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A presentation to summarize my understanding towards the journal 'Experimental evolution of multicellularity'Journal article link:http://www.pnas.org/content/109/5/1595
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Experimental evolution of multicellularityBy William C. Ratcliff, R. Ford Denison, Mark Borrello, and Michael Travisano
MulticellularityCo-ordinated multicellularity in
evolutionMulticellular organisms evolved
from unicellular ancestor◦But study was difficult because:
That happened more than 200 million years ago
Transitional form lost
This ExperimentSaccharomyces cerevisiaeEvolution through gravity
selectionCluster formation mechanism
◦Snowflake phenotypeMulticellular Trait SelectionDivision of Labour
Settling RateRapid increase of settling rate
over the course of selectionSnowflake phenotypeFitness advantage of snowflake
phenotypes over individual cells:◦Selective condition: 34%◦Without selective condition: 10% less
Formation of Snowflake Yeast
Fitness comparison between condition with and without selection
Mechanism of Cluster Formation
Increase in cell numberTheoretically
◦Aggregation of single cells◦Post-division adhesion
Calcofluor staining stains yeast bud scar
Snowflake PhenotypeStable
◦Transferred three replicates snowflake yeast population 35 times without gravitational selection, no unicellular strains invaded
Independent of pseudohyphal growth
Not mutation that made pseudohyphal morphology constitutive
Unicellular and Snowflake Yeast after Five Days Nitrogen Starvation
Selection of Multicellular Traits
Daughter cells were produced as multicellular propagules
Analysis of Cluster Size of Parent(At Reproduction) and Offspring
Most propagules start out functionally juvenile
Shift in SelectionDeterminate Growth
◦Clusters grew to a similar size and no moreEmergent multicellular trait
◦Delays propagule production until parental cluster is larger through longer juvenile phase
Selection acting on the reproduction and survival of individual clusters rather than on that of their component cells. ◦Due to change in multicellular phenotype
Shift Showed after Selection
Cluster Size Shift
Within-Cluster Division of LabourComplex multicellularity: Cell
differentiationObservation
◦ Apoptosis to increase propagules number at the expense of propagule size
Why?◦Rapid settling means slower growing
rate
Relative Growth Rate Against Settling Percentage in 7 Minutes
Production of Smaller Propagules as AdjustmentRational
◦Produce more◦Increase fecundity◦Grow faster
How?◦Asymmetric cluster division◦Propagules less than half of parent’s
biomass◦Apoptosis through ‘weak links’
Comparison between symmetric and asymmetric division
Percentage of Apoptotic Cells
• frequency of apoptotic cells within snowflake cluster was highly correlated with settling rate after 60 transfers
Large Cluster Size and Apoptosis
Larger Cluster Size
CausesFavours
Evolution of Apoptosis
or
Large Cluster Size and Apoptosis
Determine relationship among clusters of a single genotype◦No relationship
Determine if cluster size and apoptosis frequency are independently heritable◦No relationship
Large average cluster size and higher rates of apoptosis coevolved
Apoptosis against Cluster Area among Single Genotype Clusters
No Relationship
Apoptosis against Cluster Size of Selfed Offsprings
No Relationship
Location of Dead cellsTo determine if locations of
apoptosis are linked to propagule separation
Cell separation does not cause cell death
Cluster fragmentation occurs due to dead cells
Breaking of Propagules Manually
Cluster Fragmentation Occurs between A Pair of Dead Cells
Evolution and Division of Labour
ConclusionTransition from unicellularity to
multicellularity can evolve quite quickly under appropriate selective condition
Adaptation could be seen for increased settling rate selection ◦Lengthen juvenile phase◦Production of multicellular propagules
Evolution of division of labor
Conclusionmulticellular traits readily evolve
as a consequence of among-cluster selection
Potential for multicellularity evolution omay be less constrained than is frequently postulated
Thank You