Alternative splicing shapes sexual dimorphism and erodes following the loss of sex in stick insects

This study demonstrates that alternative splicing is a widespread mechanism driving sexual dimorphism in *Timema* stick insects, particularly in male gonads, and that the transition to asexuality leads to a systematic erosion of this splicing complexity, suggesting sexual selection plays a key role in maintaining isoform diversity.

The Gist

Imagine your genome (your DNA) as a massive, single cookbook. For a long time, scientists thought that to make a male and a female look and act differently, the kitchen just had to turn the volume up or down on certain recipes. If a gene was a recipe for "muscle building," maybe the male kitchen cooks it 10 times a day, while the female kitchen cooks it only once. This is called gene expression.

But this new study on stick insects (Timema) reveals a much more sophisticated trick the kitchen uses: Alternative Splicing.

Think of a gene not just as a single recipe, but as a "Master Recipe" with many optional ingredients and steps.

• The Male Chef might take the Master Recipe, skip the "sugar" step, and add extra "spice."
• The Female Chef might take the same Master Recipe, keep the sugar, but skip the "baking time" step.

Even though they are using the exact same Master Recipe (the same gene), they end up with two completely different dishes (proteins). This is Alternative Splicing.

Here is what the researchers found, broken down into simple concepts:

1. The "Testis" is the Most Creative Kitchen

The researchers looked at three parts of the insect: the leg (femur), the stomach (gut), and the reproductive organs (gonads).

• The Analogy: Imagine the leg and stomach are like a cafeteria serving the same simple lunch every day. But the reproductive organs are like a high-end, experimental restaurant where the chefs are constantly inventing new variations of every dish.
• The Finding: The reproductive organs (gonads) had the most complex "splicing" (recipe variations). Surprisingly, the male reproductive organs were even more creative and complex than the female ones. The male "chefs" were generating a wider variety of protein dishes than the females.

2. It's Not Just About Volume; It's About Variety

Scientists used to think that making males and females different was mostly about how much of a gene was turned on (volume).

• The Analogy: They thought the difference between a male and female was just turning the radio up or down.
• The Finding: This study shows that changing the channel (splicing) is just as important as turning the volume up. In the reproductive organs, the insects use "splicing" to create differences just as often as they use "volume" (gene expression). It's a dual-engine system for creating two distinct sexes.

3. The "Old Classics" vs. The "Trendy Hits"

The researchers compared five different species of stick insects to see which recipes stayed the same over millions of years and which changed.

• The Analogy: Gene expression (volume) is like the "Top 40" hits on the radio; they change quickly as trends shift. Alternative splicing (recipe variations) is like the "Classic Rock" station; the core songs stay the same, but the bands keep remixing them in new ways.
• The Finding: While the specific genes that are turned on or off change rapidly between species, there is a core group of "Master Recipes" that are always spliced differently between males and females. These are the essential instructions for making a male or a female, and nature keeps them consistent. However, the specific variations of these recipes change faster than the volume settings do.

4. The "Asexual" Experiment: What Happens When You Stop Dating?

The stick insects are special because some species have males and females (sexual), while others are all-female and reproduce without males (asexual/parthenogenetic).

• The Analogy: Imagine a band that plays complex, high-energy music because they are competing for fans (sexual selection). Then, imagine a version of that band that stops playing for an audience and just plays for themselves (asexual).
• The Finding: When the insects lost their males and stopped having sexual competition, their "kitchen" got lazy.
  • The complex, creative recipe variations (splicing) started to disappear.
  • The "noise" in the kitchen didn't get louder (which would happen if they just stopped caring); instead, the complexity simply eroded.
  • The Conclusion: The complex splicing in sexual insects isn't just random noise or a side effect of evolution. It is actively maintained by sexual selection. The pressure to be a "perfect male" or "perfect female" keeps the kitchen busy inventing new variations. When that pressure is gone, the complexity fades away.

The Big Takeaway

This paper tells us that evolution doesn't just turn genes "on" or "off." It also acts like a master chef, constantly remixing the same ingredients to create distinct male and female forms.

However, this culinary creativity requires a reason to exist. In the world of stick insects, the drive to attract a mate and reproduce (sexual selection) is the fuel that keeps the kitchen complex and diverse. Without that drive, the recipes simplify, and the unique "flavors" of being male or female begin to fade.

Technical Summary

1. Problem Statement

Sexual dimorphism arises from divergent selective pressures on males and females, often leading to intralocus sexual conflicts where a shared genome must produce distinct phenotypes. While differential gene expression (DGE) is well-established as a primary mechanism for resolving these conflicts, the role of alternative splicing (AS) in generating sexual dimorphism remains understudied. Key open questions include:

• To what extent does AS contribute to sexual dimorphism compared to DGE?
• Are sex-biased splicing patterns evolutionarily conserved or rapidly turning over?
• Do AS and DGE target different gene sets under distinct selective constraints?
• How does the loss of sexual reproduction (transition to asexuality) and the subsequent removal of sexual selection affect splicing complexity and sex-specific variation?

2. Methodology

The study utilized the stick insect genus Timema as a natural evolutionary experiment, leveraging multiple independent transitions from sexual reproduction to parthenogenesis (asexuality).

• Biological System: Five pairs of closely related sexual and parthenogenetic Timema species. Samples included males and females from three tissues: femur (somatic), gut (somatic), and gonad (reproductive).
• Sequencing Strategy:
  • Long-read Sequencing: Used PacBio Iso-Seq (Isoform sequencing) to generate full-length, non-concatemer (FLNC) reads, enabling the resolution of complete isoform structures without assembly ambiguity.
  • Short-read Sequencing: Integrated existing Illumina RNA-seq data from the same tissues to provide expression quantification and splice junction validation.
• Data Processing & Analysis Pipeline:
  • Isoform Identification: Used the PacBio Iso-Seq pipeline (SMRT Link v12) to generate high-quality (HQ) consensus transcripts.
  • Filtering: Employed SQANTI3 to filter artifacts (e.g., RT-switching, intra-priming) and retain only high-confidence isoforms supported by short-read coverage.
  • Quantification: Mapped short reads to the filtered isoform set using Salmon to generate abundance matrices.
  • Differential Analysis:
    • DGE: Identified using DESeq2 (adjusted p < 0.05, |log2FC| ≥ 1).
    • Differential Splicing (DS): Identified using DRIMSeq (Dirichlet-multinomial model) to detect changes in relative isoform abundance (Differential Transcript Usage), independent of total gene expression.
  • Evolutionary Analysis:
    • Orthology: Inferred using OrthoFinder.
    • Selection Constraints: Calculated tissue specificity ($\tau$) and rates of coding-sequence evolution ($dN$, $dS$, $dN/dS$) using PAML (CODEML).
    • Phylogenetic Comparison: Compared patterns across sexual species and against their asexual counterparts to assess the impact of relaxed sexual selection and genetic drift.

3. Key Contributions

• Isoform-Resolved Landscape: Provided the first comprehensive, long-read based atlas of alternative splicing in Timema, revealing that 40–53% of curated genes possess multiple isoforms.
• Comparative Regulatory Mechanisms: Directly compared the prevalence and evolutionary dynamics of sex-biased gene expression versus sex-biased splicing within the same phylogenetic framework.
• Evolutionary Consequences of Asexuality: Demonstrated that the loss of sexual reproduction leads to a systematic erosion of sex-specific splicing patterns, challenging simple "drift-barrier" models that predict increased noise.

4. Key Results

A. Tissue and Sex-Specific Splicing Complexity

• Gonad Dominance: The gonad tissue exhibited the highest isoform complexity (mean isoforms per gene), significantly higher than somatic tissues (femur, gut).
• Male Bias in Reproductive Tissue: Males showed significantly higher gene expression counts and isoform diversity in the gonad compared to females. This contrasts with some Drosophila studies suggesting higher splicing complexity in ovaries, potentially reflecting greater cellular heterogeneity in Timema testes.
• Functional vs. Drift: Despite asexual species having reduced effective population sizes (which theoretically increases drift and splicing errors), sexual species exhibited higher isoform diversity than asexual species. This suggests splicing complexity is functionally maintained by selection rather than driven by drift.

B. Conservation and Turnover of Sex-Biased Splicing

• Prevalence: Sex-biased splicing (DS) is as prevalent as sex-biased expression (DE) in gonads (~34% of testable genes for DS vs. 32% for DE).
• Conserved Core: A core set of 54 genes showed conserved differential splicing across all five sexual species. These genes are enriched for splicing regulators (e.g., SRm160, B52) and germline development factors.
• Rapid Turnover: While a conserved core exists, the overall rate of lineage-specific turnover for DS genes is significantly higher than for DE genes. The enrichment for conserved DS genes is only 5-fold, compared to a 54-fold enrichment for conserved DE genes.

C. Distinct Selective Constraints

• Non-Redundant Targets: DE and DS genes largely target different gene sets. Approximately 68% of DS genes show unbiased expression levels between sexes.
• Pleiotropy and Evolutionary Rate:
  • Tissue Specificity: DS genes are significantly more broadly expressed across tissues (lower $\tau$) than DE genes.
  • Evolutionary Rate: DS genes evolve slower (lower $dN/dS$) than DE genes.
  • Interpretation: This indicates that alternative splicing is a mechanism to resolve sexual conflict for genes under strong purifying selection and broad pleiotropic constraints, allowing sex-specific isoforms without altering overall gene dosage.

D. Erosion in Asexual Lineages

• Desexualization: Upon the transition to asexuality, there is a systematic reduction in isoform diversity and a loss of sex-specific splicing patterns.
• Mechanism: Asexual females and males showed decreased abundance of both female-biased and male-biased isoforms found in sexual species.
• Drift vs. Selection: Contrary to the "drift barrier" hypothesis (which predicts increased splicing noise in asexuals due to relaxed selection), the study found no increase in splicing noise. Instead, the loss of specific isoforms suggests that sexual selection actively maintains splicing complexity in sexual species.

5. Significance

• Regulatory Evolution: Establishes alternative splicing not merely as a secondary byproduct of gene expression, but as a primary, evolutionarily dynamic layer of gene regulation essential for sexual dimorphism.
• Conflict Resolution: Provides evidence that AS is a specific evolutionary strategy to resolve sexual conflict for highly constrained, broadly expressed genes where changing expression levels is deleterious.
• Impact of Asexuality: Demonstrates that the maintenance of transcriptome complexity (specifically isoform diversity) is dependent on sexual selection. The erosion of these patterns in asexual lineages highlights the cost of losing sexual selection beyond just morphological traits.
• Methodological Advance: Highlights the necessity of long-read sequencing to accurately quantify sex-biased splicing, as short-read methods often miss the full extent of isoform diversity and differential transcript usage.

In conclusion, the study posits that sexual selection drives the maintenance of complex splicing networks in reproductive tissues, and the loss of sex leads to a rapid simplification of the transcriptome, eroding the regulatory mechanisms that facilitate sexual dimorphism.