yeast multicellularity journal summary
DESCRIPTION
A presentation to summarize my understanding towards the journal 'Experimental evolution of multicellularity'Journal article link:http://www.pnas.org/content/109/5/1595TRANSCRIPT
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Experimental evolution of multicellularityBy William C. Ratcliff, R. Ford Denison, Mark Borrello, and Michael Travisano
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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
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This ExperimentSaccharomyces cerevisiaeEvolution through gravity
selectionCluster formation mechanism
◦Snowflake phenotypeMulticellular Trait SelectionDivision of Labour
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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
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Formation of Snowflake Yeast
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Fitness comparison between condition with and without selection
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Mechanism of Cluster Formation
Increase in cell numberTheoretically
◦Aggregation of single cells◦Post-division adhesion
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Calcofluor staining stains yeast bud scar
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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
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Unicellular and Snowflake Yeast after Five Days Nitrogen Starvation
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Selection of Multicellular Traits
Daughter cells were produced as multicellular propagules
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Analysis of Cluster Size of Parent(At Reproduction) and Offspring
Most propagules start out functionally juvenile
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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
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Shift Showed after Selection
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Cluster Size Shift
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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
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Relative Growth Rate Against Settling Percentage in 7 Minutes
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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’
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Comparison between symmetric and asymmetric division
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Percentage of Apoptotic Cells
• frequency of apoptotic cells within snowflake cluster was highly correlated with settling rate after 60 transfers
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Large Cluster Size and Apoptosis
Larger Cluster Size
CausesFavours
Evolution of Apoptosis
or
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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
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Apoptosis against Cluster Area among Single Genotype Clusters
No Relationship
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Apoptosis against Cluster Size of Selfed Offsprings
No Relationship
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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
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Breaking of Propagules Manually
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Cluster Fragmentation Occurs between A Pair of Dead Cells
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Evolution and Division of Labour
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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
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Conclusionmulticellular traits readily evolve
as a consequence of among-cluster selection
Potential for multicellularity evolution omay be less constrained than is frequently postulated
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Thank You