A history of symbiosis impacts the host evolutionary trajectory in experimentally evolved amoebas

This study demonstrates that the evolutionary history of symbiosis with specific *Paraburkholderia* bacteria creates path dependence in *Dictyostelium discoideum* hosts, influencing their subsequent evolutionary trajectories even after the symbionts are removed.

The Gist

The Big Idea: "You Can't Just Un-Read a Book"

Imagine you are a student who has spent years studying a very specific, difficult subject with a brilliant but demanding tutor. You learn how to think, how to solve problems, and how to work in a way that fits perfectly with that tutor's style.

Now, imagine the tutor suddenly leaves. You are on your own. The question this paper asks is: Does your history with that tutor change how you learn and grow from that point on, even though they are gone?

Most people might think, "If the tutor is gone, I'll just go back to how I was before." But this study shows that your past relationship changes your future path. Even after the partner is gone, the "ghost" of that relationship still shapes how you evolve.

The Characters in Our Story

• The Host (The Student): A tiny, single-celled organism called Dictyostelium discoideum (a social amoeba). It's like a microscopic ant that can live alone or join a team to build a mushroom-like structure.
• The Symbionts (The Tutors): Three different types of bacteria (Paraburkholderia) that live inside the amoeba.
  • Bacteria A (P. agricolaris): The "casual tutor." It has a full toolkit of skills and doesn't rely heavily on the amoeba. It's like a tutor who shows up occasionally but has their own car and lunch.
  • Bacteria B & C (P. hayleyella & P. bonniea): The "obsessive tutors." Over time, they have lost most of their own tools (their genomes are tiny) and rely entirely on the amoeba for food and survival. They are like tutors who have moved into your house and depend on you for everything.

The Experiment: The "Breakup"

Scientists took these amoebas and their bacterial roommates and put them in a lab. They used antibiotics to kick the bacteria out (a "breakup"). Then, they let the amoebas evolve for hundreds of generations in a clean environment, completely alone.

They wanted to see: Do the amoebas that used to have roommates change differently than amoebas that never had roommates at all?

What They Found: The "Path Dependence"

The researchers looked at three main things the amoebas did:

1. How fast they multiplied (Cell Proliferation).
2. How far they could travel (Slug Migration).
3. How many babies (spores) they could make (Spore Production).

Here is the surprising result:

1. The "Casual" Roommate Left a Small Mark
The amoebas that used to host the "casual tutor" (P. agricolaris) changed a little bit, but they mostly evolved in a direction similar to the amoebas that never had a roommate. It was like a student who had a casual tutor; when the tutor left, the student just went back to their normal study habits.

2. The "Obsessive" Roommates Left a Giant Mark
The amoebas that used to host the "obsessive tutors" (P. hayleyella and P. bonniea) went down completely different evolutionary paths.

• Some started multiplying faster but stopped traveling as far.
• Others started making more babies but changed how they moved.
• Crucially, the two types of "obsessive" bacteria caused the amoebas to evolve in opposite directions, even though the bacteria were very similar to each other.

The Metaphor: The "Scars" of Symbiosis

Think of the symbiotic relationship like a dance.

• When the amoeba and the bacteria danced together for a long time, they learned specific steps. They adjusted their rhythm to match each other.
• When the music stopped (the bacteria were removed), the amoeba didn't just stop dancing. It kept trying to do the steps it learned, but now it had to do them alone.
• Because the "dance" with the "obsessive" bacteria was so intense and long-lasting, the amoeba's body and behavior were permanently rewired. It couldn't just snap back to its old self.

Why This Matters

This study proves a concept called Historical Contingency. It means that history matters. You can't just erase the past.

• For Biology: It shows that even if a partnership ends, the history of that partnership leaves a permanent mark on how a species evolves. It's not just about what you have now, but who you used to be with.
• For Life in General: It suggests that our past relationships (whether with people, environments, or other species) shape our future potential in ways we can't always predict. Just because a partner leaves doesn't mean the relationship didn't change you.

The Takeaway

The paper tells us that evolution is a story, not just a snapshot. The chapters you write with your partners stay in the book, even after those partners leave the story. The amoebas that lost their bacteria didn't just return to "normal"; they became new versions of themselves, shaped by the ghosts of their past roommates.

Technical Summary

1. Problem Statement

The study addresses a critical gap in evolutionary biology regarding historical contingency in host-microbe symbioses. While it is well-established that symbiosis shapes current fitness and evolution, it remains unclear whether the history of a symbiotic relationship influences the host's evolutionary trajectory even after the symbiont is lost. Specifically, the authors investigate whether the specific species of a bacterial symbiont previously harbored by a host dictates how the host evolves in a symbiont-free environment. This tests the hypothesis of "path dependence," where past interactions create lasting evolutionary constraints or directions that persist beyond the dissolution of the partnership.

2. Methodology

The researchers utilized a combination of experimental evolution and advanced statistical modeling on the social amoeba Dictyostelium discoideum and its facultative endosymbionts from the genus Paraburkholderia.

• Study System & Dataset:

  • Host: Dictyostelium discoideum (social amoeba).
  • Symbionts: Three distinct bacterial species: P. agricolaris (full genome, less dependent), P. hayleyella, and P. bonniea (both have highly reduced genomes, indicating long-term co-evolution and higher dependency).
  • Data Source: Phenotypic data from a previous experimental evolution study (Larsen et al., 2023). Wild-collected strains were cured of their native symbionts using antibiotics and then evolved in the laboratory for ~200–300 cellular generations in the absence of bacteria.
  • Groups: Strains were categorized into four groups based on symbiont history:
    1. Non-symbiotic (never carried symbionts).
    2. Cured of P. agricolaris.
    3. Cured of P. bonniea.
    4. Cured of P. hayleyella.

• Phenotypic Traits Measured:
  Three key life-history traits representing fitness components were measured in both ancestral and evolved lines:

  1. Cell Proliferation: Vegetative growth rate (log-transformed cell count).
  2. Slug Migration: Distance traveled toward light (cm).
  3. Spore Production: Reproductive output (log-transformed spore count).

• Statistical Analysis:

  • Univariate Analysis: Linear mixed-effect models (LMMs) and robust LMMs were fitted to test for changes in individual traits. Models included fixed effects for Treatment (Ancestor vs. Evolved), Symbiosis History (Yes/No or specific species), and their interactions. Post-hoc pairwise contrasts (FDR-adjusted) and Cohen's d effect sizes were calculated.
  • Multivariate Analysis:
    • Principal Component Analysis (PCA): To visualize evolutionary trajectories in multidimensional trait space.
    • MANOVA & PERMANOVA: To test for significant differences in multivariate trait vectors between groups, accounting for strain-level variation and correlations between traits.
    • Vector Analysis: Evolutionary shifts were quantified as vectors from ancestral to evolved centroids, measuring angular deviations and magnitudes of change.

3. Key Contributions

• Demonstration of Path Dependence: The study provides empirical evidence that the specific identity of a lost symbiont dictates the direction and magnitude of subsequent host evolution, even when the host is reared in identical symbiont-free conditions.
• Differentiation by Symbiont Genomics: The research links the evolutionary response to the genomic nature of the symbiont. Hosts that lost genome-reduced, highly dependent symbionts (P. bonniea, P. hayleyella) showed distinct evolutionary trajectories compared to those that lost the less dependent P. agricolaris or non-symbiotic strains.
• Multivariate Evolutionary Shifts: The study highlights that evolutionary adaptation to symbiosis loss is not merely a collection of isolated trait changes but a coordinated shift across multiple functional axes (e.g., trade-offs between reproduction and dispersal).

4. Key Results

• Univariate Findings:

  • General Effect: Strains with a history of symbiosis showed significant evolutionary changes in cell proliferation, slug migration, and spore production after symbiont loss, whereas non-symbiotic strains showed no significant changes.
  • Symbiont-Specific Effects:
    • Cell Proliferation: Significantly increased in lines evolved without P. agricolaris and P. hayleyella.
    • Slug Migration: Significantly decreased in lines evolved without P. bonniea.
    • Spore Production: Significantly increased in lines evolved without P. hayleyella.
  • Interaction: While the overall difference between "symbiotic" and "non-symbiotic" groups was not always statistically significant in a pooled analysis, the interaction between Treatment and Specific Symbiont Species was significant for cell proliferation, indicating that the evolutionary response depends on which symbiont was lost.

• Multivariate Findings:

  • PCA Results: The first two principal components (PC1 and PC2) captured ~71% of the variation. PC1 represented a general response to evolution, while PC2 highlighted a trade-off between spore production and migration.
  • Trajectory Divergence:
    • Hosts that lost P. agricolaris evolved in a direction similar to non-symbiotic strains but with a larger magnitude of change.
    • Hosts that lost P. bonniea and P. hayleyella (the reduced-genome species) exhibited the largest magnitude of evolutionary change but moved in divergent directions from each other and from non-symbiotic strains.
  • Statistical Significance: MANOVA and PERMANOVA confirmed significant effects of Treatment, Symbiont History, and their interaction ($p < 0.05$), proving that evolutionary trajectories are contingent on the specific symbiont history.

5. Significance and Implications

• Historical Contingency in Symbiosis: The study challenges the view that hosts simply "reset" to an optimal phenotype upon symbiont loss. Instead, it suggests that symbiotic history creates a "path-dependent" evolutionary landscape.
• Genome Reduction and Evolutionary Legacy: The distinct trajectories of hosts losing reduced-genome symbionts (P. bonniea, P. hayleyella) versus the full-genome P. agricolaris suggest that long-term, intimate co-evolution (indicated by genome reduction) leaves a deeper "evolutionary scar" or legacy on the host's physiology.
• Mechanism of Dependence: The results imply that even facultative symbioses can lead to "evolutionary addiction" or lock-in effects, where the host's developmental and physiological pathways become rewired to accommodate the symbiont. When the symbiont is removed, the host does not revert to a neutral state but evolves along a new, constrained path determined by its past.
• Broader Ecological Context: These findings have implications for understanding ecosystem resilience, such as coral bleaching or agricultural microbiome management, suggesting that the recovery or evolution of a host after partner loss is heavily influenced by the specific history of that partnership.