To self or to clone? Southern European woodland strawberry genotypes self-fertilize, whereas eastern European genotypes clone in a pollinator-free common garden.

In a pollinator-free common garden, wild woodland strawberry (*Fragaria vesca*) genotypes exhibit a geographic trade-off in reproductive assurance strategies, with southern European populations favoring self-fertilization and eastern populations relying more on clonal reproduction, both associated with reduced petal size.

The Gist

Imagine a plant as a small business owner trying to keep their shop open. In the world of plants, the "shop" is reproduction, and the "customers" are pollinators (bees, flies, butterflies). Usually, these customers visit, pick up pollen, and deliver it to other plants, ensuring a diverse and healthy next generation.

But what happens when the customers stop coming? This is the "pollinator crisis" the paper talks about. The woodland strawberry (Fragaria vesca) has a brilliant backup plan: it can do two things to ensure it has offspring even when no one is visiting.

1. The "Self-Service" Option (Selfing): The plant can fertilize itself. It's like a business owner who decides to just sell to themselves. It's reliable, but the product might not be as high-quality or diverse.
2. The "Copy-Paste" Option (Cloning): The plant can send out runners (little baby plants on long stems) to grow right next to it. It's like a business opening a franchise branch right next door. It's a perfect copy, but it doesn't mix up the genetic "recipe."

The researchers wanted to know: If a strawberry plant can't get pollinators, does it choose to self-fertilize, clone itself, or try to do both? And does it matter where the plant is from?

The Big Discovery: You Can't Have It All (The Trade-Off)

The study found a fascinating "either/or" situation. It's like a plant has a limited budget of energy.

• The Southern/Coastal Strategy: Plants from Southern Europe (lower latitudes) and lower altitudes are the master self-fertilizers. They have evolved to be very good at fertilizing themselves. Their flowers are set up so the pollen-donating parts (anthers) are right next to the receiving parts (pistils), like a self-checkout kiosk that is perfectly aligned.

  • The Catch: Because they put so much energy into making these self-fertilized fruits (which are actually quite good quality), they have less energy left to send out runners. They are the "quality over quantity" plants.

• The Eastern Strategy: Plants from Eastern Europe are the master cloners. They are less efficient at self-fertilizing but are experts at sending out runners to clone themselves.

  • The Catch: They put their energy into making many copies of themselves rather than perfecting the self-fertilization process.

The Metaphor: Think of it like a car. Some cars are built to be incredibly fuel-efficient (Selfing) but have a small trunk (few runners). Other cars have huge trunks (many runners) but guzzle gas (poor selfing). The strawberry plants have to choose which engine to build based on their location.

Why Does This Happen? (The "Why" Behind the "Where")

The researchers set up a giant garden in Sweden, took 121 different strawberry plants from all over Europe, and put them in bags to block out all the bees and flies. They wanted to see how the plants behaved when forced to rely on their backup plans.

Here is what they found about the geography:

• North vs. South (Latitude): As you go further North (towards the Arctic), the plants get worse at self-fertilizing. Instead, they grow bigger petals.
  • Analogy: Imagine you are in a cold, dark room where it's hard to find people. Instead of trying to talk to yourself, you put on a giant, bright neon sign (big petals) to scream, "Hey, look at me! Come talk to me!" The plants in the North are trying harder to attract the few pollinators that might show up.
• West vs. East (Longitude): As you go East, the plants get better at cloning.
  • Analogy: In the East, the "customers" (pollinators) might be scarce or the environment might be different, so the plants decide, "Forget trying to attract customers; I'll just open more branches of my store right here."

The "Flower Size" Twist

One of the coolest findings is about flower size.

• Southern plants (good at selfing) have small petals. Why? Because they don't need to attract anyone! They are the "self-service" stores. They don't need a flashy sign.
• Northern plants (bad at selfing) have large petals. They are the "high-traffic" stores that need a massive billboard to get customers.

The Bottom Line

This paper tells us that plants are incredibly smart and adaptable. When the "pollinator economy" crashes, they don't just panic; they have different strategies depending on their history and location.

• Southern strawberries say: "I'll handle this myself. I'm efficient and can do it alone."
• Eastern strawberries say: "I'll just multiply. I'll make more of me to take over."
• Northern strawberries say: "I'll try to make myself look extra pretty to catch the eye of any passing pollinator."

The study proves that nature doesn't have a "one-size-fits-all" solution. Instead, it has a toolbox, and different plants pick different tools based on where they live, ensuring that even in a world with fewer bees, the strawberry population survives.

Technical Summary

1. Problem Statement and Premise

In the context of the global pollinator crisis, plants increasingly rely on reproductive assurance mechanisms to ensure seed set when pollinators are absent. The two primary modes of reproductive assurance are autonomous self-fertilization (selfing) and clonal propagation (vegetative reproduction). While many plants possess the capacity for both, the evolutionary trade-offs and interplay between these two strategies remain poorly understood.

The study addresses three specific gaps:

1. The Trade-off: Is there a resource or developmental trade-off between the capacity to self-fertilize and the propensity to clone?
2. Mechanisms of Selfing: What specific floral traits (e.g., herkogamy) or genetic factors (e.g., inbreeding depression) determine the capacity for autonomous selfing?
3. Geographic Drivers: Do these reproductive strategies vary along large-scale environmental gradients (latitude, longitude, altitude) that correlate with pollinator reliability?

The study focuses on the woodland strawberry (Fragaria vesca subsp. vesca), a self-compatible perennial capable of both sexual and asexual reproduction, sampled across its European distribution range.

2. Methodology

The researchers employed a common garden experiment designed to isolate reproductive strategies from pollinator influence.

• Study Material: 121 distinct genotypes of F. vesca were collected from across Europe (latitudes 37.2°–70.18° N, longitudes -23.18°–26.07° E, altitudes 0–2275 m).
• Experimental Setup:
  • Plants were grown in an outdoor common garden in Alnarp, Sweden (2020).
  • Pollinator Exclusion: Each plant was covered with organza mesh bags to strictly exclude pollinators, forcing reliance on autonomous selfing or cloning.
  • Replication: 121 genotypes were clonally replicated (2–5 ramets per genotype, totaling 363 ramets) and randomly assigned to nine blocks.
• Data Collection:
  • Reproductive Traits: Counted total runners (cloning propensity) and flowers. Measured fruit size and the proportion of fertilized achenes (seeds) in autonomous fruits as a proxy for selfing capacity.
  • Floral Morphology: Measured lateral and vertical herkogamy (distance between anthers and pistils) and petal size.
  • Hand-Pollination Treatments: A subset of 34 genotypes underwent controlled hand-pollinations (autonomous, self-cross, out-cross, and emasculation) to calculate Relative Performance (RP) and estimate early inbreeding depression.
• Statistical Analysis:
  • Used Linear Mixed Models (LMM) and Generalized Linear Mixed Models (GLMM) with negative binomial distributions for count data.
  • Genotype and block were included as random effects.
  • Geographic coordinates (latitude, longitude, altitude) were tested as predictors for reproductive traits.

3. Key Contributions and Findings

A. Trade-off Between Selfing and Cloning

• Negative Correlation: A significant trade-off exists between the capacity for autonomous selfing and the propensity to clone. Genotypes with a higher proportion of fertilized achenes (better selfers) produced significantly fewer runners.
• Driver of Trade-off: The trade-off is driven by fruit provisioning (resource allocation to seed/fruit development) rather than flower production. Higher achene set correlated with larger fruit size, suggesting a resource competition between developing high-quality autonomous fruits and producing vegetative runners.
• Flower Number: Flower production did not show a trade-off with either cloning or selfing capacity.

B. Mechanisms Determining Selfing Capacity

• Lateral Herkogamy: The capacity to autonomously self-fertilize was strongly determined by lateral herkogamy (horizontal proximity of anthers to stigmas). Reduced lateral distance significantly increased the proportion of fertilized achenes.
• Vertical Herkogamy: Vertical distance between anthers and stigmas had no significant effect on autonomous selfing success.
• Inbreeding Depression: Early inbreeding depression (measured via Relative Performance) was low (mean 0.18) and did not limit the capacity for autonomous selfing across genotypes. This suggests that the variation in selfing success is morphological, not genetic (i.e., not due to embryo abortion).

C. Large-Scale Geographic Patterns

The study revealed distinct spatial gradients for the two reproductive assurance strategies:

• Latitudinal/Altitudinal Gradient (Selfing): The capacity to self-fertilize decreased with increasing latitude and altitude. Genotypes from Southern Europe and lower altitudes were better at autonomous selfing.
• Longitudinal Gradient (Cloning): The propensity to clone increased towards the East. Eastern European genotypes produced more runners.
• Floral Display (Pollinator Attraction):
  • Petal size (a proxy for pollinator attraction) was largest in the Northwest and smallest in the Southwest.
  • There was a significant interaction between latitude and longitude: petal size increased strongly towards the west in northern populations but showed a slight increase towards the east in southern populations.
  • Flower number was higher in genotypes from lower elevations.

4. Significance and Implications

• Reproductive Assurance Specialization: The study demonstrates that F. vesca populations are not uniformly capable of all reproductive modes. Instead, there is a continental-scale divergence where Southern populations specialize in selfing and Eastern populations specialize in cloning.
• Compensatory Strategies: The results suggest that these two modes of reproductive assurance may compensate for reduced pollinator attractiveness (smaller petals) in specific regions. For instance, Eastern genotypes with smaller petals rely more on cloning, while Southern genotypes with smaller petals rely more on selfing.
• Evolutionary Drivers: The findings challenge the simple "reproductive assurance hypothesis" which predicts increased selfing/cloning in harsher (high latitude/altitude) environments. Instead, the data suggests that in harsh northern environments, selection may favor outcrossing traits (larger petals) to attract scarce pollinators, while selfing capacity declines.
• Morphological vs. Genetic Constraints: The study highlights that lateral herkogamy is a critical, often overlooked, morphological trait driving autonomous selfing, whereas inbreeding depression was not a limiting factor in this system.

Conclusion

This research provides a comprehensive view of how a single species partitions reproductive assurance strategies across its range. It identifies a clear trade-off between selfing and cloning driven by resource allocation to fruit development, mediated by specific floral morphologies (lateral herkogamy). Crucially, it reveals that these strategies are geographically structured, with selfing dominating the south/low-altitude gradient and cloning dominating the east, likely as adaptive responses to varying pollinator landscapes and historical glacial refugia.