Constraints on the G1/S transition pathway may favor selection of multicellularity as a passenger phenotype

This study demonstrates that in yeast, simple multicellularity can be maintained as a "passenger" phenotype not because it offers a direct fitness advantage, but because the underlying *ace2* genotype confers a selective benefit—specifically faster exit from quiescence—when combined with specific constraints on the G1/S cell cycle transition.

The Gist

The Big Question: Why Do Cells Stick Together?

Imagine a bustling city of single-celled yeast. Usually, when these cells divide, they split apart immediately, like two people shaking hands and walking in opposite directions. But sometimes, due to a genetic glitch, they forget to let go. They stay attached, forming little clusters or "snowflakes."

For a long time, scientists thought these clusters only formed because they were useful—maybe they helped the yeast survive predators or find food better. But this paper asks a different question: What if the yeast didn't stick together because it was helpful, but because they were accidentally dragged along for the ride?

The authors call this a "Passenger Phenotype." Think of it like a child holding onto a parent's hand. The child isn't running faster than the parent; they are just along for the ride because they are attached.

The Experiment: The "Sleep and Wake" Test

To figure this out, the scientists used a specific type of yeast mutation called ace2. This mutation makes the yeast form snowflakes (clusters). They mixed these "snowflake" yeast with normal "planktonic" (single-cell) yeast and watched them compete.

The Setup:
Imagine a race track where the runners (yeast) have to:

1. Run a lap (grow and divide).
2. Stop and sleep for a while (enter a dormant state called quiescence).
3. Wake up and start running again immediately when food is brought back.

The Discovery:

• Normal Conditions: When the race was easy, the snowflake yeast and the single-cell yeast were equally good. Neither won.
• The Hard Mode: The scientists then introduced a "speed bump" in the yeast's internal clock (by removing a gene called CLN3). This made the yeast very slow to wake up from their sleep.
• The Twist: In this "Hard Mode," the snowflake yeast suddenly won the race. They woke up and started dividing much faster than the single-cell yeast.

The Real Reason: It's Not About the Clump

Here is the most surprising part. The scientists wanted to know: Did they win because they were stuck together in a clump?

To test this, they did two things:

1. The "Fake Clump": They created yeast that stuck together (like snowflakes) but had the normal internal clock genes. These fake clumps still lost. They were slow to wake up.
2. The "Broken Clump": They took the winning snowflake yeast and physically shook them apart (sonication) so they were single cells again. Even as single cells, they still woke up faster than the normal yeast.

The Analogy:
Imagine two cars stuck in traffic. One car has a broken engine (the CLN3 mutation) and can't start. The other car has a special "turbo button" (the ace2 mutation) that helps it start faster.

• The turbo car happens to be towing a trailer (the snowflake cluster).
• The scientists thought the trailer helped the car go faster.
• But they realized: The trailer does nothing. The car wins because of the turbo button. The trailer is just a "passenger" that happens to be attached.

How Does the "Turbo Button" Work?

The scientists dug deeper to find the mechanism. They found that the ace2 mutation does two things:

1. It stops the cells from separating (creating the snowflake).
2. It accidentally turns up the volume on a different gene called KSS1, which helps the cell wake up faster.

So, the yeast didn't evolve to be multicellular. Instead, a mutation happened that made the yeast wake up faster (a huge advantage when food is scarce). Because that same mutation also made the cells stick together, the multicellular trait survived.

The "Wild" Connection

The paper also looked at yeast found in the wild (not in labs). They found a natural version of this "turbo button" gene (AMN1) in wild yeast. These wild yeast also form clusters and wake up faster. This proves that this isn't just a lab accident; it's a real evolutionary strategy that nature uses.

The Big Takeaway

This paper suggests that multicellularity (living in groups) might not always be the "goal" of evolution.

Sometimes, a species evolves a trait to solve a specific problem (like waking up faster from sleep). If that solution accidentally causes the cells to stick together, the group living style persists. It's not because the group is better; it's because the "driver" (the gene that wakes them up) is so good that it drags the "passenger" (the multicellular clump) along for the ride.

In short: Multicellularity might have started not as a superpower, but as a happy accident that got selected because it came bundled with a real superpower.

Technical Summary

1. Problem Statement

The evolution of multicellularity has occurred independently across the tree of life. While the initial formation of simple multicellular clusters (e.g., via aggregation or failure of cell separation) is well-documented, the mechanisms by which these clusters are maintained and selected for in the absence of direct selective advantages (like predation avoidance or resource access) remain unclear.

The authors investigate the hypothesis that multicellularity might not always be the direct target of selection. Instead, it could persist as a "passenger phenotype"—a side effect of selection acting on a different, linked trait. Specifically, they explore the genetic interplay between the G1/S cell cycle transition (regulated by Cyclin D/CLN3 and Retinoblastoma/Whi5) and the multicellular "snowflake" phenotype in Saccharomyces cerevisiae, caused by the deletion of the transcription factor ACE2.

2. Methodology

The study utilized the yeast snowflake model, where ace2 deletion prevents cell separation after cytokinesis, forming multicellular clusters. The researchers employed a combination of competitive fitness assays, physiological measurements, and genetic dissection:

• Competition Assays: Isogenic strains (wild-type ACE2 vs. mutant ace2) were co-cultured in a 50:50 ratio. These competitions were performed under different genetic backgrounds:
  • Wild-type (CLN3, WHI5).
  • cln3 deletion (lengthens G1 phase).
  • WHI5 overexpression (mimics cln3 defect).
  • Populations were subjected to repeated cycles of growth to stationary phase and dilution (simulating boom-bust ecological cycles) or maintained in constant exponential growth.
• Fitness Component Analysis: To identify the source of the fitness advantage, the authors measured:
  • Population doubling time (exponential growth rate).
  • Biomass yield (carbon utilization).
  • Cell survival in stationary phase (viability).
  • Quiescence exit efficiency: The time required for cells to re-enter the cell cycle (lag phase) after refeeding from stationary phase.
• Phenotype-Genotype Decoupling:
  • Quintuple Mutant: Created a strain with deletions in five cell separation genes (cts1, dse2, dse4, egt2, scw11) to mimic the snowflake phenotype without the ace2 mutation.
  • Sonication: Physically broke ace2 snowflakes into single cells to test if the multicellular structure itself was required for the fitness advantage.
• Molecular Mechanism: Investigated the regulatory network involving ACE2, KSS1 (a MAP kinase), and CLN1 (a G1 cyclin) using RT-qPCR and genetic knockouts.
• Natural Variation: Tested the AMN1^368D allele found in wild/non-laboratory yeast strains, which induces Ace2 degradation, to see if it phenocopies the ace2 deletion.

3. Key Results

A. Conditional Selection of Multicellularity

• In a wild-type background (CLN3), the ace2 snowflake phenotype showed no fitness advantage or disadvantage compared to planktonic cells over 100 generations.
• However, in a cln3 deletion background (or WHI5 overexpression), the ace2 snowflake strain was strongly selected and rapidly took over the population.
• This selection was dependent on the G1/S transition constraints; it did not occur when cells were kept in constant exponential growth, indicating the advantage is linked to the transition from quiescence to proliferation.

B. Quiescence Exit is the Critical Factor

• The fitness advantage was not due to faster doubling times, higher biomass yield, or better survival in stationary phase.
• The advantage was entirely driven by a faster exit from quiescence. ace2 cln3 cells resumed proliferation significantly faster than ACE2 cln3 cells after refeeding.
• This phenotype was robust in both diploid and haploid strains and was independent of the size of the snowflake cluster.

C. The Advantage is Genotypic, Not Phenotypic

• Multicellularity is not the driver: The quintuple mutant (which forms snowflakes but retains ACE2) did not show the quiescence exit advantage or competitive fitness in a cln3 background.
• Genotype is the driver: When ace2 snowflakes were sonicated into single cells, they retained the rapid quiescence exit advantage.
• Conclusion: The ace2 genotype confers the advantage, not the multicellular phenotype.

D. Molecular Mechanism: The Kss1-CLN1 Axis

• ACE2 normally represses KSS1 (a MAP kinase). In ace2 mutants, KSS1 expression increases.
• KSS1 positively regulates CLN1 (a G1 cyclin partially redundant with CLN3).
• In ace2 cln3 mutants, the upregulation of KSS1 leads to higher CLN1 expression, which compensates for the lack of CLN3, allowing faster G1/S transition and quiescence exit.
• Crucial Proof: Deleting KSS1 in the ace2 cln3 background abolished the fitness advantage and the rapid quiescence exit.

E. Physiological Relevance (Wild Strains)

• The AMN1^368D allele, found in wild yeast strains, causes proteasomal degradation of Ace2.
• Strains carrying AMN1^368D phenocopied the ace2 deletion: they formed snowflakes and exhibited the same rapid quiescence exit and competitive advantage in cln3 or WHI5-overexpressing backgrounds.

4. Significance and Conclusion

The "Passenger Phenotype" Hypothesis:
The study provides a mechanistic proof-of-concept that simple multicellularity can evolve and persist not because the multicellular state itself is advantageous, but because it is genetically linked to a beneficial trait.

1. Selective Pressure: Environmental conditions (e.g., nutrient fluctuations) select for genotypes that exit quiescence rapidly (e.g., via AMN1^368D or ace2 mutations) to outcompete others in boom-bust cycles.
2. Pleiotropy: The selected genotype (ace2 loss or AMN1 mutation) has pleiotropic effects. While the selection acts on the cell cycle regulator (Kss1/CLN1), the transcription factor Ace2 also controls cell separation genes.
3. Passenger Effect: Consequently, the multicellular "snowflake" phenotype is selected as a passenger, hitchhiking alongside the beneficial cell cycle adaptation.

Evolutionary Implications:

• This challenges the view that multicellularity always requires direct selective advantages (like predation avoidance) to emerge.
• It suggests that the evolution of cell cycle regulation (specifically G1/S transition) and multicellularity may be deeply intertwined through co-option and pleiotropy.
• The findings explain how multicellularity could be maintained in nature even if it carries a cost, provided it is linked to a trait under strong positive selection.

Summary Model:
In non-laboratory strains, the AMN1^368D allele degrades Ace2. This loss of Ace2:

1. Upregulates Kss1 $\rightarrow$ Upregulates CLN1 $\rightarrow$ Faster Quiescence Exit (Selected Trait).
2. Downregulates Chitinases $\rightarrow$ Cell Separation Failure $\rightarrow$ Snowflake Formation (Passenger Phenotype).

This work highlights that evolutionary trajectories can be driven by constraints in fundamental cellular processes (like the cell cycle), with complex traits like multicellularity emerging as secondary consequences.