Natural variation in male frequency fails to predict inbreeding responses in Caenorhabditis elegans

A study on nine *Caenorhabditis elegans* strains demonstrates that natural variation in male frequency fails to predict the magnitude of inbreeding depression or the extent of fitness recovery, indicating that male frequency is a poor proxy for realized outcrossing and its evolutionary benefits.

The Gist

Imagine a tiny worm called Caenorhabditis elegans as a small, self-contained factory. Most of the time, these factories run entirely on their own, making copies of themselves without needing help from anyone else. This is called "self-fertilization." However, occasionally, these factories produce a special type of worker called a "male."

For a long time, scientists have been puzzled: Why keep these males around if the factory can run perfectly fine alone?

One popular theory was that males act like a "safety valve" against a problem called "inbreeding depression." Think of inbreeding depression like a factory running on a single, worn-out blueprint for too long. Eventually, the products start having defects because there's no fresh input. The theory suggested that when the factory gets too repetitive, the males swoop in to mix things up (outcrossing), bringing in fresh blueprints to fix the defects and keep the factory running strong.

The Experiment: A Stress Test
To test this idea, researchers set up a controlled experiment with nine different strains (families) of these worms. They knew that some families naturally produced more males than others.

1. The "No-Exit" Phase: They forced nine different worm families to reproduce strictly on their own for seven generations. No males were allowed to mix things up. This was like forcing a factory to use the same worn-out blueprint for seven years straight to see how much the quality would drop.
2. The "Fresh Air" Phase: After the quality dropped, they relaxed the rules for four generations, allowing the males to mix their genes back in. This was like opening the factory doors to let in fresh blueprints to see if the quality could bounce back.

The Prediction
The scientists guessed that the families that naturally produced more males (the ones with the biggest "safety valve") would be the best at recovering. They thought these families would bounce back to full health quickly because they were used to mixing things up.

The Surprising Result
The results were a bit of a plot twist.

• The Damage: Almost all the worm families suffered a significant drop in fitness (their "products" got worse) after the seven generations of isolation.
• The Recovery: Most families did manage to recover some of their health once they were allowed to mix again.
• The Missing Link: Here is the kicker—there was no connection between how many males a family usually had and how well they recovered.

Some families with very few males bounced back just as well as families with lots of males. Conversely, having a high number of males didn't guarantee a faster or better recovery.

The Bottom Line
In simple terms, counting how many "male workers" a worm family usually has is a terrible way to predict how well that family can handle the stress of inbreeding or how fast they can fix it. The number of males doesn't reliably tell us how much "fresh mixing" actually happens in nature, nor does it predict how well the worms will survive when things get tough. The "safety valve" theory, at least as measured by male frequency, doesn't hold up in this specific test.

Technical Summary

Technical Summary: Natural Variation in Male Frequency Fails to Predict Inbreeding Responses in Caenorhabditis elegans

Problem Statement
In androdioecious species such as Caenorhabditis elegans, reproduction is dominated by self-fertilization, leaving the evolutionary persistence of males a longstanding biological puzzle. A prominent hypothesis suggests that males provide a mechanism to escape inbreeding depression by facilitating outcrossing. However, the extent to which natural variation in male frequency within populations accurately reflects realized outcrossing rates or their fitness consequences remains unclear. This study investigates whether baseline male frequency serves as a reliable predictor of a strain's susceptibility to inbreeding depression and its subsequent capacity for fitness recovery.

Methodology
The researchers conducted a controlled experimental evolution study using nine distinct C. elegans strains that exhibited natural variation in their baseline male frequencies. The experimental design involved three primary phases:

1. Inbreeding Phase: Seven generations of enforced inbreeding were applied to all strains to induce inbreeding depression.
2. Fitness Measurement: Competitive relative fitness was measured for each strain both before and after the inbreeding period to quantify the magnitude of fitness decline.
3. Recovery Phase: Inbreeding constraints were relaxed for four generations to allow for potential outcrossing and fitness recovery.

The study specifically tested the hypothesis that strains with higher baseline male frequencies would demonstrate a faster or more complete recovery of fitness following the relaxation of inbreeding, based on the assumption that higher male frequency correlates with a greater historical opportunity for outcrossing.

Key Results
The study yielded several critical findings regarding the relationship between male frequency and inbreeding dynamics:

• Substantial Variation: The nine strains exhibited significant heterogeneity in their responses to the experimental conditions. Most strains experienced significant fitness declines following seven generations of inbreeding and showed only partial recovery after four generations of relaxed inbreeding.
• Lack of Correlation: Crucially, statistical analysis revealed no correlation between the baseline male frequency of a strain and either the magnitude of inbreeding depression or the extent of fitness recovery.
• Predictive Failure: Strains with high male frequencies did not necessarily recover faster or better than those with low male frequencies, contradicting the initial hypothesis that male frequency is a proxy for realized outcrossing efficacy.

Significance and Claims
The paper concludes that natural variation in male frequency is a poor predictor of inbreeding responses in C. elegans. The authors assert that male frequency does not reliably reflect the realized rate of outcrossing or the associated fitness consequences in natural or experimental contexts. Consequently, the presence of males and their frequency alone cannot be used to infer a population's evolutionary strategy regarding the mitigation of inbreeding depression. This finding challenges the utility of male frequency as a standalone metric for understanding the evolutionary maintenance of males in predominantly selfing species.