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