Asymmetric reproductive character displacement and female polymorphism in Ischnura damselflies

This study demonstrates that asymmetric reproductive character displacement in *Ischnura* damselflies, driven by reinforcement in hybrid zones, results in morph-specific divergence in prothorax shape and size, particularly affecting gynochrome females and males in *I. elegans*.

The Gist

The Big Picture: A Case of "Wrong Door" Keys

Imagine two neighboring towns, Town A (Ischnura elegans) and Town B (Ischnura graellsii). For a long time, they lived far apart. But recently, they moved closer, and now their borders overlap. In this border region, people from Town A and Town B sometimes try to date each other.

The problem? They aren't compatible. If they try to mate, it's like trying to fit a square peg into a round hole—it doesn't work well, and the "offspring" (hybrids) are often weak or sterile. To stop this from wasting time and energy, nature has started a process called Reinforcement. It's like the towns updating their locks and keys so that people only date within their own town.

This study looks at how this "lock and key" system works in damselflies, specifically focusing on a fascinating twist: The women in these towns come in different "costumes."

The Characters: The Costumes

In these damselfly species, the females have two main "costumes" (morphs):

1. The Gynochromes: These are the "traditional" ladies. They wear colorful, distinct outfits that make them easy to spot.
2. The Androchromes: These are the "imposters." They dress up exactly like the men (the males). They wear the same blue/green colors and look very similar to the guys.

Why do they do this?
It's a survival trick. The men in the town are very aggressive and try to mate with everyone. The "imposter" ladies (Androchromes) trick the men into thinking, "Oh, that's just another guy," so they leave them alone. This gives the imposters a break from constant harassment.

The Mystery: Who is Changing the Locks?

When Town A and Town B started overlapping, the "locks" (the physical shapes of their bodies) needed to change to prevent accidental mating. The researchers wanted to know:

• Are the "imposter" ladies changing their shape to avoid the wrong town's men?
• Are the "traditional" ladies changing?
• Are the men changing their "keys" (their grasping tools) to fit only their own town's women?

The Findings: A Tale of Two Towns

The researchers used high-tech 3D scanning (like a digital caliper) to measure the shape and size of the damselflies' "grasping area" (the prothorax, which is like a collar behind the head).

1. The "Imposter" Strategy Works (Mostly)

In Town A (I. elegans), the "imposter" ladies (Androchromes) look almost identical to the men. Their "collars" are the same shape and size. This confirms that their disguise is very effective physically.

• The Twist: In the border zone, the "traditional" ladies (Gynochromes) and the "imposters" started to look more similar to each other. It's like the border town is so mixed up that the distinct costumes are blurring together.

2. The "Lock" Change is One-Sided

Here is the most surprising part. The researchers found that the "locks" were changing, but only for the "traditional" ladies in one specific border town.

• Town A (Gynochromes): In one border zone (the NC zone), the traditional ladies changed the shape and size of their collars to make it even harder for the wrong-town men to grab them.
• Town A (Androchromes): The "imposters" didn't need to change much. Because they already look like men, the men from the wrong town couldn't grab them easily in the first place. They were already "safe."
• Town B: The ladies in the second town didn't change their locks at all. They stayed the same.

The Analogy: Imagine Town A's traditional ladies realized, "Hey, the guys from Town B are trying to date us!" So, they changed their door locks (evolved a new shape). But the "imposters" in Town A were already wearing a disguise that made the Town B guys think, "That's a guy, not a lady," so they never tried to open the door. The imposters didn't need to change their locks because they were already safe.

3. The Men are Changing Too

The men in Town A (specifically in the NC border zone) also changed the shape of their "keys" (their tail appendages used to grab the ladies). This suggests that as the ladies changed their locks, the men had to evolve new keys to keep holding onto their own partners.

The "Lock and Key" Connection

The study also checked if the men's "keys" (tail) and their own "collars" (prothorax) were related. They found a link: Size matters.
If a man has a bigger tail, he tends to have a bigger collar. It's like a suit and tie; they grow together. This suggests that as the men evolve better tools to grab the right ladies, their whole body structure shifts slightly to match.

The Bottom Line

This paper tells us that evolution isn't a one-size-fits-all process.

• Disguise is powerful: The "imposter" ladies (Androchromes) are naturally protected from the wrong suitors because they look like the guys. They don't need to evolve new defenses.
• The vulnerable change: The "traditional" ladies (Gynochromes) are the ones getting harassed the most, so they are the ones evolving new shapes to block the wrong suitors.
• Asymmetry: Evolution is messy and specific. It doesn't change everyone at once; it changes the specific group that needs it most.

In short, when two species meet, the "imposters" get a free pass because their disguise works, while the "traditional" ladies have to work hard to build a better wall to keep the wrong suitors out.

Technical Summary

1. Problem Statement and Research Context

The study investigates the evolutionary mechanisms driving Reproductive Character Displacement (RCD) in two closely related, polymorphic damselfly species: Ischnura elegans and Ischnura graellsii. These species have formed two independent hybrid zones in Spain due to range expansion and secondary contact.

• The Core Issue: In these hybrid zones, reinforcement (selection against hybridization) has strengthened a mechanical barrier to mating. Previous research indicated that this reinforcement is asymmetric: it is strong in gynochrome females (the typical female color morphs) but weak or absent in androchrome females (male-mimic morphs).
• The Knowledge Gap: It was unclear whether this asymmetric reinforcement corresponds to morph-specific patterns of RCD in the morphological traits involved in the "tandem position" (the initial grasping phase of copulation). Specifically, the authors sought to determine if gynochrome females show greater morphological divergence from heterospecifics than androchrome females, and if male traits co-evolve with these female traits.

2. Methodology

The study employed geometric morphometrics to analyze shape and size variations in reproductive structures.

• Study System: Samples were collected from allopatric populations and two independent hybrid zones (Northwest [NW] and North-Central [NC]) in Spain.
• Dataset:
  • Females: 298 individuals (269 from previous data + 29 new) of both species, categorized into androchrome and gynochrome morphs.
  • Males: 60 individuals (30 per species) for prothorax analysis; caudal appendage data was integrated from a previous study (Ballén-Guapacha et al., 2024).
• Structures Analyzed:
  • Prothorax (Pronotum): The structure males grasp during tandem formation. Analyzed via posterior and lateral views using landmarks and semi-landmarks.
  • Male Caudal Appendages: The structures used to grasp the female.
• Analytical Techniques:
  • Generalized Procrustes Analysis (GPA): To align landmark configurations and remove non-shape variation.
  • Procrustes ANOVA & Linear Models: To test for differences in shape and Centroid Size (CS) among sexes, morphs, and geographic zones.
  • RCD Inference: RCD was defined as a significantly larger interspecific morphological distance in sympatry (hybrid zones) compared to allopatry.
  • Morphological Integration: Two-block Partial Least Squares (PLS) analysis to test for covariation between male caudal appendages and male prothorax.

3. Key Contributions

• Morph-Specific RCD: The study provides the first evidence that RCD can occur asymmetrically between female color morphs within the same species.
• Mechanism of Isolation: It links the "male mimicry" strategy of androchrome females directly to the mechanical isolation of the tandem position, showing that their prothorax morphology inherently resists heterospecific coupling.
• Male Trait Covariation: It extends RCD findings to male prothorax shape and investigates the functional integration between male grasping structures (appendages) and the grasping site (prothorax).

4. Key Results

A. Female Morph Differentiation and Mimicry

• I. elegans:
  • Shape: Androchrome females closely resemble males in prothorax shape across all zones. Gynochrome females differ significantly from both males and androchromes. However, the shape distinction between female morphs diminishes in the NW hybrid zone (high introgression).
  • Size: Androchromes are larger than gynochromes and resemble males in size, particularly in allopatry and the NC zone.
• I. graellsii:
  • Differentiation between morphs is weaker and more stable across zones. Both morphs differ from males in shape, but androchromes do not mimic male shape as closely as in I. elegans.

B. Asymmetric Reproductive Character Displacement (RCD)

• Prothorax Shape:
  • RCD Detected: In I. elegans females (both androchrome and gynochrome) in both hybrid zones.
  • No RCD: In I. graellsii females.
  • Male RCD: Detected in I. elegans males in the NC zone only.
• Prothorax Size (Centroid Size):
  • Asymmetric RCD: RCD in size was detected only in gynochrome females of I. elegans in the NC hybrid zone.
  • No RCD: No size-based RCD was found in androchrome females or in the NW hybrid zone for any group.
• Interpretation: The asymmetry in size-based RCD mirrors the previously observed asymmetry in mechanical reinforcement. Gynochromes, which suffer higher rates of heterospecific mating attempts, show stronger divergence in size to prevent coupling. Androchromes, already mechanically isolated by their male-like morphology, do not require further size divergence.

C. Morphological Integration

• Male Traits: A significant positive correlation was found between the size of male caudal appendages and the male prothorax in I. elegans, but not in I. graellsii.
• Shape: No significant correlation was found between the shape of male appendages and the male prothorax in either species.
• Implication: This suggests a limited, size-based covariation in male traits that may influence mating compatibility, supporting a "lock-and-key" mechanism where male traits evolve in concert to maintain species-specific coupling.

5. Significance and Conclusion

This study demonstrates that female polymorphism is a critical driver of evolutionary dynamics in hybrid zones.

1. Differential Selective Pressures: The presence of alternative female morphs (androchrome vs. gynochrome) creates distinct selective landscapes. Androchromes benefit from "pre-existing" mechanical isolation due to mimicry, reducing the need for RCD. Gynochromes, facing higher harassment, undergo strong asymmetric RCD.
2. Complexity of Reinforcement: Reinforcement is not a uniform process across a species; it can be morph-specific. The study confirms that RCD can act on shape (broadly across morphs) and size (specifically on gynochromes) differently.
3. Evolutionary Stability: The results highlight how hybridization can generate complex, morph-specific patterns of reproductive divergence, maintaining species boundaries through a combination of innate mechanical barriers (mimicry) and evolved displacement (RCD).

In summary, the paper elucidates how the interplay between sexual conflict, male mimicry, and hybridization drives asymmetric morphological evolution in damselflies, providing a nuanced view of how reproductive isolation evolves in polymorphic systems.