molecules enables cells to talk to each other + interact
This enables differentiation (signalling) and dependency (e.g. adhesion and secretion of an extracellular matrix; survival signals)
molecular tool kits:
Genome comparisons: building blocks are present in unicellular relatives of multicellular organisms
- Genome of unicellular alga or animal relative or fungal cell or amoeba encodes all these proteins
- They are used for “unicell function’
- Adhesion to a substratum environmental sensing
- Multicellular alga, plant, animals, fungua or social amoeba encodes all these proteins
- Possibly in larger protein families than unicells
- They function in a multicellular context
o Cell-cell adhesion and communication - In addition to the unicellular context
Which genes are turned on and off, and when this happens may be key as may regulation of protein activity
is multicellularity being dictated by the genome sequence?
by in large NO
Genomes are similar
Just on a different scale
Certain genes switched on/off
This regulates genes
what are Choanoflagellates:
unicellular relatives of animals
- Tiny water dwelling organisms with flagella
- Resemble collar cells (for feeding) in multicellular animals
- Choanoflagellates can be unicellular or form colonies (multicellular)
Choanoflagellates can help is understand the origins of multicellular animals
- Genome sequencing confirms that choanoflagellates are the closest living relatives of metazoans (the multicellular animals)
- Bacterial signals (sulfonolipids) promote multicellular development in choanoflagellates.
Environmental stimulus triggers multicellularity
plant multicellularity
- All land plants have a single common ancestor (we don’t know what or if unicellular or multicellular)
- Algae (freshwater and marine) can be unicellular or multicellular or both
Freshwater algae: different degrees of multicellularity despite similar genomes
can Chlamydomonas become multicellular under the right conditions
yes
- Experimental evolution of multicellularity in Chlamydomonas over many generations
- Multicellular Chlamydomonas can be selected via settlement
- Chlamydomonas also becomes multicellular (stable population) in response to predation by Paramecium
- Abiotic stressors (e.g. salt stress) can also promote multicellularity.
what controls multicellular development in the green seaweed (marine alga) Ulva
signals from two different bacteria
- Ulva multicellular development requires bacteria (symbiosis)
- The Ulva-bacterial adult organism is know as a ‘holobiont’
when did multicellularity evolve
Complex life on land (increasing oxygen in atmosphere)
Or did it?
- New fossil find of complex calcified structures pushes back putative animal multicellulairy to 890 million years ago
- Did sponges survive iin lower oxygen by living right next to photosynthetic cyanobacteria and feeding off bacterial goo?
- Or was there more oxygen than we thought?
- Is the fossil really a sponge?
multicellularity summary
Being multicellular requires…
adhesion, communication, differentiation and dependency
multicellularity summary
Many organisms possess…
molecular toolkits for multicellularity including unicellular organisms
multicellularity summary
Choanoflagellates and green algae can give clues as to…
the origins of animal and plant multicellularity respectively
multicellularity summary
The multicellular state is ancient but…
a patchy fossil records cannot give us all the answers
Molecular studies on living organisms help us to infer more about origins and evolution
